CONTINUING HIGH LAKE LEVELS ARE LEADING TO SEVERE EROSION OF WESTERN LAKE SUPERIOR’S CLAY BLUFFS IN WISCONSIN

Wave erosion at the base of coastal bluffs is a major driver of slope instability on Great Lakes shorelines. This has increased since 2013 because of the rise of lake levels, which are now very close to their all-time highs. Bluffs in most of Douglas County, the westernmost WI county on Lake Superior, are especially vulnerable because they are composed of clayey Douglas Till and locally some sand and gravel. In 1976 and in Spring of 2008, 2012, 2017, 2018, and 2019 low-level oblique air photos were taken of the shoreline in order to characterize conditions on the beach and, in places where present, the bluff above the beach. A qualitative evaluation of conditions on the bluff and on the beach was made and mapped using Esri’s ArcGIS software for the 1976 and 2008 shoreline, and now the 2019 photos. In 2008 broad beaches were common. In 2019 most of the beach was water covered and waves were breaking against the bluff base.

A total of 680 geolocated photos taken of the Douglas County shoreline in late April 2019 were examined and compared to those from 2008. In Douglas County there are 18.78 air miles of bluff. In 2008, 1.87 miles of bluff were mapped as having mostly bare slope. That number increased to 4.64 in 2019. In 2008, 6.77 miles were mapped as unstable/failing. This number was 15.38 in April 2019. In places where there is a high bluff behind the beach there has been a response to the higher lake level, but that response has not yet been felt at the top of the bluff in some places. Upper slopes of the highest bluffs appear to be vegetated with fewer bare, vegetation-free slope segments than in 2008. The lower parts of the bluffs in 2019 had become steeper and more unstable than they were in 2008. Many of the lower parts of high bluffs are now partly vegetation free and standing nearly vertical. This steep lower bluff slope segment will migrate upward to the top of the bluff in the coming years. Thus, although most structures near the top of high bluffs are not threatened at this time, they will be in the future, especially if the lake level remains high. When mapping bluff condition, bluffs with failing lower slopes were considered unstable/failing. Results for all of the Wisconsin’s Lake Superior shoreline will be posted at http://floodatlas.org/asfpm/oblique_viewer/.