HOW LARGE TREES INFLUENCE THE MAGNITUDE, FREQUENCY, AND SPATIAL STRUCTURE OF COHESIVE BANK RETREAT

Although there are many models that incorporate the influence of riparian trees on bank erosion, few acknowledge that large trees impose characteristic spatial and temporal templates to erosional processes and rates. We use data from the Brandywine Creek in Pennsylvania and the South River in Virginia to document spatial and temporal patterns describing the retreat of gently curving bends with forested riparian zones containing a mix of very large and small trees, cohesive banks, and slow rates of migration (~ 0.1 m/yr). Field data include 2.5 years of bank profile surveys, measurements of tree morphology, and LiDAR surveys repeated biannually. These banks retreat through a cycle of erosion on decadal timescales. Initially, small volumes (mean = 0.173 m³ per event) of soil are removed between large trees growing on the bank. Near trees, the rate of bank retreat is initially negligible, creating a scalloped bank morphology buttressed by large trees. As the trees are increasingly exposed to the flow, they become slowly undercut, gradually leaning into the channel and sliding down to the toe of the bank, eventually toppling into the river after a few years, which restarts the cycle. A magnitude-frequency analysis of monitoring data from the Brandwine Creek indicates that the effective bank erosion event removes only 0.11 m³; thus forested bank retreat is dominated by small soil failures. Large erosion events, many associated with trees toppling into the river, greatly increase the variance of time-averaged bank erosion rates, such that almost 4 years of monitoring are needed to measure time-averaged erosion rates to within an accuracy of 10%. Because of the limited spatial extent of our survey data and the absence of large discharges, these results are provisional. Improved process-based models to predict the retreat of forested riverbanks should account for the spatial structure of riparian forests and the interactions between the flow, the gradual removal of bank soils, and the tree’s ongoing efforts to remain attached to the bank and alive.