Cowles Bog, part of the Indiana Dunes National Lakeshore, is a remnant of the marsh system that once stretched between Gary (Lake County) to Michigan City (LaPorte County). Recognized as a National Natural Landmark, the area is named for Dr. Henry C. Cowles, whose studies of plant succession made the Indiana Dunes famous as the birthplace of North American plant ecology. The core of the Cowles Bog property consists of an interdunal marsh which surrounds a small fen. The fen, which is off limits to the public due to its sensitivity, is supported by a constant flow of lime-rich water from springs beneath a floating mat of peat moss; the vegetation here is composed of tamaracks and a remnant stand of white pines. The Cowles Bog area also includes a pristine beach habitat, black oak savanna, and a lowland forest of red maple and yellow birch. These diverse habitats attract a great variety of birds, including such wetland species as bitterns and rails.
Cowles Bog supports a number of threatened bird species, and the diverse sampling of natural habitats and its location along the Indiana Dunes National Lakeshore add to the significance of this IBA. The property is situated directly west of Indiana Dunes State Park and Beverly Shores, two other Important Bird Areas; when combined, these three areas constitute nearly 10 contiguous miles, and over 2000 hectares, of frontage along the lakeshore. Because of their location along the southern tip of the latitudinally-elongated waters of Lake Michigan, these IBAs serve as an important portal to the lower Midwest for southbound migrants and necessary stop-over habitat for northbound overflights. In addition, Cowles Bog contains a rich mosaic of representative natural communities often considered rare or endangered in the state - interdunal wetalnds (including a large fen), oak savanna, dunal ridges, and sand beach can all be found on the property. The importance of these habitats for birds, especially when considering the migratory obstacle of Lake Michigan, are quite significant when considering the amount of heavy industry (Bethlehem Steel, NIPSCO power, Port of Indiana, and Midwest Steel) which borders Cowles Bog immediately to the west. The site does contain a variety of nesting wetland birds which are listed in D1 of the selection criteria, including American and Least Bitterns (probable), Sandhill Crane, Virginia Rail, and a significant population of Marsh Wren.