Effect and Implications of Differential Isostatic Rebound on Lake Superior's Regulation Limits

Abstract

In 1902, U.S. federal authorities established water level limits for Lake Superior as part of the requirements permitting hydropower water diversions from the lake's outlet, the St. Marys River. The vertical datum to which these limits are referenced must be adjusted every 25 to 30 years due to differential isostatic rebound (i.e., crustal movement). Because crustal movement unequally changes the land-to-water relationship along Lake Superior's shoreline, the method by which the limits are established on a new datum could accelerate the natural effects of crustal movement with adverse effects on shoreline flood levels, navigation depths, and lake outflows. With the implementation of the new International Great Lakes Datum of 1985, a study was undertaken 1) to review the methods used to establish the limits on previous datums and 2) to quantify the changed land-to-water relationship between the limits established in 1902 and those referenced to the new datum. Rates of differential crustal uplift, determined from linearly regressing water level differences between gauge pairs with time, were used. The study shows that although the numerical values of the upper and flow reduction threshold limits have changed due to subsequent datum adjustments and different reference gauge locations, the relationship of the limits with respect to the lake's outlet and the lake-wide average has not changed from that established in 1902. The present method of regulating Lake Superior based on a lake-wide average water level is shown to maintain the natural changes in the land-to-water relationship around the lake due to crustal movement. Due to these natural changes, the upper regulation limit is now 0.21 m higher at Duluth, Minnesota, and 0.26 m lower at Michipicoten, Ontario, than in 1902. By 2050, these differences will be as much as 0.34 m higher and 0.43 m lower, respectively. Thus, the implications of crustal movement should be considered in long-term planning, particularly with respect to establishing flood levels along Lake Superior's southwestern shore and navigation depths along the northeastern shore.