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Tidally varying methane (CH₄) concentrations in estuaries may arise from physical advection and by chemical effects tied to varying exposure to salinity. An investigation of spatial and temporal variability in water-column CH₄ was conducted in the lower Columbia River using shipboard surveys and time series data from fixed stations. Peaks in CH₄ coincided with ebb tides at multiple sites located along the flank of the estuary adjacent to tidal flats and wetlands. High-resolution measurements taken at the outflow of a shallow lateral bay revealed that these CH₄ peaks were positively related to tidal amplitude when the lateral bay was exposed exclusively to freshwater over the tide cycle; in contrast, this relationship was inversed when brackish waters were involved. A positive relationship between tidal amplitude and CH₄ is consistent with a mechanism of tidal pumping from bottom sediments in the bay. In …