DENDROGEOMORPHOLOGICAL EVIDENCE OF FLOOD EVENTS IN THE UPPER CATCHMENT OF THE NOGUERA PALLARESA RIVER (PYRENEES, SPAIN)

3.1. Sampling and dendrochronological analysis of tree-ring growth

Two tributaries of the Upper Noguera Pallaresa are studied: the Romadriu River (in the left-hand margin) and the Flamisell River (in the right-hand margin) (Figure 1 and Table 1). In the Romadriu subcatchment, the Portainé and Ramiosa streams (tributaries of the Romadriu) were sampled between March 2014 and March and September 2015, while between the end of May and the beginning of June 2016, other areas close to the riverbanks of the Romadriu River itself were also sampled. The lower section of the Portainé stream, which forms an elongated cone of alluvial debris at its confluence with the Romadriu, has been intensively studied and the results obtained were published by Génova, et al. (2018)Génova, M., Díez-Herrero, A., Furdada, G., Guinau, M. & Victoriano, A. 2018. Dendrogeomorphological evidence of flood frequency changes and human activities (Portainé Basin, Spanish Pyrenees). Tree-ring research, 74(2): 144-161. https://doi.org/10.3959/1536-1098-74.2.144 and Furdada, et al. (2020)Furdada, G., Victoriano, A., Díez-Herrero, A., Génova, M., Guinau, M., De las Heras, A., Palau, R., Hürlimann, M., Khazaradze, G., Casas, J.M., Margalef, A., Pinyol, J. & Gonzalez, M., 2020. Flood consequences of land-use changes at a ski resort: Overcoming a geomorphological threshold (Portainé, Eastern Pyrenees, Iberian Peninsula). Water, 12(2): 368. https://doi.org/10.3390/w12020368 . The cone is covered by a very diverse hardwood forest, dominated by species such as Populus tremula L., Fraxinus excelsior L., Populus nigra L., Prunus avium L. and Betula pendula Roth, along with scattered specimens of Tilia platyphyllos, Quercus petraea (Matt.) Liebl., Juglans regia L., Acer campestre L., and Salix caprea L.). Around the Ramiosa stream, which in the same area flows into the Romadriu River, there is a similar forest, although here Quercus petraea and Fraxinus excelsior are more frequent. On the other hand, in the sampled areas of the Romadriu riverbank Populus tremula and Populus nigra dominate. The presence of a ski resort in Portainé from 2006-2008 resulted in the different behavior of flood and debris flow dynamics (Furdada, et al., 2020Furdada, G., Victoriano, A., Díez-Herrero, A., Génova, M., Guinau, M., De las Heras, A., Palau, R., Hürlimann, M., Khazaradze, G., Casas, J.M., Margalef, A., Pinyol, J. & Gonzalez, M., 2020. Flood consequences of land-use changes at a ski resort: Overcoming a geomorphological threshold (Portainé, Eastern Pyrenees, Iberian Peninsula). Water, 12(2): 368. https://doi.org/10.3390/w12020368 ), leading to the sedimentation of the current cone at the confluence of the Portainé stream with the Romadriu River. In the Ramiosa catchment, the channel flow is incised in the bedrock.

In October 2018, a small area of ~500 m2 in the Flamisell subcatchment, was sampled in a dense riparian forest growing on a large gravel bar, consisting mainly of Alnus glutinosa (L.) Gaertn., together with scattered Populus nigra trees. The area is situated approximately 1 km south of the town of Senterada and downstream of the small dam or weir of Senterada (0.12 Hm3, 26.5 m long and 3.7 m high above the channel, built between 1919 and 1920; Galí, 1922Galí, J., 1922. Los Grandes aprovechamientos hidráulicos de la Sociedad Productora de Fuerzas Motrices. Revista tecnológico-industrial, 39: 65-80.). This sampling area, located immediately downstream of the weir, was chosen because a large number of trees with damage and scars were found, and also to explore the effect of this small hydraulic structure.

The dendrochronological sampling is focused mainly on trees that presented clear signs of damage (Stoffel & Bollschweiler, 2008Stoffel, M. & Bollschweiler, M., 2008. Tree-ring analysis in natural hazards research? An overview. Natural Hazards and Earth System Science, 8(2):187-202. https://doi.org/10.5194/nhess-8-187-2008 ; Díez-Herrero, et al., 2013Díez-Herrero, A., Ballesteros-Cánovas, J.A., Bodoque, J.M. & Ruiz-Villanueva, V. 2013. A new methodological protocol for the use of dendrogeomorphological data in flood risk analysis. Hydrology Research, 44(2): 234-247. https://doi.org/10.2166/nh.2012.154 ; Stoffel & Corona, 2014Stoffel, M. & Corona, C., 2014. Dendroecological dating of geomorphic disturbance in trees. Tree-Ring Research, 70(1): 3-20. https://doi.org/10.3959/1536-1098-70.1.3 ; Génova, et al., 2015Génova, M., Máyer, P., Ballesteros-Cánovas, J., Rubiales, J.M., Saz, M.A. & Díez-Herrero, A. 2015. Multidisciplinary study of flash floods in the Caldera de Taburiente National Park (Canary Islands, Spain). Catena, 131: 22-34. https://doi.org/10.1016/j.catena.2015.03.007 , 2018Génova, M., Díez-Herrero, A., Furdada, G., Guinau, M. & Victoriano, A. 2018. Dendrogeomorphological evidence of flood frequency changes and human activities (Portainé Basin, Spanish Pyrenees). Tree-ring research, 74(2): 144-161. https://doi.org/10.3959/1536-1098-74.2.144 ), potentially caused by the impact of boulders and sediment particles or floating material carried by the floods (mainly logs or large pieces of wood). The main indicators of macroscopic damage identified consisted of scars, tree crown loss or loss of the main stem, tilting trunks, and dead trees (Figure 2). Other nearby trees were also sampled, because although they did not show any external signs of damage, they may either have been wounded at the base covered by dragged material or perhaps had sustained old internal wounds covered by callus (Génova, et al., 2015Génova, M., Máyer, P., Ballesteros-Cánovas, J., Rubiales, J.M., Saz, M.A. & Díez-Herrero, A. 2015. Multidisciplinary study of flash floods in the Caldera de Taburiente National Park (Canary Islands, Spain). Catena, 131: 22-34. https://doi.org/10.1016/j.catena.2015.03.007 , 2018Génova, M., Díez-Herrero, A., Furdada, G., Guinau, M. & Victoriano, A. 2018. Dendrogeomorphological evidence of flood frequency changes and human activities (Portainé Basin, Spanish Pyrenees). Tree-ring research, 74(2): 144-161. https://doi.org/10.3959/1536-1098-74.2.144 ).

Additional data were collected for each tree selected, including general dendrometric data (perimeter, height, dominance) and the size of the wounds. Furthermore, sketches of the sampled trees and the environment were made, their precise location was determined with differential GNSS, their detailed geomorphological position (elements or facets; such as river bank, channel bar, floodplain), and photographs were also taken.

The extraction of the dendrochronological samples (cylindrical cores) was carried out using a 400 mm long Pressler borer with an internal diameter of 5 mm (Grissino-Mayer, 2003Grissino-Mayer, H.D., 2003. A manual and tutorial for the proper use of an increment borer. Tree-Ring Research, 59: 63-79.). At least 2 samples were extracted from each selected specimen representing the trunk growth, one in the flow direction and the other in the opposite direction. Other samples close to the scars or the callus were likewise extracted to complete the information (Stoffel & Bollschweiler, 2008Stoffel, M. & Bollschweiler, M., 2008. Tree-ring analysis in natural hazards research? An overview. Natural Hazards and Earth System Science, 8(2):187-202. https://doi.org/10.5194/nhess-8-187-2008 ; Génova, et al., 2018Génova, M., Díez-Herrero, A., Furdada, G., Guinau, M. & Victoriano, A. 2018. Dendrogeomorphological evidence of flood frequency changes and human activities (Portainé Basin, Spanish Pyrenees). Tree-ring research, 74(2): 144-161. https://doi.org/10.3959/1536-1098-74.2.144 ), both the trunks and replacement branches of decapitated trees (Figure 3). The extracted samples were placed on wooden supports for conservation and drying and, once dry, the usual preparation procedures were followed before measurement. In addition, some wedges with parts of the callus and complete sections of the trunk of dead or badly damaged trees were also obtained (Génova, et al., 2018Génova, M., Díez-Herrero, A., Furdada, G., Guinau, M. & Victoriano, A. 2018. Dendrogeomorphological evidence of flood frequency changes and human activities (Portainé Basin, Spanish Pyrenees). Tree-ring research, 74(2): 144-161. https://doi.org/10.3959/1536-1098-74.2.144 ).

