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Extreme floods are anticipated to become more frequent in a future warmer climate. However, the long-term alterations in flood patterns across different regions of Europe remain unclear. In this study, we present a 15,000-year record of extreme floods in the Duero River, located in the southwestern Atlantic region. We analysed slackwater flood sediments, quantified the discharge and timing of individual flood beds over millennial time scales, and identified their potential climate influences. The composite record includes at least 62 floods grouped into ten flood-rich periods (with an average duration of 230 years). A high-frequency phase of moderate-magnitude floods (>10 events) occurred at ∼11.6–11.5 ka, following a period of reduced flood activity during the Younger Dryas. Similar clusters of Early Holocene floods (10.8–10.3 ka, 9.5 ka) coincided with or preceded meltwater pulses from the North Atlantic. The absence of palaeoflood records with discharges exceeding 6100 m3/s during the Mid-Holocene suggests a decline in winter hydro-meteorological extremes. High flood magnitudes were recorded during transition periods toward cooler and wetter conditions at 4.4, 2.3, 0.5, and 0.11 ka with discharges ranging from 7600 to 10,000 m3/s. These periods were interpreted as indicative of a southward shift in cyclone tracks in Europe driven by negative phases of the North Atlantic Oscillation (NAO). Conversely, flood magnitudes decreased during past warmer climate conditions (1.7 ka, 0.9 ka, and the present), although flood frequency remained high. The current decline in flood frequency reflects an increase in flood regulation due to dams, but it is also consistent with the prevailing positive trend in the NAO observed over the last 40 years., Ministerio de Ciencia e Innovación (España), Dirección General del Agua, Fundación Biodiversidad, Union Europea, Depto. de Geodinámica, Estratigrafía y Paleontología, Fac. de Ciencias Geológicas, TRUE, pub

Sustainable river management now encompasses a much wider concept that includes hydromorphological and fluvial habitat studies. In ephemeral streams, the geomorphological characterization of channels is complex due to episodic flows and riparian vegetation dynamics. Stream channel survey and classification at the watershed scale provide the basis for geomorphological conservation, process interpretation, assessing sensitivity to disturbance, and identifying reaches that supply and store sediment. Here, we present a stream classification based on a two-step approach: (1) automatic river segmentation based on spatial variability in channel/valley morphology from topographic measurements (LiDAR, light, detection and ranging), and (2) fluvial landform and vegetation density mapping derived from multispectral opensource satellite images (Sentinel-2) using support vector machine (SVM) and Random Forest (RF) algorithms. These analyses provide continuous, quantitative spatial values of geometric (channel/valley width, slope gradient, and route distance), landform (active channel and gravel bars with five densities of vegetation cover), and hydraulic (specific stream power) variables. Four stream types were identified in the Rambla de la Viuda catchment (1500 km2), an ephemeral gravel-bed river in eastern Spain. The spatial distribution of channel types is explained by differences in geometry (active channel width, valley width, and slope gradient) and a hydraulic parameter (specific stream power). The landforms/vegetation patterns provided insight on causal relationships between erosion and deposition processes during high flow periods and the time since the most recent large disruptive flood event. Channel type distribution provided first-order predictions about the location of reaches that supply and store sediment and thus information on sediment continuity along the river. Dam effects on downstream reaches resulted in geomorphological disequilibrium, producing narrowing

