On the Hydrodynamically-Driven Deposition of Mud in River Systems

The transport and deposition of mud by rivers is the primary agent of landscape construction and evolution in many fluvial and coastal environments. However, despite the prevalence and importance of these cohesive sediment-comprised landscapes, such as floodplains and deltas, the controls on the sedimentation of cohesive sediment transported by rivers remains relatively unconstrained. Few field studies have directly explored the controls on the flocculation of mud in freshwater environments or have examined how the transport and properties of mud flocs respond to changes in water chemistry, turbulent mixing, and sediment supply which are thought to be the primary controllers of floc size, and thus floc settling velocity. Furthermore, laboratory experiments exploring the controls on floc morphology have demonstrated that flocculation of cohesive sediment in water is a ubiquitous phenomenon which will occur under nearly all natural physical and chemical conditions, contrary to the canonical understanding that changes in water chemistry as rivers approach the ocean induce flocculation and sedimentation of mud. To address this discrepancy, a field investigation was conducted in the Mississippi River Delta in the South Pass and Southwest Pass channels to investigate the controls on the size and sedimentation of mud as the Mississippi river transitions from a fluvial to a marine environment. In-situ measurements of floc size, shear velocity, and bed sediment composition were collected along a longitudinal profile of the river from a purely fluvial, freshwater setting to a purely marine, saline environment. Results show that mud transported in suspension is flocculated under both freshwater and saline conditions, and, in agreement with laboratory results, is relatively unaffected by changes in water chemistry. As a result of this baseline flocculated state, a portion of the floc size distribution has a high enough settling velocity to allow for mud deposition on the channel bed as well as during overbank flows, even in the absence of enhanced roughness elements, such as vegetation. Thus, the deposition of mud by rivers is primarily driven by hydrodynamics, not changes in water chemistry, allowing for the construction of the muddy channels and floodplains found in both fluvial and coastal environments.