Recent climatic warming and associated glacial retreat may have a large impact on sediment release and transfer in Alpine river basins. Concurrently, the sediment transport capacity of many European Alpine streams is affected by hydropower exploitation, notably where flow is abstracted but the sediment supply downstream is maintained. Here, we investigate the combined effects of climate change and flow abstraction on morphodynamics and sediment transfer in the Borgne River, Switzerland. From photogrammetrically derived historical Digital Elevation Models (DEMs), we find considerable net aggradation of the braided river bed (up to 5 m) since the onset of flow abstraction in 1963. Reaches responded through bed level steepening which was strongest in the upper most reach. Widespread aggradation however did not commence until the onset of glacier retreat in the late 1980s and the dry and warm years of the early 1990s. Upstream flow intake data shows that this aggradation coincided with an increase in sediment supply, although aggradation accounts for no more than 25% of supplied material. The remainder was transferred through the studied reaches. Estimations of bed load transport capacity indicate that flow abstraction reduces transport capacity by 1–2 orders of magnitude. While residual transport rates vary with morphological evolution, they are in the same order of magnitude as the sediment supply rates, which is why significant transport remains. However, the reduction in transport capacity makes the system more sensitive to short‐term (annual) changes in climate‐driven hydrological variability and climate‐induced changes in intake management and sediment delivery rates.
Alpine water and sediment supply influence the sediment budget of many important European fluvial systems such as the Rhine, Rhône and Po rivers. In the light of human induced climate change and ACCEPTED MANUSCRIPT overestimation of denudation rates and thus limit the applicability of cosmogenic nuclide analysis in such glaciated settings.
The ongoing rise in sea level affects tidal propagation and circulation in estuaries, and these changes can have far reaching consequences on the sediment dynamics, water quality and extreme water levels. This study aims at anticipating the evolution of the tidal dynamics in the Tagus (Portugal) in the 21st century, in particular due to sea level rise (SLR). The existence of a resonance mode of about 8 hours in this estuary, that selectively amplifies both semi-diurnal and quarter-diurnal tidal constituents, makes the response of the Tagus estuary to SLR unique. The study was conducted with a shallow water model, forced by present and future conditions, namely higher mean sea levels and an extrapolated bathymetry based on present sedimentation rates. Model results showed that SLR will significantly affect tidal asymmetry, in particular because the intertidal area can decrease by up to 40% by the end of the 21st century. As a result, the strong ebb-dominance of this estuary will decrease significantly. This evolution of tidal asymmetry will be counteracted by the effect of sedimentation of the salt-marsh areas. Also, SLR will enhance the resonance in the Tagus estuary. As a consequence, extreme water levels will be higher than the sum of present levels with the SLR
Deltas are important coastal sediment accumulation zones in both marine and lacustrine settings. However, currents derived from tides, waves or rivers can transfer that sediment into distal, deep environments, connecting terrestrial and deep marine depozones. The sediment transfer system of the Rhone River in Lake Geneva is composed of a sublacustrine delta, a deeply incised canyon and a distal lobe, which resembles, at a smaller scale, deepsea fan systems fed by high discharge rivers. From the comparison of two bathymetric datasets, collected in 1891 and 2014, a sediment budget was calculated for eastern Lake Geneva, based on which sediment distribution patterns were defined. During the past 125 years, sediment deposition occurred mostly in three high sedimentation rate areas: the proximal delta front, the canyon-lev ee system and the distal lobe. Mean sedimentation rates in these areas vary from 0Á0246 m year À1 (distal lobe) to 0Á0737 m year À1 (delta front). Although the delta front-lev ees-distal lobe complex only comprises 17Á0% of the analysed area, it stored 74Á9% of the total deposited sediment. Results show that 52Á5% of the total sediment stored in this complex was transported toward distal locations through the sublacustrine canyon. Namely, the canyon-lev ee complex stored 15Á9% of the total sediment, while 36Á6% was deposited in the distal lobe. The results thus show that in deltaic systems where density currents can occur regularly, a significant proportion of riverine sediment input may be transferred to the canyon-lobe systems leading to important distal sediment accumulation zones.
Abstract. Suspended sediment export from large Alpine catchments (> 1000 km2) over decadal timescales is sensitive to a number of factors, including long–term variations in climate, the activation–deactivation of different sediment sources (proglacial areas, hillslopes, etc.), transport through the river system, and potential anthropogenic impacts on the sediment flux (e.g. through impoundments and flow regulation). Here, we report on a marked increase in suspended sediment concentrations observed close to the outlet of the upper Rhône River Basin in the mid–1980s. This increase coincides with a statistically significant step–like increase in basin–wide mean air temperature. We explore the potential explanations of the suspended sediment rise in terms of discharge (transport capacity) change, and the activation of different sources of fine sediment (sediment supply) in the catchment by hydroclimatic forcing. Time series of precipitation and temperature–driven snowmelt, snow cover and ice–melt simulated with a spatially distributed degree–day model, together with erosive rainfall on snow–free surfaces, are tested as possible reasons for the rise in suspended sediment concentration. We demonstrate that the abrupt change in air temperature reduced snow cover and the contribution of snowmelt, and enhanced ice–melt. The results of statistical tests showed that the onset of increased ice–melt was likely to play a dominant role in the suspended sediment concentration rise in the mid–1980s. Temperature–driven enhanced melting of glaciers, which cover about 10 % of the catchment surface, can increase suspended sediment yields through increased runoff from sediment–rich proglacial areas, increased contribution of sediment–rich meltwater, and increased sediment supply in proglacial areas due to glacier recession. The reduced extent and duration of snow cover in the catchment may also have partly contributed to the rise in suspended sediment concentration through hillslope erosion by rainfall on snow free surfaces, and by reducing snow cover on the surface of the glaciers and thereby increasing meltwater production. Despite the rise in air temperature, changes in mean discharge in the mid–1980s were statistically insignificant, and their interpretation is complicated by hydropower reservoir management and the flushing operations at intakes. Thus, the results show that to explain changes in suspended sediment transport from large Alpine catchments it is necessary to include an understanding of the multitude of sediment sources involved together with the hydroclimatic conditioning of their activation (e.g. changes in precipitation, runoff, air temperature). This is particularly relevant for quantifying climate change and hydropower impacts on streamflow and sediment budgets in high Alpine catchments.
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