We used the LINTAB measurement table with a precision of 0.01 mm and the associated computer program TSAP-Win to measure the tree rings and compile temporal tree-ring growth data series. The tree-ring series were dated by assigning dates to each value of the series using synchronization techniques to perform qualitative and quantitative, visual and statistical analyses (Cook & Kairiukstis, 1990Cook, E. & Kairiukstis, L., 1990. Methods of Dendrochronology. Applications in the Environmental Sciences. Dordrecht, Netherlands: Kluwer Academic Publishers.), as well as using the statistical functions (mainly Standard t-value and synchronicity) of TSAP-Win, and results were verified using the software Cofecha (Grissino-Mayer, 2001Grissino-Mayer, H.D., 2001. Evaluating crossdating accuracy: A manual and tutorial for the computer program COFECHA. Tree-Ring Research, 57(2): 205-221.). First, we synchronized the tree-ring series from each tree, which was then averaged and synchronized with those from the other trees of the same species.

3.2. Dendrogeomorphological analysis

We selected and analyzed marks and other evidence in the dated tree-ring series that could provide information on torrential floods (Shroder, 1980Shroder, J. F., 1980. Dendrogeomorphology: Review and new techniques of tree-ring dating. Progress in Physical Geography, 4(2):161-188. https://doi.org/10.1177/030913338000400202 ; Stoffel and Bollschweiler, 2008Stoffel, M. & Bollschweiler, M., 2008. Tree-ring analysis in natural hazards research? An overview. Natural Hazards and Earth System Science, 8(2):187-202. https://doi.org/10.5194/nhess-8-187-2008 ; Díez-Herrero, et al., 2013Díez-Herrero, A., Ballesteros-Cánovas, J.A., Bodoque, J.M. & Ruiz-Villanueva, V. 2013. A new methodological protocol for the use of dendrogeomorphological data in flood risk analysis. Hydrology Research, 44(2): 234-247. https://doi.org/10.2166/nh.2012.154 ; Ruiz-Villanueva, et al., 2013Ruiz-Villanueva, V., Díez-Herrero, A., Bodoque, J.M., Ballesteros Cánovas J.A. & Stoffel, M., 2013. Characterisation of flash floods in small ungauged mountain basins of Central Spain using an integrated approach. Catena, 110: 32-43. https://doi.org/10.1016/j.catena.2013.06.015 ; Stoffel & Corona, 2014Stoffel, M. & Corona, C., 2014. Dendroecological dating of geomorphic disturbance in trees. Tree-Ring Research, 70(1): 3-20. https://doi.org/10.3959/1536-1098-70.1.3 ; Génova, et al., 2018Génova, M., Díez-Herrero, A., Furdada, G., Guinau, M. & Victoriano, A. 2018. Dendrogeomorphological evidence of flood frequency changes and human activities (Portainé Basin, Spanish Pyrenees). Tree-ring research, 74(2): 144-161. https://doi.org/10.3959/1536-1098-74.2.144 ), mainly as follows:

Abrupt growth changes (suppressions and releases) in tree-ring growth series are responses to different possible inputs. Therefore, they were only taken into account as complementary data when they were correlated with other indications and affected more than 10 % of the trees. These indicators were analyzed using the LRM software (List Ring Measurement; Holmes, 1999Holmes, R.L., 1999. Dendrochronological Program Library (DPL). User’s Manual. Tucson, USA: Laboratory of Tree-Ring Research, University of Arizona.), which identifies the significant years in growth change according to their representativeness in groups of trees. In the most recent study carried out in the Flamisell subcatchment, where the trees were young, this analysis was complemented with a study of growth changes according to the proposal put forward by Nowacki & Abrams (1997)Nowacki, G.J. & Abrams, M.D. 1997. Radial‐growth averaging criteria for reconstructing disturbance histories from presettlement‐origin oaks. Ecological Monographs, 67(2): 225-249. https://doi.org/10.1890/0012-9615(1997)067[0225:RGACFR]2.0.CO;2 and previously used in Dendrogeomorphology (for example in Sorg, et al., 2010Sorg, A., Bugmann, H., Bollschweiler, M. & Stoffel, M., 2010. Debris-flow activity along a torrent in the Swiss Alps: Minimum frequency of events and implications for forest dynamics. Dendrochronologia, 28(4): 215-223. https://doi.org/10.1016/j.dendro.2009.11.002 and Malik, et al., 2021Malik, I., Dłużewski, M., Rotnicka, J., Wistuba, M., Krzemień, K., Muszyński, A., Rojan, E. & Ślęzak, A., 2021. Simultaneous growth releases and reductions among Populus alba as an indicator for floods in dry mountains (Morocco). Ecological Indicators, 129: 107874. https://doi.org/10.1016/j.ecolind.2021.107874 ). The percentage change in the growth of each tree from the date of the possible event (GC) was determined using the growth mean of 2 years before (M1) and the growth mean of 2 years after (M2), according to the formula:

considering the threshold for recognizing the change in each tree to be ≥25%.

On completion of the analysis of the different marks and signals registered in the tree rings, we follow the proposal by Génova, et al., (2018)Génova, M., Díez-Herrero, A., Furdada, G., Guinau, M. & Victoriano, A. 2018. Dendrogeomorphological evidence of flood frequency changes and human activities (Portainé Basin, Spanish Pyrenees). Tree-ring research, 74(2): 144-161. https://doi.org/10.3959/1536-1098-74.2.144 to date the recognized evidence of damage. The events are therefore dated with a biannual denomination, the dendrogeomorphological year, which indicates that they may have first started at the end of the formation of the previous ring; that is, during the latency period of the trees until the end of the year’s growth.

3.3. Documentary, geomorphological and instrumental data

Documentary data on recent floods and human activities were obtained from different technical reports (IGC, 2008IGC, 2008. Nota tecnica sobre la visita al barranc de Portaine i al barranc des Caners els dies 1 i 2 d’octubre de 2008 en motiu de la torrentada ocorreguda la matinada del dia 12 de Setembre de 2008. AP-187/08. Barcelona, Spain: Institut Geologic de Catalunya.; ICGC, 2010aICGC, 2010a. Estudi de la torrentada de la nit del dia 11 al 12 de Setembre de 2008 al barranc de Portaine (Pallars Sobira). AP-019/10. Barcelona, Spain: Institut Geologic de Catalunya.,bICGC, 2010b. Nota de la visita al barranc de Portaine (Pallars Sobira) arran del episodi de pluges dels dies 22 i 23 de Juliol de 2010. AP-046/10. Barcelona, Spain: Institut Geologic de Catalunya., 2011ICGC, 2011. Nota de la visita al barranc de Portaine (Pallars Sobira) arran de l’episodi de pluges del dia 5 d’agost de 2011. AP-054/11. Barcelona, Spain: Institut Geologic de Catalunya, Barcelona. and 2013aICGC, 2013a. Avaluacio de la dinamica torrencial del torrent de Portaine. AP-035/13. Barcelona, Spain: Institut Geologic de Catalunya.,bICGC, 2013b. Nota de la visita al barranc de Portaine (Pallars Sobira) arran de l’episodi de pluges del dia 23 de Juliol de 2013 AP-091/13. Barcelona, Spain: Institut Geologic de Catalunya, Barcelona.; IGC et al., 2013IGC, GEOCAT & FGC, 2013. Informe de visita de terreny de les esllavissades i barrancades a la carretera d’acces a Portaine a Juliol de 2013. AP-057/13. Barcelona, Spain: Institut Geologic de Catalunya, Gestio de Projectes S.A. and Ferrocarrils de la Generalitat de Catalunya.). Historical data on the main rainfall and past flooding were obtained from local witnesses, archives (mainly from the Arxiu Comarcal del Pallars Sobirà), newspaper libraries, and the work by Balasch, et al. 2008Balasch, C., Becat, J., Marugan, C.M., Nadal, A., Rapalino, V. & Remacha, R., 2008. Les riuades del segle XX al Pallars Sobirà: 1907, 1937 i 1982. Colección: Arxius y societat: Quaderns de divulgació històrica nº 2. Barcelona, España: Departament de Cultura i Mitjans de Comunicació, Generalitat de Catalunya.. The PREDIFLOOD (Barriendos, et al., 2014Barriendos, M., Ruiz-Bellet, J. L., Tuset, J., Mazón, J., Balasch, J.C., Pino, D. & Ayala J. L., 2014. The “Prediflood” database of historical floods in Catalonia (NE Iberian Peninsula) AD 1035-2013, and its potential applications in flood analysis. Hydrol. Earth Syst. Sci., 18: 4807-4823. https://doi.org/10.5194/hess-18-4807-2014 ) and INUNGAMA flood databases (Llasat, et al., 2013Llasat, M.C., Llasat-Botija, M., Petrucci, O., Pasqua, A. A., Rossello, J., Vinet, F. & Boissier, L., 2013. Towards a database on societal impact of Mediterranean floods within the framework of the HYMEX project. Nat. Hazards Earth Syst. Sci., 13: 1337-1350. https://doi.org/10.5194/nhess-13-1337-2013 ), handled and managed by M. Barriendos and C. Llasat (Universitat de Barcelona), respectively, were also consulted. Images provided by the company in charge of the Mal Pas hydroelectric infrastructure (https://ca.wikipedia.org/wiki/Central_hidroel%C3%A8ctrica_de_Mal_Pas) helped us to analyze the dynamics of the torrential flows that affected the Portainé stream in 2008 and 2010 (Garcia-Oteyza, et al., 2015Garcia-Oteyza, J., Genova, M., Calvet, J., Furdada, G., Guinau M. & Díez-Herrero, A., 2015. Datación de avenidas torrenciales y flujos de derrubios mediante metodologias dendrogeomorfológicas (barranco de Portaine, Lleida, Espana). Revista Ecosistemas, 24(2): 43-50. https://doi.org/10.7818/ECOS.2015.24-2.07 ; Génova, et al., 2018Génova, M., Díez-Herrero, A., Furdada, G., Guinau, M. & Victoriano, A. 2018. Dendrogeomorphological evidence of flood frequency changes and human activities (Portainé Basin, Spanish Pyrenees). Tree-ring research, 74(2): 144-161. https://doi.org/10.3959/1536-1098-74.2.144 ; Victoriano, et al., 2018bVictoriano, A., Díez-Herrero, A., Génova, M., Guinau, M., Furdada, G., Khazaradze, G. & Calvet, J., 2018b. Four-topic correlation between flood dendrogeomorphological evidence and hydraulic parameters (the Portainé stream, Iberian Peninsula). Catena, 162: 216-229. https://doi.org/10.1016/j.catena.2017.11.009 ). The collection of photographs belonging to the La Pobla de Segur Fire station, showing the impacts of the 1982 floods, were used to corroborate the high energy of the event and its destructive capacity.

