Geomorphologic impact of outburst flood cycle in braided gravel-bed rivers: confluence of Colonia and Baker rivers, Patagonia, Chile

Colonia River, as other glacial streams influenced by global warming, is affected by Glacial Lake Outburst Floods (OF). We present a study on geomorphologic features of Colonia River to comprehend the impact of OFs in sedimentary processes, analysing erosion and deposition linked to OF phenomena and frequency (since April 2008, more than 12 OFs have occurred). Interpolation analysis is carried out to create an elevation surface (DTM) from a set of sample measurements, given by contour line (2007 LiDAR) and rtkGPS point (2011 and 2012) data. These were used to create DTMs applying Delaunay triangulation. A comparisons between morphology during the current OF cycle that started on 8 April 2008, and the previous morphology (after 40 years without OFs, since end of previous cycle in 1968), is carried out through the creation of DEMs of Difference (DoDs) based on TINs. We created two DoDs, respectively between 2011-2007 (before/after the OFs cycle) and 2012-2011 (during the cycle, after the events of January and April 2012) to provide a high-resolution, spatially distributed surface model of topographic and volumetric change through time. The floodplain has been classified on the basis of the morphological characteristics: main and secondary channel(s), bars, braided area, island and delta fan features. The changes evidenced in the DoD are segregated according to these categories plus the geomorphic change taking place in each category (using GCD; Wheaton et al 2014). Issues regarding data quantity and quality will be discussed, and implications for the planned mega-dam that would flood the confluence

Magnitude and timing of Holocene glacial lake outburst floods in the Baker River, Northern Patagonian Icefield, Chile / Magnitud y frecuencia de inundaciones Holocenas generadas por vaciamiento de lagos glaciares en el Rio Baker, Campo de Hielo, Patagonico Norte, Chile

The Baker River (Southern Chile) drains a catchment of 27,000 km2 and has a mean annual discharge of 1100 m3s-1. Since last deglaciation, the morpho-sedimentology of the Baker valley has been controlled by Outburst Floods (OFs) of different scales. We apply geomorphic mapping, stratigraphy (including radiocarbon and OSL dating) and palaeoflood hydrology to reconstruct the frequency, timing and magnitude of Holocene OFs. Geomorphic mapping reveals evidence of two Holocene alluvial terraces. The oldest (highest) contains basal gravels capped by a well-developed buried Podzolic Luvisol that was radiocarbon dated to 6160±40 BP. In this alluvial sequence, at least two major floods occurred between then and 5300 BP and at least eight major floods between 5300-2500 BP. At least three Late Holocene (post 610±30 BP) GLOF event(s) inundated upper terrace surfaces along the reach downstream the confluence of the Colonia River into the Baker River. We report on the implications of this palaeoflood research in relation to the proposed HydroAysén hydroelectric scheme

Hydrological response of a dryland ephemeral river to southern African climatic variability during the last millennium

12 páginas, 6 figuras, 2 tablas.-- El PDF del artículo esta en su versión post print.A long-term flood record from the Buffels River, the largest ephemeral river of NW South Africa (9250 km2), was reconstructed based on interpretation of palaeoflood, documentary and instrumental rainfall data. Palaeoflood data were obtained at three study reaches, with preserved sedimentary evidence indicating at least 25 large floods during the last 700 yr. Geochronological control for the palaeoflood record was provided by radiocarbon and optically stimulated luminescence (OSL) dating. Annual resolution was obtained since the 19th century using the overlapping documentary and instrumental records. Large floods coincided in the past within three main hydroclimatic settings: (1) periods of regular large flood occurrence (1 large flood/~30 yr) under wetter and cooler prevailing climatic conditions (AD 1600–1800), (2) decreasing occurrence of large floods (1 large flood/~100 yr) during warmer conditions (e.g., AD 1425–1600 and after 1925), and (3) periods of high frequency of large floods (~ 4–5 large floods in 20–30 yr) coinciding with wetter conditions of decadal duration, namely at AD 1390–1425, 1800–1825 and 1915–1925. These decadal-scale periods of the highest flood frequency seem to correspond in time with changes in atmospheric circulation patterns, as inferred when comparing their onset and distribution with temperature proxies in southern Africa.The study was funded by the 6th Framework Programme of the European Commission through the project “FloodWater recharge of alluvial Aquifers in Dryland Environments”, WADE Project (contract no. GOCE-CT-2003-506680).Peer reviewe

High resolution lake sediment record reveals self-organized criticality in erosion processes regulated by internal feedbacks

