Lithological Controls on Riverbank Retreat: Four-Year Monitoring Using Erosion Pins and Terrestrial Laser Scanning

Abstract Riverbank retreat reflects interactions between bank-material properties and variable erosional forcing, but the extent to which lithological contrasts persist in multi-year retreat patterns remains unclear. We monitored six riverbanks in the Podhale region of the Western Carpathians, Poland, representing fine-grained alluvium (Type A), gravel-dominated sediments (Type B), and consolidated flysch bedrock or cemented Neogene gravels (Type C). Retreat was measured from 2019 to 2022 using erosion pins and terrestrial laser scanning (TLS). Mean annual retreat over the four-year period ranged from 39.2 to 81.4 cm for erosion pins and from 42.2 to 81.0 cm for TLS. Type B sites showed the lowest retreat, whereas higher values occurred at Type A sites and at the single Type C site, indicating that greater consolidation did not correspond consistently to lower retreat. Substantial interannual variability occurred at all sites. Despite larger differences in individual years, erosion pins and TLS identified broadly consistent multi-year spatial patterns. Lithological differences were therefore evident, but they did not account for retreat magnitude on their own. The four-year record also shows the value of combining point measurements with spatial surveys when interpreting variable riverbank erosion.