Reservoir deltas and their role in pollutant distribution in valley-type dam reservoirs: Les Království Dam, Elbe River, Czech Republic

Interpretation of catchment pollution history from sediment cores is a common approach for dam reservoirs, but relatively little is known about the spatial distribution of pollutants in the lithologically diverse depositional subsystems. We studied the depositional system of a hundred-year-old valley-type reservoir of the Les Království Dam built in 1919 on the upper reaches of the Elbe River, Czechia. Based on the analysis of pre-dam river valley morphology using historical topography maps with 1 m vertical resolution, 30 ground-penetrating radar sections on the lake and 137Cs dating of four cores, we described a deltaic sediment body with maximum thickness of ~11 m, accumulation rates ranging from 3.6 to 9.7 cm/yr, and an annual loss in reservoir capacity of 0.8%. The sediment body has a distinct delta shape in the longitudinal section, with a delta platform dissected by a channel (topsets) and delta slope (foresets) and a ponded basin (bottomsets) located near the dam. The upstream sections of the channels are prone to erosion and sediment redistribution into downstream parts of the reservoir, as indicated by ground-penetrating radar sections and distribution of 137Cs in younger sediment strata. We retrieved four cores 1.0 to 1.6 m long from different parts of the delta and analysed the sediment grain size using laser granulometry, geochemical composition (X-ray fluorescence, ICP-MS lithogeochemistry, total organic carbon (TOC)) and concentrations of polycyclic aromatic hydrocarbons (PAH). The grain size variation between the delta platform (sandy silts and silty sands) and prodelta (clayey silts) sediments strongly controls the concentrations of lithogenic elements (Al, Fe, K and Ti) but it has a relatively little effect on distribution of toxic metals (Zn, Cu, Pb) and TOC concentrations. The toxic metals show strong affinity to TOC and PAH show a distinct sawtooth vertical distribution suggesting that they were deposited from depositional events during hyperpycnal flows induced by river floods. This study shows that a hyperpycnal delta may serve as a good predicable model for toxic metal- and organic pollutant distribution in dam reservoirs. The role of hyperpycnal flows should not be overlooked in contamination studies of dam reservoirs. •Sediments in Les kralovstvi dam lake accumulate at high rates, up to 9.7 cm/yr.•Sediments form a deltaic body with distinct topsets, foresets and bottomsets.•Hyperpycnal flows during floods distributed pollutants in