Water-Level Changes and Beach Morphodynamics: Insights from Topobathymetric Monitoring of a Receding Shoreline, Illinois Beach, Lake Michigan

Mattheus, C.R.; Pearce, K.; Rosario, L., and Spitzer, E., 2025. Water-level changes and beach morphodynamics: Insights from topobathymetric monitoring of a receding shoreline, Illinois Beach, Lake Michigan. Journal of Coastal Research, 41(5), 749–769. Charlotte (North Carolina), ISSN 0749-0208. Great Lakes beach-shoreline morphodynamic responses to meter-scale lake-level fluctuations remain understudied, yet they are important to understand from a coastal resiliency perspective. This paper addresses beach morphodynamics and sand volumetrics along an unarmored stretch of sandy beach shoreline at Illinois Beach State Park, SW Lake Michigan, on the updrift end of a migrating sand-ridge promontory. Captured in topobathymetric monitoring datasets are morphodynamic patterns associated with a rise in base lake level greater than 1.5 m, from 2013 through 2020, and an approximately 1-m fall in base lake level, from 2020 through 2023. This assessment reports on the preconstruction system morphodynamics over decadal lake-level fluctuation, with ongoing geological monitoring efforts offering an opportunity to evaluate the impacts of offshore emergent breakwater construction (in 2023). A loss of approximately 54,000 m2 of ridge-and-swale terrain occurred along the historically net-erosional beach with the 2013–20 lake-level rise, manifested as an average elevation loss of 5 m between 2012 and 2020 shoreline positions. Overwash volumes accounted for less than 2000 m3 (<1.5%) of total sand loss from shoreline recession. Eroded sand was mostly exported from the proximal littoral zone, a dynamic that halted with the onset of lake-level fall (2020–23). Postpeak topobathymetric model comparisons suggest a net-volumetric balance across the area of 2012–20 shoreline retreat. Mapped 1939-present shoreline positions suggest that high-magnitude or high-rate lake-level rise events accelerate rates of terrain loss at this overall recessive site. During the latest rise in lake level, around 20,000 m2/y of terrain were lost, whereas the 1939–2023 average rate was less than 2000 m2/y, an order of magnitude lower. Although shoreline positions have advanced to a lakeward position with the post-2020 lake-level fall, this was mostly due to re-exposure with little substantial influx of sand. These are important considerations for coastal managers.