Assessing channel geometry in response to land use disturbance in a low‐relief, glacially conditioned setting

Abstract

Bankfull stage, the highest flow elevation contained by a river channel before overbank flooding occurs, is the presumed threshold for channel morphological change. At bankfull, the channel boundary experiences the highest shear stress, producing somewhat predictable hydraulic relationships and a cross‐sectional form. However, land use and glacial conditioning can profoundly impact a channel's geomorphic responses. Two common methods characterize bankfull flow: one based on flow frequency and other based on channel form. This study considers a simplified approach to identify upstream land use relative to estimates of bankfull flow versus a channel's geometric form. The approach compares archived geomorphic surveys of 140 river reaches in southern Ontario, Canada, to 2‐year flood quantiles modelled from historical flood data of 207 gauge stations. Flood frequency analysis determines that annual maximum series (AMS) datasets, fitted to optimized probabilistic distributions, underestimate discharge for low‐magnitude, high‐frequency flood events compared to partial duration series (PDS) datasets. For smaller drainage areas (<100 km2) associated with an extensive agricultural activity and/or urbanization, the estimates of bankfull discharge (Qbf) generated by cross‐sectional channel geometry are greater than the gauge‐derived Q2 values. Channels impacted by high levels of upstream land use disturbance show statistically significant lower width‐to‐depth ratios (p < 0.001) and a trend towards a finer D50 bed material, suggesting enhanced surface runoff delivering more mobile finer sediments and channel confinement or incision. This research quantifies the formative bankfull stage to better understand the link between land use and a channel's ‘natural’ hydrogeomorphic response in a low‐relief, glacially conditioned setting.