Abstract. Some mixed bedrock-alluvial dryland rivers are known to undergo
cycles of alluvial building during low flow periods, punctuated by stripping
events during rare high magnitude flows. We focus on the Olifants River,
Kruger National Park, South Africa, and present 2-D morphodynamic simulations
of hydraulics and sediment deposition patterns over an exposed bedrock
anastomosed pavement. We examine the assumptions underlying a previous
conceptual model, namely that sedimentation occurs preferentially on bedrock
highs. Our modelling results and local field observations in fact show that
sediment thicknesses are greater over bedrock lows, suggesting these are the
key loci for deposition, barform initiation and island building. During peak
flows, velocities in the topographic lows tend to be lower than in
intermediate topographic areas. It is likely that intermediate topographic
areas supply sediment to the topographic lows at this flow stage, which is
then deposited in the lows on the falling limb of the hydrograph as
velocities reduce. Subsequent vegetation establishment on deposits in the
topographic lows is likely to play a key role in additional sedimentation
and vegetation succession, both through increasing the cohesive strength of
alluvial units and by capturing new sediments and propagules.
Limited seed retention during winter inhibits vegetation establishment in spring, affecting lateral marsh expansion capacity
