Quantifying bank retreat rates with exposed tree roots

In this study we use a biometric approach based on anatomical changes in the wood of exposed tree roots to quantify riverbank erosion along South River, Va, a site where commonly applied techniques for determining bank erosion rates are either not appropriate due to the required spatial scale of analysis (i.e., erosion pins, traditional surveys, LiDAR analysis) or have failed to detect obvious erosion (i.e. photogrammetric techniques). We sampled 78 exposed roots from 24 study reaches and processed them both macroscopically (2 to 20 times magnification) and microscopically (20 to 100 times magnification), comparing the estimated erosion rates between levels of magnification and to those obtained with photogrammetric techniques. We found no statistical differences between the output of macroscopic and microscopic analyses (t-test, alpha =0.01) but encountered difficulty in identifying the year of root exhumation in some samples. Therefore, we suggest analyzing roots at both levels of magnification to increase confidence and obtain erosion rate estimates from every sample. When comparing exposed root analysis to photogrammetric techniques, the results indicate that the exposed root approach is a feasible and effective method for estimating decadal to centennial scale bank erosion. In addition to producing erosion rates statistically indistinguishable from photogrammetric techniques (t-test, alpha = 0.01), exposed root analysis demonstrated more consistent detection of erosion. The results of this study indicate that exposed tree root analysis is a robust tool that provides insights into decadal scale erosion where other commonly applied techniques may not be appropriate or easily applied.