Abstract
Rising sea levels, subsidence, and decreased fluvial sediment load
threaten river deltas and their marshes. However, the feedbacks between
fluvial and marsh deposition remain weakly constrained. We investigate
how marsh accumulation impacts the fluvial sediment partitioning between
a delta’s topset, coastal zone, and foreset by comparing a delta
experiment with proxy marsh accumulation to a control. Marsh
accumulation alters fluvial sediment distribution by decreasing the
slope in the subaerial marsh window by ~40%, creating
an ~8% larger delta top and a ~100%
larger marsh platform. The reduced slopes decrease relative delta
elevation, and fluvial incursions into the marsh trap 1.3 times more
clastic volume. The volume exported to deep water remains unchanged.
Marsh deposition shifts elevation distributions towards sea level, which
produces a hypsometry akin to field-scale deltas. Given that risk is
tied to elevation, marsh accumulation accentuates low-elevation areas,
while providing essential land-building capabilities.