Coastal Geomorphic Response to Seasonal Water-Level Rise in the Laurentian Great Lakes: An example from Illinois Beach State Park, USA

Ethan Theuerkauf, Katherine Braun, Danielle Nelson, Morgan Kaplan, Vivirito Salvatore, and Jack Williams

Hydrodynamic processes, such as fluctuating water levels, waves, and currents, are the primary natural forces driving coastal evolution across timescales ranging from minutes to millennia. In large lacustrine systems, such as the Laurentian Great Lakes, the role of water level in driving long-term (centuries to millennia) coastal evolution is well understood. However, more research is needed to explore short-term (weeks to months) beach geomorphic evolution in response to fluctuating water level. Developing a mechanistic understanding of how water level fluctuations shape coastal response across these shorter time scales is imperative for developing predictive models that inform coastal management. Here, we present measurements of geomorphic response along a lacustrine beach ridge plain to seasonal water level fluctuations during a decadal high-stand in Lake Michigan water level. Frequent beach and dune topographic change measurements revealed high spatial and temporal variability in geomorphic response to rising lake level. Sites immediately downdrift of shore protection began to erode immediately as lake level increased. The co-occurrence of peak seasonal lake levels and a modest increase in wave energy resulted in erosion and overwash at sites that resisted erosion during the initial seasonal rise in lake level. This study fills an important gap in our scientific understanding of large lacustrine systems by mechanistically linking seasonal hydrodynamic processes to geomorphic response. The results from this study can be used to parameterize coastal evolution models and will aid coastal managers in anticipating the impacts of high lake levels on beach and dune environments.