Impact of reservoir construction on the sediment budget of a downstream-linked freshwater lake

Abstract

We investigated the magnitude of reservoirs sedimentation upstream and its sediment-shadow effect on a large downstream-linked lake, Lake Tana (Ethiopia). To better understand the reservoir-induced reductions of fluvial sediment downstream, sediment budgets were developed for two scenarios, with the sediment budget by Lemma et al. (2020) serving as a baseline. The results indicated that reservoir sedimentation is extremely high with annual storage capacity losses (1–4.2%) exceeding the global average. Consequently, unless mitigation measures are implemented, the reservoirs’ dead storage volume will be filled with sediment in less than 42% of their designed life expectancy. The sediment-trapping conditions created by the reservoirs will also reduce sediment inflow from tributaries into the lake by 15–52%, overbank sedimentation on floodplains by 33–48%, and sediment deposition in Lake Tana by 16–54% annually. Overall, under the current environmental setup, the ongoing and planned reservoirs constructions could extend the expected useful lifetime of Lake Tana from 918 to 2022 years.