Vegetation influence on delta evolution and dynamics under varying
water- and sediment-discharge conditions
Abstract
The dynamics and evolution of deltas and their channel networks involve
interactions between many factors, including water and sediment
discharge and cohesion from fine sediment and vegetation. These
interactions are likely to affect how much vegetation influences deltas,
because increasing sediment discharge increases aggradation rates on the
delta and may result in sediment transport processes happening on
timescales that are faster than those for vegetation growth. We explore
how varying water and sediment discharge changes vegetation’s effect on
delta evolution. We propose two new insights into delta evolution under
different discharge conditions. First, without vegetation, we observe a
regime shift in avulsion dynamics with increasing water discharge, from
a few active channels supplemented by overbank flow and undergoing
episodic avulsion (with low discharge) to many active channels
experiencing frequent partial avulsions (with high discharge). Second,
with vegetation, increased aggradation results in more frequent
switching of the dominant channels with increased sediment discharge,
but also prevents vegetation from establishing in non-dominant channels
resulting in more frequent channel reoccupation and therefore greater
stability in channel network planform. These insights have important
implications for understanding the distribution of water, sediment, and
nutrients on deltas in the face of future changes in climate, human
modifications of fluxes of sediment and water to the coast, and
especially for restored or engineered deltas with controlled water or
sediment discharges.