Flocculated Silt, Not Clay, Dominates River Suspended Sediment Load

River mud builds fertile floodplains, combats land loss due to sea level rise, and buries vast amounts of organic carbon. These processes rely on mud settling velocity, which is set by the grain size, mineralogy, and flocculation state of mud. Here we determined these factors using a suspended sediment data compilation from lowland rivers and new measurements from the Wax Lake Delta. Results show that rivers transport mostly silt‐sized grains (∼77% by mass). Although flocculation is classically associated with clay minerals, surprisingly most silt is flocculated (∼88%). Clay minerals dominate the clay size class (<2 μm) as expected, but this size is only a small percentage of the load (2.4%). Clay minerals are sparse among flocculated sediment (∼17%), implying that physical trapping of grains and binding by organic matter instead drive mud flocculation. Flocculated silt dominates river sediment load (∼62%), indicating a new paradigm for river sediment transport.