Effect of Sea‐Level Change on River Avulsions and Stratigraphy for an Experimental Lowland Delta

Abstract

Lowland deltas experience natural diversions in river course known as avulsions. River avulsions pose catastrophic flood hazards and redistribute sediment that is vital for sustaining land in the face of sea-level rise. Avulsions also affect deltaic stratigraphic architecture and the preservation of sea-level cycles in the sedimentary record. Here, we present results from an experimental lowland delta with persistent backwater effects and systematic changes in the rates of sea-level rise and fall. River avulsions repeatedly occurred where and when the river aggraded to a height of nearly half the channel depth, giving rise to a preferential avulsion node within the backwater zone regardless of sea-level change. As sea-level rise accelerated, the river responded by avulsing more frequently until reaching a maximum frequency limited by the upstream sediment supply. Experimental results support recent models, field observations, and experiments, and suggest anthropogenic sea-level rise will introduce more frequent avulsion hazards farther inland than observed in recent history. The experiment also demonstrated that avulsions can occur during sea-level fall—even within the confines of an incised valley—provided the offshore basin is shallow enough to allow the shoreline to prograde and the river to aggrade. Avulsions create erosional surfaces within stratigraphy that bound beds reflecting the amount of deposition between avulsions. Avulsion-induced scours overprint erosional surfaces from sea-level fall, except when the cumulative drop in sea-level is greater than the channel depth and less than the basin depth. Results imply sea-level signals outside this range are removed or distorted in delta deposits.