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

Highlights: • Damming effect on C emission and storage was studied in a tributary of the Three Gorges Reservoir. • Damming resulted in higher TOC, CH₄ and CO₂ concentrations in upstream sediments. • CH₄ emissions downstream estuary were slightly lower than upstream during high reservoir levels. • CH₄ emissions prevailed by ebullition particularly at the estuary during low reservoir levels. 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.