The detailed geomorphological mapping, together with the positions of the affected trees at the mouth of the Portainé stream to the Noguera Pallaresa River, were presented and analyzed by Victoriano, et al., (2018b)Victoriano, A., Díez-Herrero, A., Génova, M., Guinau, M., Furdada, G., Khazaradze, G. & Calvet, J., 2018b. Four-topic correlation between flood dendrogeomorphological evidence and hydraulic parameters (the Portainé stream, Iberian Peninsula). Catena, 162: 216-229. https://doi.org/10.1016/j.catena.2017.11.009 , who related the different geomorphologic units with the relative position of the damaged trees and the hydraulics of the floods. The results obtained have been completed and compared with those of the Flamisell study area. Vertical aerial photographs and multi-temporal orthophotographs (period 1946-2018) were collected in both the Flamisell and Romadriu study areas. Photointerpretation was carried out to detect changes due to fluvio-torrential dynamics using criteria consisting of tree mass density, coloration, and texture (indicative of sediment transported and deposited shortly before the photograph was taken, etc.). The 1982 vertical aerial photographs, obtained one month after the flood, provide only limited information due to the shadows in the narrower and deeper valleys. Although it was not possible to interpret the entire valley floor of the Romadriu River, the photointerpretation of the shadow-free sections provided relevant information regarding the effects of the flood (see Results section).

Although there are numerous meteorological stations around the Flamisell and the Romadriu subcatchments, none of them are located within the Flamisell area and only those of Salòria and Portainé (in operation since 2011) are situated within the Romadriu subcatchment. Furthermore, many of them have short time series or considerable gaps. Figure 4 shows the stations from which data have been used in this work. Original data have been used (both from https://www.meteo.cat/observacions/xema, with point-selective daily data from June 2009 to the present, and from MeteoPirineu) and data cited in the works by Palau, et al., (2017)Palau, R.M., Hürlimann M., Pinyol, J., Moya, J., Victoriano, A., Génova, M. & Puig, C., 2017. Recent debris-flows in the Portainé catchment (Eastern Pyrenees, Spain). Analysis of monitoring and field data focusing on the 2015 event. Landslides, 14(3): 1161-1170. https://doi.org/10.1007/s10346-017-0832-9 , Trapero, et al., (2013a)Trapero, L., Bech, J. & Lorente, J., 2013a. Numerical modelling of heavy precipitation events over Eastern Pyrenees: Analysis of orographic effects. Atmospheric Research, 123: 368-383. https://doi.org/10.1016/j.atmosres.2012.09.014 and Bech, et al., (2011)Bech, J., Pineda, N., Rigo, T., Aran, M., Amaro, J., Gaya, M., Arus, J., Montanya, J. & Van der Velde, O., 2011. A Mediterranean nocturnal heavy rainfall and tornadic event. Part I: Overview, damage survey and radar analysis. Atmos. Res., 100: 621-637. https://doi.org/10.1016/j.atmosres.2010.12.024 . Palau, et al., (2017)Palau, R.M., Hürlimann M., Pinyol, J., Moya, J., Victoriano, A., Génova, M. & Puig, C., 2017. Recent debris-flows in the Portainé catchment (Eastern Pyrenees, Spain). Analysis of monitoring and field data focusing on the 2015 event. Landslides, 14(3): 1161-1170. https://doi.org/10.1007/s10346-017-0832-9 take precipitation values from various meteorological stations in a detailed study of the 21/Aug./2015 event. Bech, et al., (2011)Bech, J., Pineda, N., Rigo, T., Aran, M., Amaro, J., Gaya, M., Arus, J., Montanya, J. & Van der Velde, O., 2011. A Mediterranean nocturnal heavy rainfall and tornadic event. Part I: Overview, damage survey and radar analysis. Atmos. Res., 100: 621-637. https://doi.org/10.1016/j.atmosres.2010.12.024 defined isohyets from real data of 24 h accumulated precipitation in the regional 1-2/Nov./2008 event, while Trapero, et al. (2013a)Trapero, L., Bech, J. & Lorente, J., 2013a. Numerical modelling of heavy precipitation events over Eastern Pyrenees: Analysis of orographic effects. Atmospheric Research, 123: 368-383. https://doi.org/10.1016/j.atmosres.2012.09.014 performed simulations of the same event using the data from Bech, et al., (2011)Bech, J., Pineda, N., Rigo, T., Aran, M., Amaro, J., Gaya, M., Arus, J., Montanya, J. & Van der Velde, O., 2011. A Mediterranean nocturnal heavy rainfall and tornadic event. Part I: Overview, damage survey and radar analysis. Atmos. Res., 100: 621-637. https://doi.org/10.1016/j.atmosres.2010.12.024 , considering 24 h of accumulated precipitation. From these two works, we selected the ranges of precipitation registered in the set of stations analyzed. In addition, monthly and annual daily maximum rainfall data have been obtained from the Anuaris de dades meteorològiques (Yearbooks of meteorological data) of the Catalan Meteorological Service (https://www.meteo.cat/wpweb/climatologia/serveis-i-dades-climatiques/anuaris-de-dades-meteorologiques/).