In Alpine regions, speleothem development on karst systems largely occurs during warm interglacial or interstadial phases due to their limited growth during cold stages. Still, recent attention has been given to the role of clastic sediments in caves, less dependent on temperature conditions. Yet, only a small number of caves worldwide preserve both speleothems and detrital deposits. Here we present an outstanding record of fine-grain laminated sediments and carbonate speleothems from the Granito cave (South Central Pyrenees, Spain) associated with seasonal to annual hydroclimatic pulses through the Last Glacial Cycle (LGC). Analysis of cave clastic facies together with new absolute dates on glacial deposits along the valley has provided new insights into the karst-glacial interactions in relation with long-term landscape evolution, with support of geomorphological, sedimentological, mineralogical, palynological, geochemical and geochronological data (U/Th series, OSL and IRSL).The Granito cave was formed before 153.6±1.2 ky (Marine Isotopic Stage, MIS 6), as indicated by the oldest stalagmites dated in the cave, although speleothem formation occurred preferentially during MIS 5 and the Holocene interglacial stages. A 60 m thick clastic deposit was IRSL dated from 71.8±5.6 ky to after 26.5±2.5 ky corresponding with the maximum ice extent (MIE) during the LGC and subsequent glacial stabilization at 30.3±1.7 – 36.2±2.2 ky. The sediment infill was produced by water ponded in the cave coeval with the presence of glacial ice blocking the cave entrance. The cave clastic sequence includes channel, slackwater and backswamp facies, interpreted to result from pulses of sediment-laden water flow under vadose and phreatic flow regimes. The cave sediment’s mineralogical composition points to an exogenous source associated with glacial till sediments overlying the hillslope above the cave and along the Ara River valley. The palynological results obtained from the cave sediments s

Two contiguous alluvial valleys (Maria and Guadalupe sectors) of the upper Guadalentin River (SE Spain) show distinct Holocene alluvial architecture (cut-and-fill, nested and stepped fill terraces), revealing a strong control of valley morphology and bedrock barriers on sediment delivery, storage and preservation. The headwater valley (Maria sector) preserves four morphosedimentary alluvial units corresponding to two palaeogeographic stages. Morphosedimentary unit (MSU1), the most extensive, is a major depositional valley infill dominated by sand and silt texture, showing three stratigraphic sets dated 9000-8500, 5300-4800, 4000-3000 cal BP. This vertically stacked alluvial accumulation occurs upstream of a narrow bedrock channel, that limited sediment connectivity with the downstream Guadalupe sector. Between 3000 and 2350 cal BP, a 15-m deep incision on MSU1 occurred, and subsequent cut-and-fill cycles generated three alluvial terraces dated at 2350-1900 cal BP (MSU2), 1800-700 cal BP (MSU3), and post-400 years (MSU4). In the downstream valley (Guadalupe sector), six flights of alluvial terraces (MSUg1 to MSUg6) were formed by episodic aggradation and quasi-continuous incision, in response to baselevel changes of the lower Guadalentin River during the Holocene. Alluvial chronologies in these two contiguous valleys show out-of-phase sedimentation periods under low connectivity conditions (9-3 ka), and in-phase cut-and-fill cycles after valley re-connection (post-3.0 ka). The late Holocene alluvial activity periods also coincide with morpho-stratigraphic data from the lower Guadalentin, indicating that fluvial connectivity throughout the catchment was only completed in the late Holocene.

Climate change implies changes in the frequency and magnitude of flood events. The influence of climate variability on flooding was evaluated by an analysis of sedimentary (palaeofloods) and documentary archives. A 500-year palaeoflood record at Montilea River (657 km(2) in catchment area), eastern Spain, revealed up to 31 palaeofloods with a range of discharges of 20-950 m(3) s(-1), and with at least five floods exceeding 740-950 m(3) s(-1). This information contrasts with the available gauged flood registers (since year 1971) with an annual maximum daily discharge of 129 m(3) s(-1). Our palaeoflood dataset indicates flood cluster episodes at (1) 1570-1620, (2) 1775-1795, (3) 1850-1890, and (4) 1920-1969. Flood rich periods 1 and 3 corresponded to cooler than usual (about 0.3 degrees C and 0.2 degrees C) climate oscillations, whereas 2 and 4 were characterised by higher inter-annual climatic variability (floods and droughts). This high inter-annual rainfall variability increased over the last 150 years, leading to a reduction of annual maximum flow. Flood quantiles (>50 years) calculated from palaeoflood+gauged data showed 30%-40% higher peak discharges than those using only instrumental records, whereas when increasing the catchment area (1500 km(2)) the discharge estimation variance decreased to-15%. The results reflect the higher sensitivity of small catchments to changes on flood magnitude and frequency due to climate variability whereas a larger catchment buffers the response due to the limited extent of convective storms. Our findings show that extended flood records provide robust knowledge about hazardous flooding that can assist in the prioritization of low-regret actions for flood-risk adaptation to climate change.

