Morphological control on sedimentation rates and patterns of delta floodplains in the Swiss Alps

Glacially overdeepened Alpine valleys and their perialpine lakes are very effective sedimentary sinks that include landforms as alluvial fans, fan deltas, and deltas. The sedimentary archives of these landforms can provide long sedimentary records of erosion and aggradation processes in a catchment. Focusing on longer time series is vital to improve the conventional instrument-based models of sediment transfer, as longer time series record the effective low frequency sediment discharges triggered by high magnitude floods. The research reported here combines statigraphical analysis of the sedimentary records of two lake deltas in the Bernese Alps with geostatistical modeling of the sedimentation storage of delta plain deposits. From the three-dimensional sediment storage model, the sedimentary environments and aggradation dynamics of the Aare and the Lutschine Rivers were reconstructed in 500-year time slices over the last 2500 years. The spatial distributions of the aggradation rates of the two deltas show similar trends, with decreasing downstream rates from delta apex to the delta front at Lake Brienz, which is related to the adaption of the river's longitudinal profile to the Pleistocene overdeepened valley morphology. Local modification of this trend was detected at narrowing of the valley floor caused by landslides. The quantification of delta plain sediment volumes provides evidence that aggradation processes have decreased (Aare) or remained nearly stable (Lutschine) in the delta plains during the last 1500 years, despite the strong human impact in the form of land use. This decrease and the possible compensation of major human-induced catchment erosion may be attributed to a more or less efficient regulation of the river channels during the last millennium, which transferred the sediment load into Lake Brienz. The sediment volumes obtained and the historical progradation of delta lobes support the previous hypothesis of the connectivity from source (headwater catchments) to sink (perialpine lakes) in the main valleys of the Bernese Alps. The results of our study provide new insight into the quantitative estimation of sediment storage for geomorphic process modeling and into the evolution of alpine deltas during the late Holocene. (C) 2013 Elsevier B.V. All rights reserved.