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The Wisconsin glaciation — the torrent, the lake, the marsh
The making: The flatness that made the portage possible is glacial work. In the last ice age the lobes of the Laurentide ice sheet — the Michigan, Saginaw and Huron-Erie lobes of the geological literature — covered and then uncovered this country, and their meltwater did the landscaping. The survey literature, from Leverett's 1899 U.S. Geological Survey monograph forward, names the central event the Kankakee Torrent: about 19,000 years ago, by the modern dating, glacial meltwater ponded behind the moraines broke through in a flood that scoured the Kankakee valley and spread the outwash — the sand and gravel plain, the Kankakee Outwash Plain, that still floors the country west and south of the bend. As the ice drew back, a second water body formed in the valley: glacial Lake Kankakee, in existence by about 14,000 years ago, a shallow lake over a basin the geologists measure at more than 3,000 square miles of northern Indiana and the Illinois border. Its northern waters, in Edward Barrett's 1916 state geology report, stretched "to the marshes southwest of South Bend" — that is, the lake's edge stood at this city's doorstep.
The marsh: The lake could not cut an outlet through the limestone ledge at Momence, in present Illinois, and so as it shallowed it became not dry land but marsh: the Grand Kankakee Marsh, one of the largest freshwater marshes in the United States. Its size is carried here as the literature gives it, in a range rather than a false precision: about 400,000 acres of wetland within the Kankakee watershed in the Fraser and Bleuer geological framework (1991), and 500,000 acres in the standard lake-history accounting — a mosaic of marsh, swamp, wet meadow, savanna and open water strung along a river whose natural, unchannelized course meandered some 240 to 250 miles from headwaters barely 100 feet above its Illinois mouth. The river was straightened and the marsh drained in the decades after this section closes; what matters to the pre-city record is the world that was here when people were: a wet, level, game- and fish-rich country whose headwater ponds west of the bend were the far end of the portage, and whose near-level divide — the Kankakee rises in this county at an elevation the surveyors put only a little above the St. Joseph's own level — is the physical fact under everything else in this section.
Sources: Leverett, The Illinois Glacial Lobe, USGS Monograph 38 (1899), the Kankakee valley chapters; Curry, Hajic, Clark et al., "The Kankakee Torrent and other large meltwater flooding events during the last deglaciation, Illinois, USA" (2014) — the torrent's dating; Barrett, "Dunes of Northwestern Indiana," 41st Annual Report, Indiana Department of Geology and Natural Resources (1916) — Lake Kankakee's northern waters to "the marshes southwest of South Bend"; Fraser & Bleuer, "Geologic Framework of the Aquifers in the Kankakee River Lowland," Indiana Geological Survey (1991) — the 400,000-acre wetland figure and the outwash plain; the Lake Kankakee geological literature as summarized from Barrett (1916) and Leverett — the 500,000-acre marsh and the Momence ledge.