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How the St. Joseph was made out of the Kankakee's headwaters — Montgomery's river-robbing, the sluiceway, and the modern basin study
River robbing: The present river exists because a tributary stole a trunk stream. In Montgomery's account, the glacial Dowagiac, which had entered the great Kankakee at the site of South Bend from the north, itself reversed: at a point just below Niles it "doubled upon itself at an angle of forty-five degrees, followed the abandoned channel of its former tributary and discharged its waters into Lake Michigan," leaving a worn channel three to four miles wide and thirteen miles long leading back to the Kankakee at South Bend. The arithmetic decided the rest. "The distance from South Bend… to St. Joseph, Michigan, is thirty-eight miles, with a fall of one hundred and forty-one feet. From South Bend to Momence, Illinois, the distance is ninety-two miles with a fall of ninety-three feet." With the fall by the new route three and a half times greater, the first annual floods split at the bend, the new channel cut rapidly through the old river deposits, "finally taking all the waters of the once mighty Kankakee, and leaving the valley from South Bend to the Desplaines a geological monument." A sandbar thrown diagonally across the old channel at South Bend — built of sediment from Wenger Creek, twelve to fourteen feet high, the ridge the settlers knew as the Bluff — "sealed forever" the Kankakee valley, "and the upper stream became a distinct river, the beautiful St. