Comparing the Cohesive Effects of Mud and Vegetation on Delta Evolution

Cohesive sediment exerts a significant influence on delta evolution, increasing shoreline rugosity and decreasing channel mobility. Vegetation has been assumed to play a similar role in delta evolution, but its cohesive effects have not been explicitly studied. We use the model DeltaRCM to directly explore two effects of vegetation: decreasing lateral transport of sediment and increasing flow resistance. We find that vegetation and cohesive sediment do alter delta morphology and channel dynamics in similar ways (e.g., more rugose shorelines and deeper, narrower, less mobile channels) but that vegetation may have additional implications for deltaic sediment retention and stratigraphy, by confining flow and sand in channels. Our results suggest that sediment composition is a first-order control on delta morphology, but vegetation has a stronger influence on channel mobility time scales. To fully understand the cohesive influences acting on a delta, vegetation influence should be considered in addition to fine sediment. Plain Language Summary Human use of deltaic land relies on the existence of stable wetland habitat. Both vegetation and mud influence delta stability by making sediment more difficult to erode. Both introduce cohesion-mud because individual grains stick together, and vegetation because roots hold the sediment together. Vegetation also introduces friction, which has a similar effect to cohesion (decreasing erosion). When it is more difficult to erode channel banks, channels remain in the same location for longer, influencing where water and sediment are transported on the delta. Most studies consider only mud, but we compare the effects of these two sources of cohesion using a simple model that routes water and sediment on a delta. We find that mud and vegetation can affect deltas in similar ways. However, vegetation also changes the patterns in which sand and mud are deposited on deltas and can cause more sediment to be transported to the ocean instead of deposited on the delta. Additionally, we find that cohesive sediment is more important in determining delta shape but that vegetation is more important in controlling how much channels move. To fully understand how cohesion influences deltas, we should consider the effects of both mud and vegetation.