Four-decades of bed elevation changes in the heavily regulated upper Atchafalaya River, Louisiana, USA

Previous studies have established that under natural conditions, alluvial river confluence zones experience channel scour followed by mid-channel bar development. Less attention is given to bed evolution downstream of large alluvial river confluences under engineered conditions, such as discharge regulation and levee confinement. Here we present four decades of morphologic changes along the 69-km uppermost Atchafalaya River, a downstream distributary of the confluence of the Mississippi River Outflow channel and the Red River. We aim to find the answer to one critical question: how does the channel bed downstream of an engineering-controlled confluence respond to regulated flow? We utilize single-beam bathymetry data collected in 1967, 1977, 1989, 1998, and 2006 to quantify riverbed deformation of the reach after the flow regulation began in 1963. Suspended sediment load and stream power during the four decadal periods are calculated. Results show that between 1967 and 2006, extensive bed degradation occurred and the average bed elevation reduced by 3.8 m. A total volume of 105 ± 26 × 106 m3 sediment was scoured from the uppermost Atchafalaya riverbed over this 40-year period, implying that channel erosion in this river reach has contributed to downstream coarse sediment delivery and associated deltaic growth in the Atchafalaya Bay. Bed aggradation only occurred during 1989–1998 in response to excess sediment input from the Mississippi River, likely attributable to the 1993 long-lasting Mississippi River flood. But the same amount of riverbed deposit eroded in the following eight years, demonstrating how quickly a large regulated river can diminish a perturbation owing to excess sediment input. These findings not only reveal the complexity of morphologic adjustments of a river channel in response to intensive engineering disturbances but also provide useful information for future modeling studies and management plans for the Atchafalaya and other large, engineered alluvial river confluences. •Riverbed change downstream of a highly engineered river confluence was assessed.•Decadal changes in riverbed volume and mass across the channel were quantified.•Scouring was extensive within 40 km downstream of the engineered conference.•No mid-channel bar was deposited in the confluence zone of the large alluvial river.•The confluence engineering did not prevent downstream channel bed enlargement.