Large-Scale Channel Migration in the Sittang River Estuary.

An estuary is a dynamic environment where marine and fluvial processes meet to form complex and transient morphology. The estuary morphology is largely determined by net sediment transport by two-way tidal flows, but the hydrodynamics also depends on the morphology of the tidal channels. The estuary inherently accommodates cyclic processes that are internally generated through hydro-morphodynamic interactions. In addition, the estuary evolves in response to changes in external forces by natural and anthropogenic factors. Morphological changes under the different controls often hinder the comprehension of the evolutionary processes of estuaries. Here we explored morphological changes in the Sittang River estuary, Myanmar, which has great morphological dynamism from extreme tidal energy and large sediment inputs, through field surveys and satellite imagery analysis. We identify an autocyclic process in a sedimentary system driving large-scale channel migration in decadal to multidecadal cycles. We show that drastic changes of the estuary morphology occasionally occur with rapid bank erosion through modulation of the cyclic channel migration under conflicting tidal and fluvial forces. This extreme case with minimal human intervention highlights channel migration as a key process in morphological evolution of tide-dominated estuaries undergoing active infilling.