Lack of methane hotspot in the upstream dam: Case study in a tributary of the Three Gorges Reservoir, China

Abstract

River damming has seen a growing trend in demand worldwide and the impounded reaches are considered hotspots of greenhouse gas emissions. However, it remains unclear how the spatial distribution of C-gas in sediments and methane (CH4) emissions of dammed tributary changes under different operation periods of the Three Gorges reservoir (TGR). We measured CH4 and carbon dioxide (CO2) concentrations in sediment and CH4 emissions from a dammed river of the TGR, and evaluated the effect of damming on the spatial variability of carbon in the sediment and on CH4 flux. It was found that damming led to a distinct spatial pattern of total organic carbon (TOC) in the sediment, which resulted in higher CH4 and CO2 in upstream sediment compared to the downstream. During the TGR impounding period, the upstream CH4 diffusive flux (0.253–0.427 mg m−2 h−1) across the water-air interface was higher than in the downstream (0.093 mg m−2 h−1), which was consistent with the spatial variation of CH4 in the sediments. However, the CH4 emission was predominantly by ebullition and the flux in the downstream (169.173 mg m−2 h−1) was significantly higher than upstream (12.23–123.05 mg m−2 h−1) in the discharging period. This can be attributed to a sharp increase in TOC in downstream sediment due to riparian zone soil erosion on both banks, which was caused by periodic large fluctuation in the water level, and a shallow water depth in the downstream. This study adds to our understanding of effects of the TGR's operation on CH4 emissions from a dammed tributary and suggests that the water level fluctuation of tributaries which has direct influence on ebullition and methane oxidation caused by manipulation of the TGR should not be overlooked.