A river delta’s shape and kinematics are dictated by the three-dimensional evolution of its distributary channels on the delta front, yet measurements of this evolution are scarce. We supply four bathymetric surveys documenting this evolution for Gadwall Pass—part of the Wax Lake Delta, one of the few rapidly prograding regions of the greater Mississippi Delta in coastal Louisiana, United States. This distributary channel extends 2–6 km beyond the sub-aerially emergent delta (dependent on water surface elevation) and bifurcates into four similarly sized distributary channels (average channel width = ∼150 m) in this sub-aqueous reach. Distributary channel growth proceeds primarily through erosion of the unchannelized foreset deposit, and growth patterns differ between high and low river flow. During high river flow, high upstream sand supply acts to aggrade the bed both inside and outside of the channel network. Erosion during high flow is focused at the sand shoals that define the sidewalls of the bifurcate channels, causing channel network rearrangement into a single primary channel with the remaining secondary channels branching off of it. During low river flow, bed erosion is focused at channel tips and the beds of all of the subaqueous distributary channels, leading to a bayward extension of each channel tip by ≥0.87 channel-widths. Channel-bottom erosion during low river flow is enhanced by tidally modulated currents that support sand suspension and transport in the subaqueous channels during ebb tide while receiving only a small sand supply from upstream.