Evidence for Mud as Flocculated Bed-Material Load Versus Washload in a River Delta

Abstract

Mud dominates the particulate load of sediment and organic carbon from continents to oceans, but mud concentration and transport rate remain notoriously difficult to predict. In rivers, mud is thought to be transported as washload—particles so small that they are absent from the riverbed, washed through the river like passive tracers, and controlled by external inputs rather than local sediment entrainment from the bed. However, freshwater flocculation in rivers can aggregate mud grains into larger particles that behave hydrodynamically more like sand. If correct, this finding opens the door to describe mud transport as bed-material load—particles in dynamic interchange between the bed and water column—for which robust theory exists. Here we present evidence that mud behaves as flocculated bed-material load rather than washload in the freshwater Wax Lake Delta (WLD), a major distributary of the Mississippi River Delta. Grain size-specific concentration-depth profiles indicate that mud is flocculated in WLD. In situ turbidity sensors, airborne hyperspectral imaging (AVIRIS-NG), and concentration-depth profiles show that mud concentration varies temporally and spatially in response to shear stress variations, consistent with bed-material load dynamics. Furthermore, mud exists in the channel bed (median 14% mud by volume) and dominates the bed on deltaic islands (median 90%). Bed-material entrainment theory explains observed near-bed mud concentrations using a formulation that accounts for floc growth and densification near the bed. Together, these findings support a unified treatment of sand and flocculated mud as bed-material load in lowland rivers and deltas.