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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.
Lemma et al. (Wed,) studied this question.