Two contiguous alluvial valleys (Maria and Guadalupe sectors) of the upper Guadalentín River (SE Spain) show distinct Holocene alluvial architecture (cut-and-fill, nested and stepped fill terraces), revealing a strong control of valley morphology and bedrock barriers on sediment delivery, storage and preservation. The headwater valley (Maria sector) preserves four morphosedimentary alluvial units corresponding to two palaeogeographic stages. Morphosedimentary unit (MSU1), the most extensive, is a major depositional valley infill dominated by sand and silt texture, showing three stratigraphic sets dated 9000–8500, 5300–4800, 4000–3000 cal BP. This vertically stacked alluvial accumulation occurs upstream of a narrow bedrock channel, that limited sediment connectivity with the downstream Guadalupe sector. Between 3000 and 2350 cal BP, a 15-m deep incision on MSU1 occurred, and subsequent cut-and-fill cycles generated three alluvial terraces dated at 2350–1900 cal BP (MSU2), 1800–700 cal BP (MSU3), and post-400 years (MSU4). In the downstream valley (Guadalupe sector), six flights of alluvial terraces (MSUg1 to MSUg6) were formed by episodic aggradation and quasi-continuous incision, in response to baselevel changes of the lower Guadalentín River during the Holocene. Alluvial chronologies in these two contiguous valleys show out-of-phase sedimentation periods under low connectivity conditions (9–3 ka), and in-phase cut-and-fill cycles after valley re-connection (post-3.0 ka). The late Holocene alluvial activity periods also coincide with morpho-stratigraphic data from the lower Guadalentín, indicating that fluvial connectivity throughout the catchment was only completed in the late Holocene.

High-resolution topographic models have revolutionized monitoring of river changes by comparing sequential river topographic surveys (i.e. change detection). Nevertheless, much more may be obtained from this innovative quantification of changes. In this paper, we enhance the interpretation of geomorphic processes by presenting a new method for understanding of sources and sinks of sediment, river sediment transfers and functional sediment connectivity. Repeat digital elevation models (DEMs) obtained by photogrammetry were used to quantify topographic change after two floods by creating a DEM of difference (DoD) of a 6.5 km-long reach of Rambla de la Viuda stream, an ephemeral gravel-bed river in eastern Spain. The proposed method involved dividing the channel into 10 m-long longitudinal strips that were used to systematically draw boundaries between the erosional and depositional areas of the DoD. The analysis objectively: (i) drew a series of erosional and depositional segments, from 120 to 1360 m in length; (ii) estimated ranges of source-to-storage sediment transport distances, 320–670 m in the upstream and middle reaches and up to 2030 m in the lower reach; and (iii) obtained values of functional connectivity (i.e. the ratio between the sediment exported (erosion) and retained (deposition), ranging from 10 to 10). The variability in these three parameters along the river was found to be related to the level of channel disturbance by in-stream mining during the 1990s and 2000s. Additionally, this method indicates that the main process responsible for self-adjustment of the present morphosedimentary conditions is intra-reach erosion of banks and channel beds. Thus, this study proposes a new methodology to characterize morphological change, sediment transfer and connectivity that may serve as environmental indicators of the hydromorphological integrity of rivers with potential application to the European Water Framework Directive. © 2019 John Wiley & Sons, Ltd.