Meteorological data for the Portainé stream have been collected and analyzed by Victoriano, (2018)Victoriano, A., 2018. Integración de métodos para la caracterización hidrogeomorfológica de la actividad torrencial en una cuenca de montaña (Portainé, Pirineos Orientales). Departament de Dinàmica de la Terra i de l’Oceà, Universitat de Barcelona, Barcelona, 2018. Tesis Doctorals en Xarxa. http://hdl.handle.net/10803/668463 and Furdada, et al. (2020)Furdada, G., Victoriano, A., Díez-Herrero, A., Génova, M., Guinau, M., De las Heras, A., Palau, R., Hürlimann, M., Khazaradze, G., Casas, J.M., Margalef, A., Pinyol, J. & Gonzalez, M., 2020. Flood consequences of land-use changes at a ski resort: Overcoming a geomorphological threshold (Portainé, Eastern Pyrenees, Iberian Peninsula). Water, 12(2): 368. https://doi.org/10.3390/w12020368 . The most significant data were selected for the present work and are presented in the following sections. Moreover, reports on “Singular weather events” (https://www.meteo.cat/wpweb/climatologia/butlletins-i-episodis-meteorologics/episodis-meteorologics/) were consulted and cross-checked with rainfall and gauging data whenever necessary or possible (due to short series, which do not cover the periods of interest or contain data gaps). For example, for the Flamisell, when no “Singular weather events” were found that could explain the damage to the vegetation (e.g. in the dendrogeomorphological year 2009-10, see section 4.3) the dates of rainfall events of interest (see section 4.3) were explored in the meteorological yearbooks. For these dates, using the Automatic Stations Map, the precipitation in each one of the stations was selected and compared. It should be noted that the significant rainfall events addressed by Bech, et al., (2011)Bech, J., Pineda, N., Rigo, T., Aran, M., Amaro, J., Gaya, M., Arus, J., Montanya, J. & Van der Velde, O., 2011. A Mediterranean nocturnal heavy rainfall and tornadic event. Part I: Overview, damage survey and radar analysis. Atmos. Res., 100: 621-637. https://doi.org/10.1016/j.atmosres.2010.12.024 and Trapero, et al., (2013a)Trapero, L., Bech, J. & Lorente, J., 2013a. Numerical modelling of heavy precipitation events over Eastern Pyrenees: Analysis of orographic effects. Atmospheric Research, 123: 368-383. https://doi.org/10.1016/j.atmosres.2012.09.014 , and also described in the “Singular weather events”, are indeed well known, but their hydrological and derived geomorphological effects have not been studied before.

There are no gauging stations in the Romadriu subcatchment, so there are no discharge data for this subcatchment. In the Flamisell subcatchment, there are 3 gauging stations (Table 2).

We use data from station 9267 - Capdella (in operation since 1989) - as it provides the longest discharge series and is the station that contains the periods of the establishment of the riparian trees and the dendrogeomorphological years with detected damage on these trees (see Results section), as well as being relatively close to the sampling area (~18 km upstream). For this station, data are available on daily average discharges and monthly maximum instantaneous discharges (m³/s). Although no data exist for the following periods: 26/Nov./1990 - 18/Jan./1991; 19/Jul./2000 - 19/Aug./2000; 31/Jul/2002 - 05/Sep./2002; 01/Oct./2006 - 30/Sep./2007; 01/Oct./2008 - 28/Mar./2011 and while these data do not reflect the total daily discharges, they nevertheless indicate the days on which significant flooding occurred.

At station 9181 - La Pobla de Segur (in operation from 1965 to 1991) - approx. 9 km below the sampling area, at the confluence with the Noguera Pallaresa, there are no data from 01/Nov./1982 to 01/Oct./1983 due to the destruction of the gauging station caused by the November 1982 flood. The series ends when the period of establishment of riparian trees begins, so no useful data was available for interpreting the damage to the vegetation. In any case, the data of the monthly contributions in hm³ of the Flamisell were retrieved to obtain an order of magnitude of the contributions of this subcatchment. These data were compared with the 1982 Flamisell flood discharge reconstruction obtained by Balasch, et al., (2008)Balasch, C., Becat, J., Marugan, C.M., Nadal, A., Rapalino, V. & Remacha, R., 2008. Les riuades del segle XX al Pallars Sobirà: 1907, 1937 i 1982. Colección: Arxius y societat: Quaderns de divulgació històrica nº 2. Barcelona, España: Departament de Cultura i Mitjans de Comunicació, Generalitat de Catalunya.. The 9274 - Sallente P.P. station - provided no useful data for the present work.

All the data obtained, both documentary and geomorphological, were unfortunately incomplete, and the rainfall and discharge data series have gaps or are short. Faced with these limitations, the methodology used is based on the compilation and chronological ordering of each type of data and, subsequently, on their comparison and joint analysis. Geomorphological and forest density changes in the sampled areas were analyzed from the middle of the 20^th century to the beginning of the 21^st using aerial photographs and orthophotos from different periods. They were then compared with documentary data on floods (using historical and testimonial data and photographs of the events) and cross-checked with the dendrogeomorphological years with the detected damage to trees. The few available instrumental series (meteorological and hydrological) were also compared to the establishment of the riparian forest, the documented torrential flood events, and the dated damage to the trees. All these analyses contribute to the understanding of the dynamics of subcatchments of different dimensions facing floods of different magnitudes. As Kondolf & Piegay (2003)Kondolf, G.M. & Piégay, H., (eds.), 2003. Tools in fluvial Geomorphology, Chichester, UK: John Wiley & Sons Ltd. https://doi.org/10.1002/0470868333 suggest, working with the convergence of evidence between the various data sources and comparing multiple zones increases knowledge and takes the complexity of the systems into account. In our work, the areas under study consist of different spatial scales, have undergone their own geomorphological and vegetation colonization evolution, and present a particular response to torrential flooding.

4.1. Riparian forest characteristics: tree species, sizes, and ages

110 trees of 11 different species (Acer campestre, Alnus glutinosa, Fraxinus excelsior, Juglans regia, Pinus sylvestris, Populus nigra, Populus tremula, Prunus avium, Quercus petraea, Salix caprea, and Tilia platyphyllos) are analyzed, including; a) 81 trees in the Romadriu subcatchment (9 trees in the Ramiosa stream, 15 trees in the Romadriu river, and the 57 trees in the Portainé stream already studied in our previous work; and b) 29 trees in the Flamisell subcatchment. Almost all of them are angiosperms belonging to very different families and with different types of xylem, from ring pore (e.g.: Quercus petraea, Fraxinus excelsior) to diffuse pore (e.g.: Alnus glutinosa, Tilia platyphyllos) or with intermediate characteristics (e.g.: Populus spp., Prunus avium). Among the sampled species, in which 4 or more trees are analyzed (Table 3), poplars (Populus nigra) and oaks (Quercus petraea) stand out as the thickest. It should be noted that, since the oaks have a slightly lower average perimeter, they can reach twice the age of the poplars. Quaking aspen (Populus tremula) almost doubled the perimeter and the tree-ring width mean of the common ash (Fraxinus excelsior), despite having the same estimated mean age, while the alders studied (Alnus glutinosa), which present perimeters similar to common ash, rarely exceeded twenty years of age (Table 3).

In the Flamisell subcatchment, both poplars and alders have the greatest average ring width (although the fact that these are comparatively younger trees must be taken into account), while in the Romadriu subcatchment, the poplar is also the species with the largest tree-ring width average.

Large differences are identified between the average and maximum ages obtained in the Romadriu subcatchment, compared to those obtained in the Flamisell (Table 3), and therefore also in terms of the dates of forest establishment. In the Romadriu, most of the trees belonging to the most predominant species (Populus tremula, Fraxinus excelsior, Populus nigra) that comprise the riparian forest were established in the 1950s or thereafter. Other trees were older, establishing themselves between the early 20^th century and the 1920s: oak (Quercus petraea), lime (Tilia platyphyllos), and walnut (Juglans regia); while only a few others, such as cherry trees (Prunus avium), were much younger. In contrast, the alders and poplars on the Flamisell riverbanks were much more recent, their establishment being estimated as in the 1990s.

4.2. Temporal and spatial evidence of flooding

4.1.1. Dating the tree damage

 ⌅

Table 4 shows the main evidence of damage recorded (scars and growth suppression) in the two subcatchments under study, and the dendrogeomorphological year in which it was estimated that this damage occurred. For the Romadriu subcatchment, we have selected data since 1960 from our previous work in the Portainé stream (Génova, et al., 2018Génova, M., Díez-Herrero, A., Furdada, G., Guinau, M. & Victoriano, A. 2018. Dendrogeomorphological evidence of flood frequency changes and human activities (Portainé Basin, Spanish Pyrenees). Tree-ring research, 74(2): 144-161. https://doi.org/10.3959/1536-1098-74.2.144 ) and we have added the damage detected in the Ramiosa stream and other sampled areas of the Romadriu River.

Table 4.  Scars and suppressions detected and dated in the subcatchments studied. N: Number of trees with evidence, P: Percentage of trees about the total sampled on each subcatchment (only indicated if ≥1%), DGY: Dendrogeomorphological Year.
Tabla 4.  Cicatrices y supresiones detectadas y fechadas en las subcuencas estudiadas. N: Número de árboles con evidencias, P: Porcentaje de árboles en relación al total muestreado (sólo se indica si ≥1%), DGY: Año dendrogeomorfológico.

