Coastal geomorphic response to seasonal water-level rise in the Laurentian Great Lakes: An example from Illinois Beach State Park, USA

Abstract

Hydrodynamic processes, such as fluctuating water levels, waves, and currents, shape coastlines 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, additional research is needed to explore short-term (weeks to months) beach geomorphic response to fluctuating water level. Developing a process-focused understanding of how water level fluctuations shape coastal response across these shorter time scales is imperative for 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 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. None of the sites in this study returned to their initial morphology following seasonal lake level rise. Given that peak water levels were nearly identical in 2017 and 2018, yet the majority of erosion at our sites occurred in 2017, we postulate that erosion associated with seasonal lake level rise is primarily a function of the change in annual maximum water level from year to year, rather than solely the elevation of the water level.