Climate change implies changes in the frequency and magnitude of flood events. The influence of climate variability on flooding was evaluated by an analysis of sedimentary (palaeofloods) and documentary archives. A 500-year palaeoflood record at Montlleó River (657 km2 in catchment area), eastern Spain, revealed up to 31 palaeofloods with a range of discharges of 20–950 m3 s−1, and with at least five floods exceeding 740–950 m3 s−1. This information contrasts with the available gauged flood registers (since year 1971) with an annual maximum daily discharge of 129 m3 s−1. Our palaeoflood dataset indicates flood cluster episodes at (1) 1570–1620, (2) 1775–1795, (3) 1850–1890, and (4) 1920–1969. Flood rich periods 1 and 3 corresponded to cooler than usual (about 0.3 °C and 0.2 °C) climate oscillations, whereas 2 and 4 were characterised by higher inter-annual climatic variability (floods and droughts). This high inter-annual rainfall variability increased over the last 150 years, leading to a reduction of annual maximum flow. Flood quantiles (>50 years) calculated from palaeoflood+gauged data showed 30%–40% higher peak discharges than those using only instrumental records, whereas when increasing the catchment area (1500 km2) the discharge estimation variance decreased to ~15%. The results reflect the higher sensitivity of small catchments to changes on flood magnitude and frequency due to climate variability whereas a larger catchment buffers the response due to the limited extent of convective storms. Our findings show that extended flood records provide robust knowledge about hazardous flooding that can assist in the prioritization of low-regret actions for flood-risk adaptation to climate change.

Pedofeatures associated with ancient cold climatic conditions have been recognized in soils on terraces in the Monegros area (central Ebro Basin, Spain), at a latitude of 41°49′N and an altitude of 300 m a.s.l. Eleven soil profiles were described on fluvial deposits corresponding to the most extensive terrace (T5) of the Alcanadre River, Middle Pleistocene in age (MIS8–MIS7). Each soil horizon was sampled for physical, chemical, mineralogical and micromorphological analyses. Macromorphological features related to pedocryogenic processes were described: involutions, jacked stones, shattered stones, detached and vertically oriented carbonatic pendents, fragmented carbonatic crusts, laminar microstructures, succitic fabric, silt cappings on rock fragments and aggregates, and irregular, broken, discontinuous and deformed gravel and sandy pockets. Accumulations of Fe–Mn oxides, dissolution features on the surface of carbonatic stones, and calcitic accumulations were identified related to vadose–phreatic conditions. The observed periglacial features developed under cold environmental conditions in exceptional geomorphic and hydrological conditions. This soil information may have potential implications in studies of paleoclimate in the Ebro Valley as well as in other Mediterranean areas.

In ephemeral rivers, channel morphology represents a snapshot at the end of a succession of geomorphic changes caused by floods. In most cases, the channel shape and bedform migration during different phases of a flood hydrograph cannot be identified from field evidence. This paper analyses the timing of riverbed erosion and deposition of a gravel bed ephemeral river channel (Rambla de la Viuda, Spain) during consecutive and moderate- (March 2013) and low-magnitude (May 2013) discharge events, by applying a morphodynamic model (Delft3D) calibrated with pre- and post-event surveys by RTK-GPS points and mobile laser scanning. The study reach is mainly depositional and all bedload sediment supplied from adjacent upstream areas is trapped in the study segment forming gravel lobes. Therefore, estimates of total bedload sediment mass balance can be obtained from pre- and post-field survey for each flood event. The spatially varying grain size data and transport equations were the most important factors for model calibration, in addition to flow discharge. The channel acted as a braided channel during the lower flows of the two discharge events, but when bars were submerged in the high discharges of May 2013, the high fluid forces followed a meandering river planform. The model results showed that erosion and deposition were in total greater during the long-lasting receding phase than during the rising phase of the flood hydrographs. In the case of the moderate-magnitude discharge event, deposition and erosion peaks were predicted to occur at the beginning of the hydrograph, whereas deposition dominated throughout the event. Conversely, the low-magnitude discharge event only experienced the peak of channel changes after the discharge peak. Thus, both type of discharge events highlight the importance of receding phase for this type of gravel bed ephemeral river channel.

