Methane emissions due to reservoir flushing: a significant emission pathway?

Reservoirs represent a globally significant source of the greenhouse gas methane (CH4), which is emitted via different emission pathways. In some reservoirs, reservoir flushing is employed as a sediment management strategy to counteract growing sediment deposits that threaten reservoir capacity. Reservoir flushing utilizes the eroding force of water currents during water level drawdown to mobilize and transport sediment deposits through the dam outlet into the downstream river. During this process, CH4 that is stored in the sediment can be released into the water and degas to the atmosphere, resulting in CH4 emissions. Here, we assess the significance of this CH4 emission pathway and compare it to other CH4 emission pathways from reservoirs. We measured seasonal and spatial CH4 concentrations in the sediment of Schwarzenbach Reservoir, providing one of the largest datasets on CH4 pore water concentrations in freshwater systems. Based on this dataset we determined CH4 fluxes from the sediment and estimated potential CH4 emissions due to reservoir flushing. CH4 emissions due to one flushing operation can constitute 7 %-14 % of the typical annual CH4 emissions from Schwarzenbach Reservoir, whereby the amount of released CH4 depends on the seasonal timing of the flushing operation and can differ by a factor of 2. Larger flushing events that mobilize deeper sediment layers lead to non-linear increases in CH4 mobilization. This suggests that regular flushing of smaller sediment layers releases less CH4 than removal of the same sediment volume in fewer flushing events of thicker sediment layers. However, additional indirect CH4 emissions pathways contributing to the total CH4 emissions may vary with the flushing operation. In other reservoirs with higher sediment loadings than Schwarzenbach Reservoir, reservoir flushing could cause substantial CH4 emissions, especially when flushing operations are conducted frequently. Our study recognizes CH4 emissions due to reservoir flushing as an important pathway, identifies potential management strategies to mitigate these CH4 emissions and emphasizes the need for further research.