CHARACTERISTIC RIPARIAN TREE AND INTERRELATED BANK GEOMETRIES DIFFERENTIATE ERODING RIVER BANKS FROM STABLE OR ACCRETING RIVER BANKS

We tested the hypothesis that riparian trees and the banks on which they grow will develop characteristic geometries which reflect bank stability (eroding, non-eroding) that can be used to classify a bank as eroding or non-eroding. These geometries include the trunk deflection angle (the deviation of a tree’s trunk from vertical), diameter at breast height (dbh), and percent undercutting (the length of exposed space underneath a tree, divided by the dbh, multiplied by 100). All data was collected from trees growing on banks of the South River, Virginia, a steep, gravel-bed, bedrock river with a history of industrial mercury contamination. Study reaches were classified as eroding, accreting, or stable by measuring changes in bank location over time using aerial imagery, calculating bank stability indices, studying the history of dams downstream (only at sites upstream of dams), examining cores (only to detect accretion), and using data from previous terrestrial LIDAR studies (only to detect erosion). We measured trees on four banks: an eroding bank, an accreting bank, a stable bank, and a bank where the history of change could not be resolved from historical aerial photographs (but where continued dam degradation downstream between 2002 and 2005 suggests a recent onset of erosion). The median percent undercutting of the erosional bank (95%) is significantly larger (p=0.05) than the median percent undercutting of the stable (0%) or accreting (0%) banks. The trunk deflection angles are also significantly different: on the eroding bank, trees had a lower median deflection angle (0°) than on the stable (21°) and accreting (19.5°) banks. The median values of percent undercutting (77%) and deflection angle (7°) for the site above the recently breached dam are statistically indistinguishable from the median values of these parameters at the eroding bank. This bank, therefore, has likely started to erode in recent years. Our results define characteristic geometries developed by trees on eroding banks and the banks themselves that can be used to document bank erosion with less financial and time commitments and in some cases finer resolution than more traditional methods.