[EN]: Monitoring detailed topographic changes in rivers is an essential tool for understanding the morpho-sedimentary behavior of rivers. However, as the resolution of topographic reconstruction techniques improves, survey costs and the time consumed increase. In this paper, the emerging Structure from Motion Multi-View Stereo (SfM-MVS), a high resolution but low cost technique, is scrutinized to assess whether it constitutes a viable option for ‘change detection’ in ephemeral rivers. To that end, three photogrammetric flights were carried out to reconstruct the subsequent digital elevation models (DEMs) and quantify fluvial change caused by two floods in 2015 along a 6.5 km reach in an ephemeral gravel-bed river (Rambla de la Viuda; eastern Spain). The application of SfM-MVS resulted in a maximum point cloud density of 1295.4 pts/m2 and a DEM resolution of 3.11 cm/pix. Individual flights registered errors of 0.101, 0.171 and 0.083 m. Level of detection (LoD) of DEMs of difference (DoDs), i.e. topographic change detection, resulted in 0.198 and 0.190 between the first and second, and second and third flights respectively. Orthomosaics were also successfully created at a maximum resolution of 2.50 cm/pix. Analysis of the best configuration of SfM-MVS for ephemeral river monitoring indicated that a high overlap of photographs and, therefore, a large number of projections were critical for an efficient workflow and a high-quality model. To ensure model quality and survey efficiency, the following configuration is recommended: (1) overlap index of more than 20 projections, (2) flight distribution at two heights, and (3) the use of ground control points. SfM-MVS topographies and DoDs showed a discontinuous pattern represented by a succession of erosive-depositional sequences. Evolution of one of these sequences has been studied in detail and the legacy of a past mining pit was pointed out to be the principal driver for this morphosedimentary pattern. Change detection qua, [ES]: Para llegar a comprender el comportamiento morfosedimentario de los ríos es necesario realizar un seguimiento detallado de los cambios topográficos que tienen lugar en sus cauces. Sin embargo, cuanto mayor es la resolución de la técnica utilizada para la reconstrucción topográfia, el coste y el tiempo necesario para cubrir una cierta superficie se incrementa. En este trabajo se ha utilizado y comprobado la utilidad de la fotogrametría digital automática (Structure from Motion Multi-View Stereo: SfM-MVS), una técnica que puede alcanzar gran resolución a un bajo coste. Para ello se han llevado a cabo tres vuelos fotográficos para reconstruir el modelo digital de elevaciones (MDE) de un tramo de 6.5 km de la Rambla de la Viuda, un río efímero situado en el Levante español. Estos MDE fueron comparados entre sí para cuantificar las modificaciones producidas por dos inundaciones ocurridas en 2015. Se obtuvo una nube de puntos con una densidad máxima de 1295.4 pts/m2 y una resolución para el MDE de 3.11 cm/pixel. Los errores asociados a las tres topografías fueron de 0.101, 0.171 y 0.083 m, respectivamente. El nivel de detección de cambios (LoD) fue de ±0.198 y ±0.190 para los modelos digitales de las diferencias entre las superficies (DoDs), obtenidos entre el primero y el segundo, y el segundo y el tercer vuelo respectivamente. Como subproducto del procesamiento de las topografías se obtuvieron ortofotos con una resolución máxima de 2.50 cm/pix. En relación a la metodología se observó que para un flujo de trabajo eficiente y una reproducción topográfica fidedigna es crucial seguir estas recomendaciones: (1) mantener un alto grado de superposición entre fotografías, manteniendo un valor de número de proyecciones superior a 20, (2) distribuir el plan de vuelo en dos pasadas a dos alturas distintas, para ganar resolución a la par que superposición y (3) el uso de puntos de control fijos al sustrato rocoso, para evitar la gran inversión de tiempo que supone desplegar l

The present study aims at the appraisal and latter development of a comprehensive methodology that enables a better assessment of extraordinary river Duero floods impact on infrastructures in general and on historical heritage areas in particular. Due to the long-­‐lasting historical and sedimentary records of the area, estimations and evaluations integrate different data sources and time scales. Flood data for the Douro River at Zamora have been compiled (760 years). During this period a total of 11 catastrophic floods were identified (e.g. 1739 and 1860), as well as over 30 extreme floods (e.g. 1636). Documentary data provided information on: 1) flood levels; 2) classification of events according to flood water level and damage. Temporal changes in the magnitude and frequency of floods have a strong correlation with climatic variations. Flood frequency analysis using gauged and historical floods shows differences in the discharge figures calculated by the Water Authority in Zamora.

