SEDIMENT TRANSPORT AND CHANNEL DYNAMICS OF THE LOWERMOST MISSISSIPPI RIVER IN RECENT DECADES

Abstract

Sediment transport and channel dynamics of large alluvial rivers in their lower reaches can strongly influence physical, chemical and biological conditions in their deltaic regions. In this paper, we report the latest assessment on sediment transport and riverbed dynamics of the last 500-km reach of the Mississippi River before entering the Gulf of Mexico. This river reach is also termed as the Lowermost Mississippi River (LmMR), which begins downstream of its division node to the Atchafalaya River. Our assessment shows that in the past 2-3 decades, suspended sediment and bed material loads in the LmMR were significantly higher at the upstream location than those at the downstream locations. Nearly 70% of the riverine sand is trapped within the LmMR. Continuous riverbed aggradation (up to 10 m) occurred below the Mississippi-Atchafalaya diversion, presenting favorable backwater conditions for an avulsion. Backwater effects have mainly controlled riverbed deformation in the LmMR, while flow reduction may have also contributed to channel aggradation in the lowermost reach. Our assessment reveals the considerable complexity of morphodynamical responses of a large alluvial river to human interventions, strongly suggesting that future river engineering and management of the world large alluvial rivers need to look for effective strategies that will improve sediment transport.