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.