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Flash floods represent one of the most common natural hazards in mountain catchments, and are fre-quent in Mediterranean environments. As a result of the widespread lack of reliable data on past events, the understanding of their spatio-temporal occurrence and their climatic triggers remains rather limited. Here, we present a dendrogeomorphic reconstruction of past flash flood activity in the Arroyo de los Puentes stream (Sierra de Guadarrama, Spanish Central System). We analyze a total of 287 increment cores from 178 disturbed Scots pine trees (Pinus sylvestris L.) which yielded indications on 212 growth disturbances related to past flash flood impact. In combination with local archives, meteorological data, annual forest management records and highly-resolved terrestrial data (i.e., LiDAR data and aerial imag-ery), the dendrogeomorphic time series allowed dating 25 flash floods over the last three centuries, with a major event leaving an intense geomorphic footprint throughout the catchment in 1936. The analysis of meteorological records suggests that the rainfall thresholds of flash floods vary with the seasonality of events. Dated flash floods in the 20th century were primarily related with synoptic troughs owing to the arrival of air masses from north and west on the Iberian Peninsula during negative indices of the North Atlantic Oscillation. The results of this study contribute considerably to a better understanding of hazards related with hydrogeomorphic processes in central Spain in general and in the Sierra de Guadarrama National Park in particular.

The occurrence of El Niño has been generally considered the main driver of hydro‐geomorphic processes in Peru. However, the climatic characterization of hydro‐geomorphic events (HGE) occurring in the absence of El Niño remains scarce. Information contained in the DesInventar disaster database suggests a widespread occurrence of HGE associated to cold‐neutral sea surface temperature (SST) in the central Pacific and south tropical Atlantic. Here, we aim at characterizing synoptic patterns associated with HGE that have occurred over last 35 years related to the different El Niño types and focusing as well on the non‐Niño phases. We use the ERA‐Interim reanalysis climate data and implement self‐organizing maps to assess the link between HGE in Peru and specific synoptic patterns. Results suggest that synoptic patterns associated with La Niña and neutral conditions play an important role in the occurrence of hydro‐geomorphic disasters in Peru during the austral summer. A total of 21% of the events are associated only with the 1972–1973, 1982–1983 and 1997–1998 El Niño events and are mainly focused in the northern Pacific coast of the country (i.e., Tumbes, Piura and Lambayeque) while more than 36% of the recorded events in the database were associated with La Niña and neutral conditions between 1970 and 2013. La Niña‐related events were more relevant in the Andean–Amazonian regions, whereas neutral conditions were related to more frequent HGEs in the southern regions (south of the 13.25°S) along the Peruvian Pacific coast. These outcomes imply an enhanced understanding of the synoptic mechanisms leading to the occurrence of HGE and contribute to a better understanding of the triggers of HGE causing disaster no exclusively related to El Niño‐like years in Peru.

• Flash floods represent a regional natural hazard in Guadarrama range.• Dendrochronology allows fill the lack of systematic data in mountain environments.• Regional studies and quality samples allow reducing the number of collected samples.• We complement existing records with 8 events covering the last~200 years.• Forest management could limit the amount of proxy evidence of flash flood events. Flash floods are a common natural hazard in Mediterranean mountain environments and responsible for serious economic and human disasters. The study of flash flood dynamics and their triggers is a key issue; however, the retrieval of historical data is often limited in mountain regions as a result of short time series and the systematic lack of historical data. In this study, we attempt to overcome data deficiency by supplementing existing records with dendrogeomorphic techniques which were employed in seven mountain streams along the northern slopes of the Guadarrama Mountain range. Here we present results derived from the tree-ring analysis of 117 samples from 63 Pinus sylvestris L. trees injured by flash floods, to complement existing flash flood records covering the last~200 years and comment on their hydro-meteorological triggers. To understand the varying number of reconstructed flash flood events in each of the catchments, we also performed a comparative analysis of geomorphic catchment characteristics, land use evolution and forest management. Furthermore, we discuss the limitations of dendrogeomorphic techniques applied in managed forests. G R A P H I C A L A B S T R A C T a b s t r a c t a r t i c l e i n f o

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<p>With the substantial glacier mass reduction projected by the end of the century, the formation and rise of periglacial lakes has to be expected. Even though these changes often occur in remote areas, they can nevertheless have catastrophic impacts on populations and infrastructure through processes such as glacial lake outburst floods (GLOF). GLOFs are the result of complex geomorphic changes and subject to various timescales, thus urging the need for a multidimensional approach. The present study combines two approaches to analyze natural hazards in the secluded San Rafael National Park in Chilean Patagonia (North Patagonian Icefield). The Grosse glacier outlet was chosen after interpreting satellite imagery and historical pictures showing a historical emptying of a lateral lake, which was also supported by local testimonies. Dendrogeomorphology was primarily used with an automatic detection approach to identify possible dates of occurrence of past GLOFs at the Grosse outlet. A total of 105 disturbed Nothofagus trees were sampled highlighting 6 event years between 1958 and 2011. The second method aimed at complementing the tree-ring-based findings with UAV imagery acquired during fieldwork and the mapping of geomorphic evidence of past GLOFs. Huge boulders and deposits are one of the signs recognized as remnants of past lake outbursts and were thus used to differentiate small, rainfall-induced floods from high magnitude events. More precisely, through an object-based strategy, we mapped deposits and extrapolated a theoretical flow orientation. Whereas the first method allowed to select dates of potential events, the second facilitated identification and mapping of the spatial extent of past high-energy events. Analysis of imagery also allowed detection of the occurrence of a 200-m wide breach in the frontal moraine as well as the vanishing of a lateral lake estimated to be 1.8 × 10<sup>6 </sup>m<sup>2</sup> in the 1950s, which we date to 1958 using tree-ring records. When used together the two approaches can represent a valuable contribution to historical records and help future assessments of natural hazard at Grosse glacier, but also in other high-mountain environments.</p>