Hydrodynamically‐Driven Deposition of Mud in River Systems.

The riverine transport and deposition of mud is the primary agent of landscape construction and evolution in many fluvial and coastal environments. Previous efforts exploring this process have raised uncertainty regarding the effects of hydrodynamic and chemical controls on the transport and deposition of mud, and thus the constructions of muddy coastal and upstream environments. As such, direct measurements are necessary to constrain the deposition of mud by river systems. Here, we combine laboratory evidence and a field investigation in the Mississippi River delta to explore the controls on the riverine transport and deposition of mud. We show that the flocculation of mud, with floc diameters greater than 10 μm, in freshwater is a ubiquitous phenomenon, causing the sedimentation of mud to be driven by changes in local hydrodynamics, and thus providing an explanation for how river systems construct landscapes through the deposition of mud in both coastal and upstream environments. Plain Language Summary: Muddy landscapes are some of the most common and vital environments on Earth's surface, such as floodplains, deltas, and estuaries. Due to their small size, and thus easily suspendable nature, mud particles are thought to be extremely difficult to deposit under typical flow conditions in rivers. As such, the means by which rivers deposit mud has been the subject of much study and debate. Canonically, the deposition of cohesive sediment occurs as rivers approach the ocean, where increases in salinity induce the aggregation of clay and silt particles to form heavier floccules, or "flocs." Recent studies have also explored the influence of organic material on mud aggregation. However, the means by which rivers are able to construct muddy landscapes upstream, in the absence of saline or organic‐rich water, remains uncertain. To address this discrepancy, we utilize a combination of laboratory evidence and a field investigation on the Mississippi River delta to show that all mud is transported as flocs, not as individual grains, and as such, the deposition of mud by river systems is controlled by the ability of the river flow to suspend existing mud flocs, and not by abrupt changes in water chemistry inducing flocculation in coastal regions. Key Points: Flocculation of mud in water is a ubiquitous phenomenonSalinity may have little to no effect on mud floc size as rivers enter marine environmentsDeposition of flocculated mud in river systems is driven by changes in hydrodynamic stress [ABSTRACT FROM AUTHOR]