Vertical Hydrologic Exchange Flows Control Methane Emissions from Riverbed Sediments.

CH emissions from inland waters are highly uncertain in the current global CH budget, especially for streams, rivers, and other lotic systems. Previous studies have attributed the strong spatiotemporal heterogeneity of riverine CH to environmental factors such as sediment type, water level, temperature, or particulate organic carbon abundance through correlation analysis. However, a mechanistic understanding of the basis for such heterogeneity is lacking. Here, we combine sediment CH data from the Hanford reach of the Columbia River with a biogeochemical-transport model to show that vertical hydrologic exchange flows (VHEFs), driven by the difference between river stage and groundwater level, determine CH flux at the sediment-water interface. CH fluxes show a nonlinear relationship with the magnitude of VHEFs, where high VHEFs introduce O into riverbed sediments, which inhibit CH production and induce CH oxidation, and low VHEFs cause transient reduction in CH flux (relative to production) due to reduced advective CH transport. In addition, VHEFs lead to the hysteresis of temperature rise and CH emissions because high river discharge caused by snowmelt in spring leads to strong downwelling flow that offsets increasing CH production with temperature rise. Our findings reveal how the interplay between in-stream hydrologic flux besides fluvial-wetland connectivity and microbial metabolic pathways that compete with methanogenic pathways can produce complex patterns in CH production and emission in riverbed alluvial sediments.