Reconstruction of high-frequency erosion variability beyond the instrumental record requires well-dated, high-resolution proxies from sediment archives. We used computed tomography (CT) scans of finely laminated silt layers from a lake-sediment record in southwest Oregon to quantify the magnitude of natural landscape erosion events over the last 2000years in order to compare with palaeorecords of climate, forest fire, and seismic triggers. Sedimentation rates were modeled from an age-depth relationship fit through five C-14 dates and the 1964AD Cs-137 peak in which deposition time (yrmm(-1)) varied inversely with the proportion of silt sediment measured by the CT profile. This model resulted in pseudo-annual estimates of silt deposition for the last 2000years. Silt accumulation during the past 80years was strongly correlated with river-discharge at annual and decadal scales, revealing that erosion was highly responsive to precipitation during the logging era (1930-present). Before logging the frequency-magnitude relationship displayed a power-law distribution that is characteristic of complex feedbacks and self-regulating mechanisms. The 100-year and 10-year erosion magnitude estimated in a 99-year moving window varied by 1.7 and 1.0 orders of magnitude, respectively. Decadal erosion magnitude was only moderately positively correlated with a summer temperature reconstruction over the period 900-1900AD. Magnitude of the seven largest events was similar to the cumulative silt accumulation anomaly, suggesting these events returned the system' to the long-term mean rate. Instead, the occurrence of most erosion events was related to fire (silt layers preceded by high charcoal concentration) and earthquakes (the seven thickest layers often match paleo-earthquake dates). Our data show how internal (i.e. sediment production) and external processes (natural fires or more stochastic events such as earthquakes) co-determine erosion regimes at millennial time scales, and the extent to which such processes can be offset by recent large-scale deforestation by logging. Copyright (c) 2018 John Wiley & Sons, Ltd

Determining the relationship between fluvial activity and climatic variability during the Holocene in Spain

A database of radiocarbon dates from Holocene fluvial environments in Spain has been compiled. The dates have been classified according to the different types of depositional environments and ensembles from which the samples were collected. The sampling locations have been categorised according to geographical region, elevation and drainage basin area. The results of the analysis indicate a number of phases of increased fluvial activity: 11,280-10,230 cal. BP; 9565-8785 cal. BP; 7975-7090 cal.BP; 5740-4135 cal. BP; 3880-3085 cal. BP; 2895-1820 cal. BP; and 1300-0 cal. BP. A review of the radiocarbon dating evidence combined with palynological data of Holocene vegetation changes indicate that increased fluvial activity and geomorphc effectiveness coincide with periods with a more open landscape indicating the importance of sediment supply in the formation of the depositional landforms. Detailed analysis of the record has enabled the relative forcing of climatic variability and anthropogenic impacts to be determined. The major climatic perturbations of the Holocene that are recorded in the fluvial record are the 8200 BP, 2650 BP and Little Ice Age periods of climatic deterioration, with increased flood frequency also related to the Medieval Warm Period.This research was carried out as part of the project: “Past hydrological events related to an understanding of global change”, funded by the International Council for Science (ICSU). The authors would like to thank Mark Macklin and Eric Johnstone for their input regarding the methodological approach that came from their analysis of the UK fluvial record (in preparation).Peer Reviewe

Registros fluviales en la Península Ibérica durante la Pequeña Edad del Hielo

En este trabajo se analizan los datos cronológicos de radiocarbono procedentes de sedimentos fluviales en diferentes cuencas españolas para los últimos 1000 años. Los registros sedimentarios de llanura aluviales muestran un periodo principal de aluvionamiento ocurrido entre 800 y 500 cal BP, mientras que los registros de paleocrecidas se concentran antes y después del mismo, particularmente entre 1000-800 cal. BP y 520-250 cal BP. Este segundo periodo con mayor frecuencia de paleocrecidas aparece relacionado con la Pequeña Edad del Hielo, y coinciden con los obtenidos mediante registros documentales. Los resultados señalan la elevada sensibilidad de los registros de paleocrecidas que constituyen evidencias directas (¿no proxy¿) de eventos hidrológicos extremos, mientras que los depósitos aluviales representan la respuesta del conjunto de la cuenca a los cambios ambientales, hidrológicos y climáticos.Esta investigación ha sido financiada por el International Council for Science (ICSU) proyecto “Past hydrological events related to understanding global change”. Los autores agradecen a Mark Macklin y Eric Johnstone (Universidad de Gales, Aberystwyth) por su aportación en el desarrollo metodológico presentado en este trabajo, y que procede de su experiencia del análisis del registro fluvial del Reino Unido. Igualmente, agradecemos la información adicional sobre dataciones de radiocarbono inéditas proporcionadas por Mayte Rico (CSIC, Madrid), Dave Passmore (University of Newcastle) y Jose Ortega (Universidad Europea de Madrid).Peer Reviewe

The Holocene fluvial chronology of Spain: Evidence from a newly compiled radiocarbon database