Romadriu Subcatchment					Flamisell Subcatchment
Scars		Suppressions		DGY	Scars		Suppressions		DGY
N	P	N	P		N	P	N	P
1	--	9	8%	1973-74	5	17%	19	66%	2007-08
3	3%-	7	6%	1976-77	3	10%	19	66%	2009-10
1	--	9	8%	1987-88	2	7%	17	59%	2011-12
3	3%-	28	25%	1992-93	21	72%	18	62%	2014-15
8	7%	34	31%	1997-98
4	4%	16	15%	1999-00
2	2%	27	25%	2005-06
20	18%	13	12%	2007-08
6	5%	28	25%	2009-10

In the Flamisell subcatchment, the dates of the damage are very recent because it is a young forest, and thus, the greatest tree growth suppressions occurred in 2008, 2010, and 2015 (Figure 5). We may therefore highlight the number and percentage of external injuries dated in the period 2014-15 (72 % of the trees, Table 4), while the dendrogeomorphological years 2007-08 and 2014-15 are those that present a greater negative percentage change in the mean tree-ring widths, 59 % and 45 %, respectively (Figure 5).

Figure 5.  A) Individual tree-ring width series and the average of the trees sampled in the Flamisell subcatchment, indicating the years in which the greatest tree growth suppression occurred. B) Growth changes detected in each sampled tree for the dendrogeomorphological years 2007-08 and 2014-15.

Figura 5.  A) Series individuales y promedio de anchura de anillos de los árboles muestreados en la subcuenca del Flamisell, indicando los años en los que se produjo la mayor supresión del crecimiento de los árboles. B) Cambios de crecimiento detectados en cada árbol muestreado para los años dendrogeomorfológicos 2007-08 y 2014-15.

4.1.2. Geomorphological and landscape changes

 ⌅

The colonization of the forest in the Portainé stream (Romadriu subcatchment), obtained by analyzing orthophotos, was presented in Génova, et al. (2018)Génova, M., Díez-Herrero, A., Furdada, G., Guinau, M. & Victoriano, A. 2018. Dendrogeomorphological evidence of flood frequency changes and human activities (Portainé Basin, Spanish Pyrenees). Tree-ring research, 74(2): 144-161. https://doi.org/10.3959/1536-1098-74.2.144 . Since 1956, when only a few stands of trees were observed, there has been a progressive establishment and development of the forest, both in the cone (riparian forest) and on the surrounding slopes (oak grove). As of 1982, in Portainé and Ramiosa (see the following paragraph), and especially since 1990 in Romadriu as well (Génova, et al., 2018Génova, M., Díez-Herrero, A., Furdada, G., Guinau, M. & Victoriano, A. 2018. Dendrogeomorphological evidence of flood frequency changes and human activities (Portainé Basin, Spanish Pyrenees). Tree-ring research, 74(2): 144-161. https://doi.org/10.3959/1536-1098-74.2.144 ), a dense forest mass may already be observed, although the most recent orthophotos indicate that it has also been affected in the Portainé stream due to the last floods and debris flows.

Despite the shadows that exist in many sectors, the post-flood aerial photographs of December 1982 show that the Romadriu channel was significantly affected, since a large amount of sediment was eroded and transported along the channel, leading to damming at the confluence with the Noguera Pallaresa, where the riparian forest was destroyed (Figure 6). On the other hand, no alterations of the forest mass were observed at the mouths of the Portainé and Ramiosa streams. It should be borne in mind that the small Mal Pas gravity dam, located just upstream from the confluence of the Portainé stream, was built between 1995 and 1997 (as were all the hydroelectric infrastructures built in the Romadriu valley), so they were not affected during this torrential flood.

Figure 6.  Historical photographs of the effects of the 1982 flood in the Romadriu Valley. A) The dam at the confluence of the Romadriu River with the Noguera Pallaresa River, seen from upstream of the Noguera Pallaresa. B) The mouth of the Romadriu River seen from the Noguera Pallaresa River, completely devastated. Source: Arxiu Comarcal del Pallars Sobirà.

Figura 6.  Fotografías históricas de los efectos de la inundación de 1982 en el Valle de Romadriu; A) La presa en la confluencia del río Romadriu con el río Noguera Pallaresa, vista desde aguas arriba del Noguera Pallaresa. B) La desembocadura del río Romadriu vista desde el río Noguera Pallaresa completamente devastada. Fuente: Arxiu Comarcal del Pallars Sobirà.

Regarding the colonization in the Flamisell River corridor, the photograph in Figure 7 shows that the sampling area was completely devoid of vegetation in 1922 when the construction of the hydroelectric infrastructure was completed (1919-1920).

Figure 7.  Sampling area on the banks of the Flamisell River indicated in a photograph published in 1922, after the construction of the Senterada-La Pobla de Segur complex (1919-1920). The total absence of riparian vegetation is observed after the construction of the infrastructure (Source: Galí, 1922Galí, J., 1922. Los Grandes aprovechamientos hidráulicos de la Sociedad Productora de Fuerzas Motrices. Revista tecnológico-industrial, 39: 65-80.).

Figura 7.  Área de muestreo en la ribera del río Flamisell indicada en una fotografía publicada en 1922 tras la construcción del complejo Senterada-La Pobla de Segur (1919-1920). Se observa la ausencia total de vegetación de ribera tras la construcción de esta infraestructura (Fuente: Galí, 1922Galí, J., 1922. Los Grandes aprovechamientos hidráulicos de la Sociedad Productora de Fuerzas Motrices. Revista tecnológico-industrial, 39: 65-80.).

The orthophotos (Figure 8) show that there were very few trees in 1956, while in 1982 (image obtained one month after the flood) the flood had affected the entire channel as well as the adjacent plain, and had deposited sediment (in gray) where the trees sampled would later establish themselves. The historical photographs in Figure 9 show the effects of the flood in Senterada and illustrate its great destructive power. During this event, the Flamisell contributed 16,4 hm³, which is ~25 % of the total annual discharge of the Noguera Pallaresa at La Pobla de Segur. The convergence of these data explains how the complete destruction of riparian forest occurred. Subsequently, in 1988 (Figure 8), a possible beginning of tree colonization can be observed (it is the closest image in time to the establishment of most of the sampled trees). In the images corresponding to the years 2012, 2014, and 2017, the floods detected by the dendrogeomorphological analysis show no evidence of having affected the development of the forest mass. The same occurred with the flood of June 2008 (Figure 10), unknown until this work and for which no rainfall data is available, and which may well have been detected by wounds and other damage to the trees.

Figure 8.  Orthoimages of the Flamisell River sampling area. The weir and the settling tank for the water intake (for hydroelectric exploitation) can be seen at the north and northeast; in the center, the pedestrian footbridge that connects the path with the settling tank. In fuchsia circles: position of the sampled trees; in pale pink circles: their position before their establishment (Orthoimages property of Institut Cartografic i Geologic de Catalunya; CC by 4.0).

Figura 8.  Ortoimágenes de la zona de muestreo del río Flamisell. Al norte y noreste se observa el azud y el tanque de sedimentación para la toma de agua (para aprovechamiento hidroeléctrico); en el centro, la pasarela peatonal que conecta el camino con el decantador. En círculos fucsias: posición de los árboles muestreados; en círculos rosa pálido: su posición antes de su establecimiento (Ortoimágenes propiedad del Institut Cartografic i Geologic de Catalunya; CC by 4.0).

Figure 9.  Photographs of the effects of the 1982 flood in Senterada, approximately 1 km upstream from the Senterada weir and the Flamisell River sampling area. The photographs illustrate the energy of the event and its destructive power. Source: collection of photographs of the impacts of the 1982 flood, Firemen of La Pobla de Segur.

Figura 9.  Fotografías de los efectos de la inundación de 1982 en Senterada, aproximadamente 1 km aguas arriba del vertedero de Senterada y del área de muestreo del río Flamisell. Las fotografías ilustran la energía del evento y su poder destructivo. Fuente: recopilación de fotografías de los impactos de la riada de 1982, Bomberos de La Pobla de Segur.

Figure 10.  Photographs of the June 2008 flood in the Senterada weir, which affected the sampling area. A: View of the weir from downstream; B: View of the weir from upstream; C and D: View of the trees in the sampling area closest to the channel (Source: Maite Arilla).

Figura 10.  Fotografías de la inundación de junio de 2008 en el azud de Senterada, que afectó al área de muestreo. A: Vista del azud desde aguas abajo; B: Vista del azud desde aguas arriba; C y D: Vista de los árboles de la zona de muestreo más cercana al canal (Fuente: Maite Arilla).

