Kieran Dunne (TU Delft) 'Hydrodynamically-driven deposition of mud in river systems'

Abstract: The riverine transport and deposition of mud is the primary agent of landscape construction and evolution in many fluvial and coastal environments, such as floodplains, deltas, and estuaries. Canonically, mud is deposited by rivers as they enter marine environments due to the increase in salinity. This change in water chemistry is proposed to lead to the onset of flocculation, the subsequent deposition of fine, cohesive sediment, and the construction of muddy, coastal landscapes. 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 . In contrast, seasonality appears to have a strong effect on the size of transported flocs, with flocs observed to be larger in the summer than the winter, despite comparable hydrodynamic conditions. In combination, we show that the sedimentation of mud is primarily driven by changes in local hydrodynamics, with biological activity proposed as a primary source of spatial and temporal variability, to provide an explanation for how river systems construct landscapes through the deposition of mud in both coastal and upstream environments.