Alluvial rivers are dynamic elements of the landscape in the Pacific Northwest. They expand, contract, and migrate across the bottom of valleys in response to changing flow and vegetation. Channel dynamics fundamentally structure river and floodplain ecosystems. Human activities that affect channel migration—including river regulation, channel revetments, and land use—have the potential for impacting river ecosystems. Active channel width and lateral movement of the active channel centerline was analyzed in 17 km valley segment of the middle Green River in western Washington from aerial photographs for 26 years of unregulated flows (1936–1961) and 41 years of flood regulation by Howard Hansen Dam. Area-based measures proved more robust for characterizing channel dynamics than cross-sectional measurements, though cross-sectional measurements are useful for resolving local processes. Prior to regulation from 1936 to 1961, the active channel width varied from 82 m to 120 m (median 94 m) and the channel migrated laterally a total of 68 m. After flood regulation from 1961 to 2002, the active channel width was generally smaller, varying from 84 m to 52 m (median 69 m) and the channel migrated 48 m. Streamflow greater than about 250 m³/s are most effective for forcing migration and have been reduced since dam construction. The river has re-occupied areas with increasing frequency since dam construction. High flows are essential to create new channel and floodplain habitats in the middle Green River, but land cover/use and revetments in the river corridor are also influential factors for maintaining channel dynamics.