4.3. Comparison with instrumental records

Three tables summarizing significant meteorological data selected from different sources and with different criteria are presented below. Tables 5 and 6 present selected meteorological data recorded in the dendrochronological years 2007-08 and 2009-10 (when there was damage to the vegetation in both Romadriu and Flamisell subcatchments), and in the dendrochronological year 2014- 2015 (when damage to the vegetation of the Flamisell subcatchment was verified). The data correspond to the dates of rainfall events of interest, both because of the damage caused to the vegetation (known from technical reports), and because these events are considered significant on a regional scale as well as in the “Singular weather events” of METEOCAT, and consequently may have caused damage to the vegetation.

Table 5 includes selected rainfall data and its classification as local or regional events of the rainfall events of interest described above. Table 6 presents the rainfall comparison of the events of interest described above with the monthly and annual rainfall maximums of the network (Figure 4); it specifically shows: a) the maximum monthly rainfall in 24 h in the months of the dates of rainfall events of interest defined above; b) the maximum precipitation in 24 h of the year, indicating the month and day of occurrence; and c) the total accumulated precipitation in each one of the selected rainfall events. When the month in which the maximum precipitation in 24 h occurs coincides (or fails to coincide) with the dates of the rainfall events of interest, it is highlighted in dark blue or light blue, respectively.

Tables 5 and 6 highlight the large variability of rainfall in the different stations for each episode. Table 6 shows that only in the regional episode of November 1^st and 2^nd, 2008, was the maximum annual 24 h precipitation recorded at all stations (except Sort, where there was no record). Considerable variability is found in the other two regional events since while some stations register their maximum annual rainfall, others do not even register a monthly maximum. It is also evident that, in the four local events, half of the stations did not even register a maximum monthly precipitation in 24 h. This underlines the large variability of precipitation in these mountainous areas.

Table 7 presents the accumulated rainfall at Meteocat stations for three selected dates in 2010, when no “Singular weather events” were found that could explain the damage to the Flamisell vegetation in this dendrogeomorphological year. As explained in the methodology section, the days on which monthly maximum precipitation occurred in each of the weather stations in Table 6 were explored in the meteorological yearbooks and the precipitation in each of the stations presented in Table 7 was obtained from the Automatic Stations Map. This allowed us to discard the rainfall event of 02/Nov./2010 and to consider 09/June/2010 as the one that most likely caused the flood and the damage.

The existing discharge data (average daily discharge and maximum instantaneous monthly discharge peaks) at gauging station 9267-Capdella (Flamisell subcatchment) are presented below. Figure 11 shows the comparison of these data with the establishment riparian forest period and with the years of damage to trees, and the dates of some notable peak flows were highlighted. There is no clear, univocal relationship among the peak discharges or some of the notable historical flows and the damage caused.

The monthly contributions in hm³ of the Flamisell at station 9181 - La Pobla de Segur (1965-1991) were obtained (Table 8). The contribution of the 1982 flood (16.4 hm³) was greater than the average monthly contribution, reinforcing the evidence of the great destructive power of this flood. On the other hand, as mentioned before, the weir of Senterada has a capacity of 0,12 hm³, which corresponds to a storage capacity of barely 0.24% of the maximum monthly contribution and is less than the minimum monthly contribution.

5.1. Dendrogeomorphology and flood events

The first objective of a dendrogeomorphological study is to adequately select the sampling areas and species, a process that entails a series of limitations. In the search for areas where trees can provide a record of past floods, forested areas near riverbanks and streams are selected if they include externally damaged trees affected by floods. Various factors act on the evolution and development of riparian forests, but large flood events can destroy or partially eliminate them, with the loss of evidence of previous events and the subsequent establishment of a new forest of uniform age or “cohort” (Oliver & Larson, 1996Oliver, C.D. & Larson, B.C., 1996. Forest Stand Dynamics. New York, USA: John Wiley & Sons.). It can be especially significant in narrow and steep channels in mountainous areas. In fact, in these contexts, the flood events that are best recorded as damage to trees are those of intermediate magnitude, since the larger ones may destroy all the vegetation and the smaller ones leave little evidence (Ruiz-Villanueva, et al., 2010Ruiz-Villanueva, V., Díez-Herrero, A., Stoffel, M., Bollschweiler, M., Bodoque, J.M. & Ballesteros-Cánovas, J.A., 2010. Dendrogeomorphic analysis of flash floods in a small ungauged mountain catchment (Central Spain). Geomorphology, 118(3-4): 383-392. https://doi.org/10.1016/j.geomorph.2010.02.006 ; Génova, et al., 2015Génova, M., Máyer, P., Ballesteros-Cánovas, J., Rubiales, J.M., Saz, M.A. & Díez-Herrero, A. 2015. Multidisciplinary study of flash floods in the Caldera de Taburiente National Park (Canary Islands, Spain). Catena, 131: 22-34. https://doi.org/10.1016/j.catena.2015.03.007 ).

In this sense, the estimated dates of the establishment of the trees in the different areas studied in the upper catchment of the Noguera Pallaresa River present great differences. In the Romadriu subcatchment and, more specifically, in the riparian forest of the Portainé stream, it was determined that most of the trees had progressively established themselves in the alluvial cone since the early 1950s (Génova, et al., 2018Génova, M., Díez-Herrero, A., Furdada, G., Guinau, M. & Victoriano, A. 2018. Dendrogeomorphological evidence of flood frequency changes and human activities (Portainé Basin, Spanish Pyrenees). Tree-ring research, 74(2): 144-161. https://doi.org/10.3959/1536-1098-74.2.144 ). During the previous decades, the western margin of the stream probably underwent intensive cultivation (remains of the ancient presence of fruit trees are still found today, García-Oteyza, et al., 2015Garcia-Oteyza, J., Genova, M., Calvet, J., Furdada, G., Guinau M. & Díez-Herrero, A., 2015. Datación de avenidas torrenciales y flujos de derrubios mediante metodologias dendrogeomorfológicas (barranco de Portaine, Lleida, Espana). Revista Ecosistemas, 24(2): 43-50. https://doi.org/10.7818/ECOS.2015.24-2.07 ). When agricultural activity ceased, the area was colonized by natural vegetation, as evidenced by the analysis of orthoimages from different years (Génova, et al., 2018Génova, M., Díez-Herrero, A., Furdada, G., Guinau, M. & Victoriano, A. 2018. Dendrogeomorphological evidence of flood frequency changes and human activities (Portainé Basin, Spanish Pyrenees). Tree-ring research, 74(2): 144-161. https://doi.org/10.3959/1536-1098-74.2.144 ). On the other hand, the maximum ages estimated in this work for the trees of the Flamisell subcatchment are much more recent, which indicates an almost simultaneous establishment in the 1990s (Table 3), and is, therefore, a clear indication of the great devastation caused by the flood of 1982.

The large flood of 1982 was the last of the three exceptionally destructive events that occurred in the Noguera Pallaresa during the 20^th century, in 1907, 1937, and 1982 (Balasch, et al., 2008Balasch, C., Becat, J., Marugan, C.M., Nadal, A., Rapalino, V. & Remacha, R., 2008. Les riuades del segle XX al Pallars Sobirà: 1907, 1937 i 1982. Colección: Arxius y societat: Quaderns de divulgació històrica nº 2. Barcelona, España: Departament de Cultura i Mitjans de Comunicació, Generalitat de Catalunya.), although no dendrogeomorphological evidence of the first two has been identified. The extensive devastation caused by the 1982 event has been documented, for example, in the Servei Geològic de Catalunya, (1983)Servei Geològic de Catalunya, (eds.), 1983. Efectes geomorfològics dels aiguats del novembre de 1982. Imformesa 1. Barcelona, España: Generalitat de Catalunya, Dpt. De Política Territorial i Obres Publiques, Barcelona., in Corominas & Alonso, (1990)Corominas, J. & Alonso, E.E., 1990. Geomorphological effects of extreme floods (November 1982) in the southern Pyrenees. InProc. of II Lausanne Symposia, August Hydrology in mountainous region II. Artificial reservoirs, water and slope 295-302 pp., and in Balasch, et al., (2008)Balasch, C., Becat, J., Marugan, C.M., Nadal, A., Rapalino, V. & Remacha, R., 2008. Les riuades del segle XX al Pallars Sobirà: 1907, 1937 i 1982. Colección: Arxius y societat: Quaderns de divulgació històrica nº 2. Barcelona, España: Departament de Cultura i Mitjans de Comunicació, Generalitat de Catalunya.. Documentary information indicates that the analyzed area of the Flamisell subcatchment was deforested when the weir was built in the 1920s (Figure 7); likewise, with a subsequent aerial photograph taken in 1956, and especially with further photographs of the 1982 flood up until the late 1980s (Figure 8), from when the establishment of the riparian forest started again. However, even though during this event the transport of materials in the Romadriu River was enormous, producing a ~10 m high dam that blocked its mouth in the Noguera Pallaresa (Figure 6), scars on trees established earlier in this subcatchment have not been dated. Most of these trees were over ten years old at that time (Table 3) and already constituted a more or less homogeneous forest in the Portainé stream, and they did show other evidence (suppressions and releases) affected growth from the following year in more than half of the trees analyzed (Génova, et al., 2018Génova, M., Díez-Herrero, A., Furdada, G., Guinau, M. & Victoriano, A. 2018. Dendrogeomorphological evidence of flood frequency changes and human activities (Portainé Basin, Spanish Pyrenees). Tree-ring research, 74(2): 144-161. https://doi.org/10.3959/1536-1098-74.2.144 ), but not in the other studied areas of the Romadriu basin.

