Fine-scale quantification of stream bank geomorphic volume loss caused by cattle access

Unrestricted cattle access to streams and rivers can be a significant source of pollution in fluvial systems, contributing to bank erosion and fine sediment inputs. Despite this pressure, observational data are scarce. This study quantified stream bank geomorphic modifications caused by cattle access at fine scale using motion-capture cameras and Terrestrial Laser Scanning (TLS) campaigns. Continuous monitoring of rainfall, discharge, conductivity and turbidity further augmented this dataset. The application of these techniques extended over a five-month grazing period in agricultural sub-catchments with intensive cattle production. At low flow, high-resolution water quality data showed that the frequency of cattle activity in and around stream margins was associated with elevated turbidity signals downstream. However, when elevated turbidity coincided with high flow events, it was not possible to distinguish between local erosion and upstream sediment transfers. TLS results indicated a loss of 0.141 m³ to 1.035 m³ stream bank material, which equates to 0.067 m³ m^-2 - 0.092 m³ m^-2 of stream bank area (between 27% and 41% in the <2 mm fraction) over the study period from sites with 130 to 1154 discrete cattle access hits. Multiple linear regression showed that the observed geomorphic volume loss could not be explained by natural processes alone (hydrometeorology), but was more significantly related to cattle-access frequency as the principal driver. The geomorphic volume loss had the potential to impact 29 m² to 197 m² of stream bed with fine sediment (<2 mm) from the three study sites. Grazing parcels adjacent to streams in the study sub-catchments were enumerated at 18.4 parcels km^-2 and so the results of this investigation potentially scale to a considerable fine sediment risk. Regulations and time-limited incentives to exclude cattle access to stream channels should therefore expect to reduce sediment pressures where these measures are targeted at access points.