A critical analysis of 74 radiocarbon dates, selected from a total of 102 published and unpublished dates from Holocene fluvial environments in Spain, has identified a number of periods of increased fluvial activity in Spain, namely: 11,170–10,230; 9630–8785; 7980–6860; 5800–4800; 3880–3085; 2895–1820; 1300–0 cal BP, the latter period reflecting sustained fluvial response to increased human impact. The radiocarbon samples were classified according to type of depositional environment: (a) alluvial overbank, (b) flood basin, (c) alluvial channel gravels, (d) fluvio-torrential deposits and (e) slackwater flood deposits (palaeofloods). Dates from slackwater flood deposits indicated at least five phases of increased frequency of large magnitude floods during the Holocene: 10,855–10,230; 9530–8780; 2880–2430; 975–790; and 520–265 cal yr BP. Flood basin type deposition occurred from 7980 to 4830 cal yr BP. The main clusters of dates from alluvial floodplain deposits occurred at 2750–2150 and 930–520 cal yr BP. The record is discussed in relation to other palaeoenvironmental archives including pollen records of Holocene vegetation change and palaeoclimatic proxies such as the North Atlantic drift ice record. Comparison with the latter indicates that major Holocene flooding coincided with cold climate phases during 9530–9280, 2880–2430 and 520–265 cal yr BP; a cooling phase at 10,855–10,230; and warming phases at 9030–8780 and 975–790 cal yr BP.This research was funded by the International Council for Science (ICSU) project “Past hydrological events related to understanding global change” coordinated by Prof Ken GregoryPeer reviewe

Palaeoflood hydrology and its role in applied hydrological sciences

This paper is a review of the methodology of palaeoflood hydrology. In particular, we focus on recent developments and the credibility of the palaeoflood data produced. The use of slackwater flood deposits as a physical record of water surface elevations reached by past floods enables the calculation of robust palaeodischarge estimates for floods that occurred during recent centuries or millennia. Over these time intervals the chronological precision from numerical age dating, such as radiocarbon, is sufficient for the structuring of the palaeoflood discharge data into different threshold levels that are exceeded by floodwaters over specific periods of time, the input data necessary for new methodologies of flood frequency analysis. The value of palaeoflood hydrology to hydrological sciences is discussed through its application in varying multidisciplinary research themes. We demonstrate the use of palaeoflood hydrology in: (1) flood risk estimation; (2) determination of the maximum limit of flood magnitude and non-exceedances as a check of the probable maximum flood (PMF) and its application in producing regional, long-term envelope curves; (3) Holocene climatic variability and (4) assessing sustainability of water resources in dryland environments where floods are an important source of water to alluvial aquifers.Our palaeoflood research has been funded by the European Commission through the projects ‘Systematic, Palaeoflood and Historical data for the improvEment of flood Risk Estimation—SPHERE’ (contract no. EVG1-CT-1999-00010) and ‘FloodWater recharge of alluvial Aquifers in Dryland Environments—WADE (contract no. GOCE-CT-2003-506680). The Spanish Commission of Science and Technology funded the project ‘Incorporation of palaeoflood and historical flood data in the calculation of dam safety—PALEOCAP’ (CICYT project no. REN2001-1633-RIES).Peer reviewe

Late Holocene fluvial chronology of Spain: The role of climatic variability and human impact

A database of published and unpublished radiocarbon dates from Late Holocene fluvial environments in Spain was critically analysed in order to obtain a fluvial chronology for this period. The principal types of depositional environment represented in the record were slackwater flood deposits (28 dates), alluvial overbank facies (12 dates) and alluvial channel facies (7 dates). The sedimentary context of each date was classified in order to identify those samples that specifically dated change in the fluvial environment. Analysis of the different depositional environment data-sets using summed probability plots identified phases of alluviation at 2800–2350 and 800–500 cal. BP. Increased frequency of large magnitude floods occurred at 2850–2500, 1000–800 and 520–250 cal. BP. The results indicate that in Mediterranean regions slackwater flood deposits are valuable archives of climatic variability even during periods of increased human impact.This research was funded by the International Council for Science (ICSU) project “Past hydrological events related to understanding global change” and the INQUA project “The significance of past hydrological events: using existing 14C data to reconstruct Holocene fluvial activity in temperate and tropical regions.”Peer reviewe

Palaeoflood hydrology: insight into rare events and extreme flood discharges

Les études paléohydrologiques permettent la reconstitution de chronologies de crue à partir des traces laissées par les sédiments de crue ou l'érosion du courant. Ces marques géologiques donnent une indication sur le niveau atteint par des crues anciennes, sur des périodes de quelques siècles à plusieurs millénaires. La datation des sédiments s'effectue à partir des matériaux présents dans les dépôts de crue (isotope 14 du carbone ou thermoluminescence de grains de minéraux). Les données paléohydrologiques peuvent ensuite être exploitées d'un point de vue probabiliste, en considérant que les événements recensés ont dépassé un ou plusieurs seuils de niveau. Les différentes applications récentes de la paléohydrologie aux événements de crue ont porté sur : (1) l'analyse fréquentielle des crues ; (2) la détermination d'un niveau maximum atteint par les crues et sa cohérence avec la Crue Maximale Probable (PMF) ; et (3) meilleur connaissance de la relation entre les crues extraordinaires et la variabilité climatique.Peer reviewe