River deltas get muddier as they grow

Larger deltas tend to be muddier than smaller deltas. This observation has been attributed to allogenic controls, such as increased fine sediment supply from larger catchments. Here we explore a complementary hypothesis: larger deltas are better able to retain mud on delta plains, and thus deltas become progressively muddier as they grow. We test this hypothesis by analyzing sand and mud retention dynamics during delta growth using Delft3D numerical experiments combined with field data. Our modeling results indicate that while sand retention initially increases and quickly saturates, mud retention continues to increase over a much longer period of delta evolution. This divergence occurs for two related reasons: (1) the area occupied by delta plain between channels grows faster than the area occupied by channels; (2) the different advection lengths for sand and mud affect their relative transport through the changing delta geometry as it grows. Comparative analysis reveals an increase in total sediment retention with delta growth in both models and field observations, a trend driven in our simulations by enhanced mud retention. This enhanced mud retention manifests as deposit fining; while upstream supply remains the primary control on deposit grain size, delta growth acts as a systematic secondary autogenic mechanism for delta muddification. We conclude that the progressive muddification of deltas is not solely dictated by external sediment supply but is also driven by internal morphodynamic feedbacks that effectively trap mud as deltas expand.