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.

先前的研究已经确定，在自然条件下，冲积河流汇合区会经历河道冲刷，然后经历河道中段的发育。在工程条件下，如流量调节和大堤限制，对大型冲积河汇流下游的河床演化给予的关注较少。在这里，我们展示了沿69公里的最上游阿察法拉亚河（密西西比河流出河道与红河汇合处的下游分支）的四十年形态变化。我们旨在找到一个关键问题的答案：工程控制汇合处下游的河床如何对调节流量做出反应？我们利用1967年，1977年，1989年，1998年和2006年收集的单波束测深数据来量化流量调节于1963年开始后河段的河床变形。计算了四个十年期间的悬浮泥沙负荷和水流功率。结果表明，在1967年至2006年之间，发生了广泛的床层退化，平均床高降低了3.8 m。总体积为105±26×10⁶ 米³在这40年期间，沉积物从最上层的阿恰法拉亚河河床冲刷出来，这表明该河段的河道侵蚀促成了下游粗沙沉积物的输送，并伴随着阿查法拉亚湾的三角洲增长。由于密西西比河的大量泥沙输入，河床在1989年至1998年期间才发生淤积，这可能归因于1993年持久的密西西比河洪水。但是在随后的八年中，同样数量的河床沉积物被侵蚀了，这表明，由于过多的泥沙输入，一条大型的调节河能够以多快的速度减少扰动。