Spatial and temporal patterns in three-dimensional flow structure are linked to channel processes and morphology in many environments. However, there is not yet an understanding of how the flow structure is influenced by channelized and gradually distributed lateral outflows that are often prevalent in river deltas. This study presents an analysis of three-dimensional flow structure data collected from Wax Lake Delta, a naturally developing river-dominated delta in the northern Gulf of Mexico. Three hydrographic surveys were conducted using a boat-mounted acoustic Doppler current profiler at two sites: a channelized outflow zone and a distributary channel experiencing unchannelized lateral outflow. The flow structure was analyzed to identify secondary circulation cells induced by both types of lateral outflow. For channelized outflow, coherent cells were observed. However, minimal presence of coherent structures was observed for unchannelized lateral outflow. The results suggest that the formation of detectable secondary circulation cells may depend upon a threshold value of the ratio of the lateral momentum flux along the length of the outflow zone and primary flow momentum flux. The threshold lies in between 0.211 and 0.375 km−1 for the conditions tested. This research contributes novel field measurements of flow structure in an actively prograding river delta and offers important implications for coastal restoration by linking three-dimensional flow structure to lateral outflow.@en