5.2. Dendrogeomorphology and flood events

In the dendrogeomorphological dating of flood events, we use the proposal put forward by Génova, et al. (2018)Génova, M., Díez-Herrero, A., Furdada, G., Guinau, M. & Victoriano, A. 2018. Dendrogeomorphological evidence of flood frequency changes and human activities (Portainé Basin, Spanish Pyrenees). Tree-ring research, 74(2): 144-161. https://doi.org/10.3959/1536-1098-74.2.144 ; i.e. a biannual denomination that corresponds to the dendrogeomorphological year. This biannual dating shows that the event, and therefore the damage caused, may have occurred from the beginning of the dormant period of the trees, which in these latitudes approximately coincides with the end of the summer period, and up until the end of the annual growth. Furthermore, this establishes a coincidence with the hydrological year in Spain, which covers the period that runs from October 1^st to September 30^th of the following year.

Numerous dendrogeomorphological indicators have made it possible to date important events in the Romadriu subcatchment (Table 4), to which must be added an event that occurred in 1969-70, when many trees in the Portainé stream may have been decapitated (Génova, et al., 2018Génova, M., Díez-Herrero, A., Furdada, G., Guinau, M. & Victoriano, A. 2018. Dendrogeomorphological evidence of flood frequency changes and human activities (Portainé Basin, Spanish Pyrenees). Tree-ring research, 74(2): 144-161. https://doi.org/10.3959/1536-1098-74.2.144 ). The new data presented here do not modify the dates of the evidence gathered from the Portainé stream but increase its amount and reliability, as in the case of the dendrogeomorphological year 1997-98 (Table 4). Among all these indicators, those dated 1997-98, 2007-08, and 2009-10 stand out, due to the greater abundance of recorded evidence, especially scars. While some coincident damages were identified in the Flamisell subcatchment, the greatest correspond to the dendrogeomorphological year 2014-15, when up to 72 % of the trees analyzed presented wounds and bark removal that in some cases reached up to a height of 2 m (Figure 2).

The coincidence of damage in the same dendrogeomorphological year in both subcatchments (Flamisell and Romadriu) does not necessarily imply that they were the consequence of the same precipitation episode that generated simultaneous floods, but may correspond to different events. As already indicated in the previous section, the dendrogeomorphological record of flood events entails limitations, since there does not necessarily have to be a direct correlation between the confirmed dates of the major events and the number and extent of dendrogeomorphological evidence (Ruiz-Villanueva, et al., 2010Ruiz-Villanueva, V., Díez-Herrero, A., Stoffel, M., Bollschweiler, M., Bodoque, J.M. & Ballesteros-Cánovas, J.A., 2010. Dendrogeomorphic analysis of flash floods in a small ungauged mountain catchment (Central Spain). Geomorphology, 118(3-4): 383-392. https://doi.org/10.1016/j.geomorph.2010.02.006 ). Thus, for example, the effects of successive events can be masked when two floods occur in the same dendrogeomorphological year, or when the later event of greater magnitude destroys all the dendrogeomorphological evidence of a less intense previous event (Ballesteros-Cánovas, et al., 2013Ballesteros-Cánovas J.A., Sanchez-Silva, M., Bodoque, J.M. & Díez-Herrero, A. 2013. An integrated approach to flood risk management: a case study of Navaluenga (Central Spain). Water Resources Management, 27(8): 3051-3069. https://doi.org/10.1007/s11269-013-0332-1 ). Moreover, the most recent damages (which occurred one or two years before) are very difficult to date, because the callus still cannot be properly identified (Génova, et al., 2018Génova, M., Díez-Herrero, A., Furdada, G., Guinau, M. & Victoriano, A. 2018. Dendrogeomorphological evidence of flood frequency changes and human activities (Portainé Basin, Spanish Pyrenees). Tree-ring research, 74(2): 144-161. https://doi.org/10.3959/1536-1098-74.2.144 ).

Another limitation is related to the impossibility of developing reference chronologies to contrast the undisturbed and disturbed tree-ring series, due to the lack of sufficient undisturbed trees in the studied sites. This has been offset by with the contrast of the data on growth anomalies between different species with different ecological ranges. Therefore, whenever possible, it will be more explanatory as well to use temporally detailed documentary and instrumental data that can be related to dendrogeomorphological evidence.

5.3. Comparison of different data sources

When contrasting dendrogeomorphological evidence and data from instrumental records in mountainous areas, it is necessary to take into account that the rainfall in those areas is strongly conditioned by the orography, which generates high-intensity rainfall nuclei, while in neighboring areas the rainfall may be much lower. In the November 1982 event, the inhabitants of the Romadriu Valley explained that the storm stagnated and precipitated directly, hitting the south-facing slope (opposite to the slope where the Portainé and Ramiosa streams are located). This matches the work by Trapero, et al., (2013b)Trapero, L., Bech, J., Duffourg F., Esteban, P. & Lorente, J., 2013b. Mesoscale numerical analysis of the historical November 1982 heavy precipitation event over Andorra (Eastern Pyrenees). Nat. Hazards Earth Syst. Sci., 13: 2969-2990. https://doi.org/10.5194/nhess-13-2969-2013 , in which simulated rainfall at the head of the Romadriu was much higher than in the small streams of Portainé and Ramiosa. The extreme rainfall of 1982 at the head of the Romadriu subcatchment produced the erosion and transport of readily available sediment, which generated the aforementioned obstruction at the confluence with the Noguera Pallaresa. On the other hand, in the case of the Portainé and Ramiosa torrents, only local erosions on the forested banks may have occurred, as well as the possible uprooting and transport of some specimens, consequently giving rise to suppressions and releases, but without enough damage to enable detection in the current riparian forest.

The dendrogeomorphological evidence of 1997-1998 in Portainé (Génova, et al., 2018Génova, M., Díez-Herrero, A., Furdada, G., Guinau, M. & Victoriano, A. 2018. Dendrogeomorphological evidence of flood frequency changes and human activities (Portainé Basin, Spanish Pyrenees). Tree-ring research, 74(2): 144-161. https://doi.org/10.3959/1536-1098-74.2.144 ) and in other areas of the Romadriu subcatchment are correlated with significant regional rainfall and flooding. In the Flamisell subcatchment, gauging station 9267 - Capdella recorded a maximum daily instantaneous flow on 18/Dec./1997, although no dendrogeomorphological evidence of this exists. Curiously, it was during the establishment of poplars and alders in the sampled area (1992-1999) when the highest flow peaks of the series occurred, including that of 1997 (Figure 11). Given their youth, shortness, and flexibility, it is likely that the trees leaned against possible floaters, so it is not possible for damage or other evidence to be identified.

