Water column can shift from sinks to sources of CH4 with increasing river size

CH₄ production in freshwaters has been considered to occur primarily in anoxic sediments. However, recent discovery of oxic CH₄ production in some lake waters makes the origins and controls over the fate of cumulative CH₄ in water column elusive. Especially, whether CH₄ can be produced in oxic water column of river systems remains unclear, and the role of water column in CH₄ emissions is poorly understood across different river sizes. Here, we present water column contributions to riverine CH₄ emissions based on 4-year national-wide in situ measurements across six large river networks with stream order from 3^rd to 8^th. We find water column acts as a contributor of CH₄ net production in 58% observations, indicating the occurrence of CH₄ production in oxic water column, which is probably attributed to CH₄ production in anoxic interface of suspended particles and production by phytoplankton. Water column can account for 7% of riverine CH₄ emissions on average across all observations. Water-air CH₄ fluxes decreased exponentially with stream order. In contrast, water column contribution increased with stream order and its roles vary with river size and shift from CH₄ sinks in streams to CH₄ sources in large rivers. Water column can consume 6% of CH₄ released from sediment in rivers of size lower than 6^th, while contribute 12% to water-air CH₄ fluxes in higher-order rivers. This shift is mainly attributed to the increase of river depth and higher concentrations of suspended particles, which may facilitate net CH₄ production in water column of large rivers. Our findings suggest oxic CH₄ production in water column represents a hitherto overlooked source of methane and can be important for CH₄ cycling and emissions in river systems, especially for large rivers.