Geologic Variables

The geomorphology variable expresses the relative erodibility of different landform types (Table 1). These data were derived using digital orthophotos provided by NPS GIS resources and Michigan and Indiana State GIS organizations (Table 2). The Apostle Islands NL, Indiana Dunes NL, and Sleeping Bear Dunes NL contained several geomorphology types, including low to very low change-potential rock bluffs, moderate change-potential alluvial and glacial deposits and beaches backed by cliffs, and high and very high change-potential gravel and sand beaches and spits (Figure 6A, Figure 6B, and Figure 6C).

Figure 6A. Lakeshore geomorphology for Apostle Islands National Lakeshore. The colored shoreline represents the variations in coastal geomorphology within the park. Click on figure for larger image.

Figure 6B. Lakeshore geomorphology for Indiana Dunes National Lakeshore. The colored shoreline represents the variations in coastal geomorphology within the park. Click on figure for larger image.

Figure 6C. Lakeshore geomorphology for Apostle Islands National Lakeshore. The colored shoreline represents the variations in coastal geomorphology within the park. Click on figure for larger image.

Shoreline erosion and accretion rates for Indiana Dunes NL and Sleeping Bear Dunes NL National Lakeshores were derived from Great Lakes shoreline recession data (Stewart, 1994) (Table 2). There were no historic shoreline data available for Apostle Islands NL so erosion rates were estimated based on NPS publications (Woolpert, 2003), significant wave heights, and lakeshore lithology (Figure 7A, Figure 7B, and Figure 7C). Shoreline change-potential was not assigned to the very low or very high categories. For Apostle Islands NL and also the Manitou Islands in Sleeping Bear Dunes NL, where historic rates were not available, rock cliff areas were assigned low shoreline change-potential because rock cliffs are not likely to experience significant erosion or accretion annually. Unconsolidated sediments were generally classified as moderate shoreline change-potential. Beaches along the exposed coast were classified as high change-potential. The shoreline change-potential within each 1-minute grid cell was averaged to determine the shoreline change-potential used here. Shoreline change-potential for the 3 parks ranges from low change-potential (areas not experiencing significant shoreline change annually, > +1m/yr) to high change-potential (areas where shoreline change is > +2 m/yr).

Figure 7A. Shoreline change for Apostle Islands National Lakeshore. The colored shoreline represents the estimated rate of shoreline change. Click on figure for larger image.

Figure 7B. Shoreline change for Indiana Dunes National Lakeshore. The colored shoreline represents the rate of shoreline change. Click on figure for larger image.

Figure 7C. Shoreline change for Sleeping Bear Dunes National Lakeshore. The colored shoreline represents the rate of shoreline change. Click on figure for larger image.

Regional coastal slope is an indication of the relative potential for change to inundation and the rapidity of shoreline retreat because low-sloping coastal regions should retreat faster than steeper regions (Pilkey and Davis, 1987). The regional slope of the coastal zone was calculated from a grid of topographic and bathymetric elevations extending 5 km landward and lakeward of the shoreline. Elevation data were obtained from the National Geophysical Data Center (NGDC) as gridded topographic and bathymetric elevations at 0.1-meter vertical resolution for 30-arc second grid cells (Table 2). Regional coastal slopes for the Apostle Islands NL, Indiana Dunes NL, and Sleeping Bear Dunes NL fall within the very low to very high change-potential category (< 0.30% - > 1.20%) (Figure 8A, Figure 8B, and Figure 8C).

Figure 8A. Regional coastal slope for Apostle Islands National Lakeshore. The colored shoreline represents the regional slope of the land, 5 km landward and lakeward of the shoreline. Click on figure for larger image.

Figure 8B. Regional coastal slope for Indiana Dunes National Lakeshore. The colored shoreline represents the regional slope of the land, 5 km landward and lakeward of the shoreline. Click on figure for larger image.

Figure 8C. Regional coastal slope for Sleeping Bear Dunes National Lakeshore. The colored shoreline represents the regional slope of the land, 5 km landward and lakeward of the shoreline. Click on figure for larger image.

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