Characterizing changes in channel morphology associated with base level fall: Application on Le Sueur and Maple Rivers

Many fluvial systems have undergone significant changes in their morphology due to base level fall since the last glaciation. Channels in such transient fluvial systems continuously adjust their morphology and may still be incising until reaching a new equilibrium condition. This study examines morphometric change detection indices and techniques to quantify and analyze the recent variations in the channel properties of the lower portions of the Le Sueur and Maple Rivers, in response to upstream knickpoint migration. The results reveal that channel reaches upstream of the knickzones remain relatively stable, exhibiting low channel bed gradients and minimal morphological change. In contrast, the local channel gradient increases about ∼3 fold near the knickpoints and continues with predominantly high rates into the downstream direction. Together, the morphometric indices indicate that the channel geometry of the Maple and Le Sueur Rivers downstream of the knickpoints is actively adjusting to accommodate the rapid base level drop. Because of incomplete adjustment processes, both rivers experienced remarkable change in channel geometry between 2008 and 2015, albeit with differing adjustment patterns. In particular, the percentage change in channel properties along the knickzone reaches up to 120% for the channel cross-sectional area, 60% for the channel width, and 75% for the mean hydraulic depth. These changes are accompanied by a sharp increase in cross-sectional stream power and impose boundary shear stress along the knickzones which amplify sediment transport capacity and drive further modifications to channel size and shape. This study provides a quantitative framework for assessing geomorphic responses to base level drop and provides insights into the feedback mechanisms between hydraulic forces and channel morphology. The findings have broader implications for understanding channel evolution under non-equilibrium conditions and for guiding river management in similar fluvial systems.