The study area and sites visited are shown in the map below.
Map of study area with sites labeled
The study area is relatively sparsely populated and has few coastal structures. The western end includes the Brule River State Forest and two small towns, Herbster and Port Wing. Development along the shoreline is mainly vacation homes and some harbors/marinas. The eastern end has some large towns, Bayfield and Washburn, a small city, Ashland, and greater shoreline development. The eastern end also includes the Red Cliff and Bad River Indian Reservations and the Apostle Islands National Lakeshore.
The climate of the study area is moist continental mid-latitude. Spring thaw and storms are believed to be significant contributors to the subaerial erosion of the bluffs. Strong northeasterly winds blow in the late fall and early winter giving rise to powerful waves traveling toward the Wisconsin and Minnesota coastlines. Shorefast ice develops in the winter, and lake ice coverage may remain greater than 50% through March (Driver et al. 1992). This ice protects the bluffs from wave action. However, the shoreline is subjected to sediment rafting alongshore and offshore, and nearshore sediment is also eroded by ice and scoured by wave action at the ice foot (Barnes et al., 1994).
Lake Superior is the largest of the Great Lakes by both volume and surface area. Lake Superior has the largest fetch distance (563 km) and deepest average depth (147 m) of the Great Lakes (GLIN, 2003), allowing Lake Superior to be the most powerful of the Great Lakes with measured deep water significant wave heights up to 7 m and periods up to 20 s (NOAA, 2004).
Lake Superior has experienced long-term water level fluctuations of over 1 meter since 1918 as shown in the Figure below. Seasonal water level fluctuations are typically around 0.35 meters with the highest lake levels occurring in late summer and early fall and the lowest levels in late winter and early spring. The seasonal and long-term water level fluctuations are mainly climatically driven, the result of changes in precipitation, runoff and evaporation. Short-term water level changes on Lake Superior are not affected by tides but are susceptible to seiches and wind setup. Outflow from Lake Superior is regulated by the International Joint Commission (IJC), and the IJC has attempted to maintain the mean monthly lake levels between 182.38 and 183.48 m IGLD (Rasid et al. 1992). However, meteorological effects have overridden human attempts to control lake levels. Isostatic rebound from glacial unloading also contributes to a slow, long-term rise in the water levels of Lake Superior (Mickelson et al. 2004); the Sault St. Marie outlet is rising faster than western Lake Superior causing this rise.
Water level fluctuations in Lake Superior