Bank erosion history of a mountain stream determined by means of anatomical changes in exposed tree roots over the last 100 years (Bílá Opava River — Czech Republic)

The date of exposure of spruce roots as a result of bank erosion was investigated on the Bílá Opava River in the northeastern Czech Republic. Following the exposure of roots, wood cells in the tree rings divide into early wood and late wood. Root cells within the tree rings also become smaller and more numerous. These processes permit dating of the erosion episodes in which roots were exposed. Sixty root samples were taken from seven sampling sites selected on two riverbed reaches. The results of root exposure dating were compared to historical data on hydrological flooding. Using the root exposure dating method, several erosion episodes were recorded for the last 100 years. The greatest bank erosion was recorded as consequence of an extraordinary flood in July 1997. In the upper, rocky part of the valley studied, bank erosion often took place during large floods that occurred in the early 20th century. In the lower, alluvial part of the valley, erosion in the exposed roots was recorded only in 1973 and has been intensive ever since. It is suggested that banks in the lower part are more frequently undercut, which leads to the falling of trees within whose roots older erosion episodes were recorded. Locally, bank erosion is often intensified by the position of 1- to 2-m boulders in the riverbed, which direct water into the parts of the banks where erosion occurs. Selective bank erosion could be intensified by debris dams and hillslope material supply to the riverbed.