In the small Portainé subcatchment, documentary data made it possible to determine in detail the exact date of recent local floods (García-Oteyza, et al., 2015Garcia-Oteyza, J., Genova, M., Calvet, J., Furdada, G., Guinau M. & Díez-Herrero, A., 2015. Datación de avenidas torrenciales y flujos de derrubios mediante metodologias dendrogeomorfológicas (barranco de Portaine, Lleida, Espana). Revista Ecosistemas, 24(2): 43-50. https://doi.org/10.7818/ECOS.2015.24-2.07 ; Génova, et al., 2018Génova, M., Díez-Herrero, A., Furdada, G., Guinau, M. & Victoriano, A. 2018. Dendrogeomorphological evidence of flood frequency changes and human activities (Portainé Basin, Spanish Pyrenees). Tree-ring research, 74(2): 144-161. https://doi.org/10.3959/1536-1098-74.2.144 ; Furdada, et al., 2020Furdada, G., Victoriano, A., Díez-Herrero, A., Génova, M., Guinau, M., De las Heras, A., Palau, R., Hürlimann, M., Khazaradze, G., Casas, J.M., Margalef, A., Pinyol, J. & Gonzalez, M., 2020. Flood consequences of land-use changes at a ski resort: Overcoming a geomorphological threshold (Portainé, Eastern Pyrenees, Iberian Peninsula). Water, 12(2): 368. https://doi.org/10.3390/w12020368 ). In 2008 and 2010, two floods occurred each year, which were recorded in the dendrogeomorphological years 2007-08 and 2009-10. In the Flamisell subcatchment, the documentary data (Figures 10 and 11) do not clearly coincide with a notable flow peak at the gauging station, although a certain increase in discharge was recorded in spring. Furthermore, the meteorological event of 1-2/Nov./2008 was of a regional nature (Bech, et al., 2011Bech, J., Pineda, N., Rigo, T., Aran, M., Amaro, J., Gaya, M., Arus, J., Montanya, J. & Van der Velde, O., 2011. A Mediterranean nocturnal heavy rainfall and tornadic event. Part I: Overview, damage survey and radar analysis. Atmos. Res., 100: 621-637. https://doi.org/10.1016/j.atmosres.2010.12.024 ; Trapero, et al., 2013aTrapero, L., Bech, J. & Lorente, J., 2013a. Numerical modelling of heavy precipitation events over Eastern Pyrenees: Analysis of orographic effects. Atmospheric Research, 123: 368-383. https://doi.org/10.1016/j.atmosres.2012.09.014 ) and may have produced a flood peak, although no discharge data is available. Therefore, the wounds in the Flamisell sampling area could have been caused by at least one of these two events or by both, and it is possible that the regional event affected both subcatchments (Romadriu and Flamisell). Damage was caused to the trees of Romadriu and Flamisell in the dendrogeomorphological year 2009-10. The meteorological events that affected Portainé were of a local nature, while the damage to the Flamisell vegetation could be explained mostly by the regional rainfall on 06/09/2010 (Table 7). From 2010 to 2015, the rainfalls recorded in the Romadriu subcatchment were not extraordinary. The local precipitations occurred in July 2011 and 2013 and in August 2014 and 2015 (less than a 10-year return period, although very intense (Furdada, et al., 2020Furdada, G., Victoriano, A., Díez-Herrero, A., Génova, M., Guinau, M., De las Heras, A., Palau, R., Hürlimann, M., Khazaradze, G., Casas, J.M., Margalef, A., Pinyol, J. & Gonzalez, M., 2020. Flood consequences of land-use changes at a ski resort: Overcoming a geomorphological threshold (Portainé, Eastern Pyrenees, Iberian Peninsula). Water, 12(2): 368. https://doi.org/10.3390/w12020368 ), causing debris flows in the Portainé stream (ICGC, 2013aICGC, 2013a. Avaluacio de la dinamica torrencial del torrent de Portaine. AP-035/13. Barcelona, Spain: Institut Geologic de Catalunya., bICGC, 2013b. Nota de la visita al barranc de Portaine (Pallars Sobira) arran de l’episodi de pluges del dia 23 de Juliol de 2013 AP-091/13. Barcelona, Spain: Institut Geologic de Catalunya, Barcelona.; IGC et al., 2013IGC, GEOCAT & FGC, 2013. Informe de visita de terreny de les esllavissades i barrancades a la carretera d’acces a Portaine a Juliol de 2013. AP-057/13. Barcelona, Spain: Institut Geologic de Catalunya, Gestio de Projectes S.A. and Ferrocarrils de la Generalitat de Catalunya.; Palau, et al., 2017Palau, R.M., Hürlimann M., Pinyol, J., Moya, J., Victoriano, A., Génova, M. & Puig, C., 2017. Recent debris-flows in the Portainé catchment (Eastern Pyrenees, Spain). Analysis of monitoring and field data focusing on the 2015 event. Landslides, 14(3): 1161-1170. https://doi.org/10.1007/s10346-017-0832-9 ), and some evidence of damage was observed in September 2015 (Génova, et al., 2018Génova, M., Díez-Herrero, A., Furdada, G., Guinau, M. & Victoriano, A. 2018. Dendrogeomorphological evidence of flood frequency changes and human activities (Portainé Basin, Spanish Pyrenees). Tree-ring research, 74(2): 144-161. https://doi.org/10.3959/1536-1098-74.2.144 ).

On 17-18/June/2013, there were significant rainfalls and snowmelt in the Val d’Aran and the headwaters of the Noguera Pallaresa and Flamisell, which caused significant flooding in the Val d’Aran (Victoriano, et al., 2016Victoriano, A., García-Silvestre, M., Furdada, G. & Bordonau, J., 2016. Long-term entrenchment and consequences for present flood hazard in the Garona River (Val d’Aran, Central Pyrenees, Spain). Nat. Hazards Earth Syst. Sci., 16: 2055-2070. https://doi.org/10.5194/nhess-16-2055-2016 ). At gauging station 9267 - Capdella, a significant flow peak was observed, higher than those of 2007, 2008, 2011, and 2015, although no flooding is documented for that occasion. In the sampling area (18 km downstream) there were no tree damages, possibly due to the attenuation of the flood. Conversely, in those other years, the precipitations may possibly have affected a significantly greater area of the subcatchment and the flow increased downstream, thereby causing damage to the trees.

The “Singular weather events” of 27/Nov.-01/Dec./2014 and 10-25/March/2015 could have caused the serious damage detected in the Flamisell forest (the dendrogeomorphological year 2014-15). The gauging station data indicate slight but unremarkable increases in flow. In both events, rainfall data presented maxima at the stations located to the north and west of the subcatchment (Tables 5 and 6Carreras, J., 1993. Flora i Vegetació de Sant Joan de l’Erm i de la Vall de Santa Magdalena (Pirineus Catalans). Institut Estudis Ilerdencs, 3: 1-321.). The contribution of flow from the Bòssia River (the tributary on the right bank of the Flamisell that drains ~20 % of the subcatchment; Figure 1) may have led to a significant increase in discharge but went unrecorded at the gauging station because it was located upstream from the confluence.

Despite the floods that caused wounds and other damage between 2008 and 2015 in the forest sampled in the Flamisell subcatchment, the vegetation has since then been densifying, which implies that: 1) the floods that caused the damage were of magnitudes less than that of 1982 when the riverside forest was devastated; 2) the weir has no regulatory role in or influence on these floods of intermediate magnitudes (nor, of course, in extreme ones), and its lamination capacity when a flood occurs is practically nil. In consequence, it does not prevent injuries or other evidence on trees.

Furthermore, it should be noted that, in general, the data from the 9267 - Capdella gauging station provide little or no useful data for interpreting the damage to the vegetation in the sampling area. None of the notable flow peaks indicated in Figure 11, except those of 25 and 27/May/2008, seem to correspond to the damage recorded. This is most probably because an extensive area of the subcatchment is downstream from the station, so the events that affect the headwaters are attenuated downstream and those that are regional and affect the entire subcatchment are underestimated. In the sampling zone, which is close to the mouth, dendrogeomorphological damage is recorded when extraordinary floods occur, but in the case of extreme floods, such as that of 1982, the evidence is destroyed. In addition, the information of a meteorological nature entails the limitation of the location of the stations. As commented above, the orography of the area gives rise to a highly heterogeneous rainfall pattern (Tables 5 and 6Carreras, J., 1993. Flora i Vegetació de Sant Joan de l’Erm i de la Vall de Santa Magdalena (Pirineus Catalans). Institut Estudis Ilerdencs, 3: 1-321.). Thus, the data from stations outside the catchment of interest provide only a general idea of the events of a regional nature (and not always), and if they are of a local nature they may not be recorded. If the catchment is very small, as is the case of the Portainé, the best indicator of its dynamics is the dendrogeomorphological data; a rain gauge will only provide really relevant data when it is located at the headwaters. If the subcatchment is larger, as is the case of Flamisell, the local events or those that affect only part of the subcatchment may generate attenuated flows downstream.

The wide and diverse set of data analyzed enables a more comprehensive knowledge of torrential flood dynamics in both subcatchments, which respond to a very inhomogeneous mountain rainfall events. Moreover, the documentary data, both scientific and testimonial, reinforce, complete, and outline the interpretation of the dendrogeomorphological results.