Comparing Paleohydrograph Reconstructions from Subsurface Stratigraphy and Topography at the Sault Ste. Marie Strandplain

Abstract

The Great Lakes are currently at high water levels, which are negatively impacting coastal infrastructure, coastal ecosystems, and stakeholders that rely on the lakes. To better understand natural fluctuations, which includes high lake levels, geoscientists study ancient shorelines to reconstruct paleohydrographs. Reconstructing past lake level elevations from a specific subsurface sedimentary contact or foreshore base (FSB) contact is the most accurate way to gain insight into ancient lake levels. The objective of this thesis is to establish an alternative method to use topographic elevations as a proxy for the FSB in the reconstruction of inferred paleohydrographs from the Sault Ste. Marie (SSM) strandplain. Light detection and ranging (LiDAR) data was used to obtain topographic elevations for this topographic reconstruction. Topographic elevations measured in the field were compared to LiDAR data and these topographic elevations were also compared to FSB elevations measured in cores. Elevation trends and patterns were statistically analyzed and visually analyzed in graph to justify that topographic elevations from LiDAR could be used as a proxy for the FSB or past lake level elevation, but so far this only applies to the SSM strandplain deposited during the Nipissing phase. The field measured topographic swale elevations could be used as an alternative to FSB elevations when a correction factor of 1.49 m was subtracted from each individual swale elevation. LiDAR data was then used to obtain one swale elevation for every beach ridge in the SSM strandplain and then a correction factor of 1.49 m was applied to the LiDAR swale elevations. Results from this thesis found that an inferred paleohydrograph reconstructed from LiDAR swale elevations was an appropriate alternative to infer ancient lake level elevations and trends. However, this has only been shown to apply for the SSM strandplain deposited during the Nipissing phase. Further comparisons at different sites and for different ages of strandplains need to be investigated. In summary, this thesis determined that LiDAR swale elevations can potentially provide an alternative method to reconstruct paleohydrographs, and thus gain valuable insight into natural lake-level trends and patterns to help place current high levels and potential future lake-level fluctuations into context for stakeholders.