This paper discusses the advances in the development of new data and methods that allow the reconstruction of long series of extreme flood data, and the application of flood analysis tools in conditions of climatic variability (past records and future projections). The work focuses on one case studies located in the Rambla de la Viuda (Maria Cristina Dam), with Mediterranean climate.The aim is to compare the effects of climate change on future extreme floods, with changes in flood frequency laws determined from paleo-­‐grazed and historical data. This methodology aims to improve the impact of climate change on hydrological extremes and their application to the design of sensitive infrastructures, with emphasis on dams. Hydro-­‐ climatic indicators will be estimated using historical, instrumental and climate series data projected with CMIP5 models.

This paper provides a new methodological approach to analyse secular patterns of flooding (magnitude and frequency) from sedimentary evidence (palaeofloods), taking into account changes in channel geometry, and their links to historical environmental changes and the inherent social and demographic evolution within the catchment. A case study analysis was focused in Rambla de la Viuda (drainage area of 1500 km2) whose stream flow is related to extreme rainfalls. A 500 years sedimentary archive was reconstructed from eight stratigraphic profiles comprising continuous sequences of slackwater flood deposits interbedded with episodic colluvial and edaphic horizons. Discharge estimates associated to sedimentary flood evidences were obtained from one-dimensional hydraulic modelling. The stratigraphic units were sampled to characterise their geochemical and paleobotanical (phytoliths) contents. Palaeoflood chronology was obtained from radiocarbon and luminescence (OSL) dating, supported by documentary data (written historical documents). A high frequency and high magnitude palaeoflood period took place during the 15th-middle 16th century, which seem to correlate in time with general wetter conditions. Three short-term environment stability conditions (land use and climatic) also made possible the development of three paleosols. The lowest flood magnitude and discharges in the sedimentary record was found between the mid-17th to mid-18th centuries, under prevailing drier environmental conditions. Episodic high magnitude flooding took place at late 18th century, correlating in time with palaeovegetation and geochemical evidences of important changes on land use (deforestation and grazing). Poorer developed soils were found at upper stratigraphic sequences (19th century) characterised by thick units of colluvium deposits, usually culminating sequences of short-lived continuous slackwater flood units. Despite of the potential human influence (land-use) on soil hydrology, the lo

River incision into bedrock resulting from the combined effects of tectonic uplift and climate governs long-term regional landscape evolution. We determined spatial and temporal patterns of post-orogenic stream incision from a sequence of well-preserved staircase terraces developed over the last 1 Ma in the Central Pyrenees and its southern foreland Ebro basin (NE Spain). Extensive remnants of ten vertically separated terraces (Qt1 to Qt10, from oldest to youngest) were mapped along 170 km of the Cinca River valley, transverse to the Pyrenean mountain belt. Multiple outcrops appear in the upper reach of the valley (Ainsa sector, 50 km from headwaters) as well as in the lower reach (Albalate sector, 125 km from headwaters). Fluvial incision into bedrock was calculated using (i) differentially corrected GPS measurements of the altitude of straths and (ii) numerical dating of alluvial sediments from the lower terraces (Qt5 to Qt9) by Optically Stimulated Luminescence, previously reported by Lewis et al. (2009), and supplemented with new dates for the upper terraces (Qt1, Qt2 and Qt3) based on paleomagnetism and supported by soil development. Considering altitude differences and the elapsed time between successive well preserved terrace couples (Qt3–Qt7, Qt7–Qt9 and Qt9-Active channel), mean bedrock incision rates ranged from 0.76 to 0.38 m ka, at the upper reach of the valley (Ainsa section), and from 0.61 to 0.20 m ka, at the lower reach (Albalate section). River incision along the valley produced vertically separated, near-parallel longitudinal terrace profiles evidencing a rapid near-uniform regional uplift as response to (i) the tectonic lithospheric thickening in NE Iberia and (ii) the erosional download rebound related to the Ebro basin exorheism. Moreover, a subtle upstream divergence of strath profiles may have been a consequence of an increase in uplift rate toward the head of the valley. Additionally, incision rates changed over time as indicate results from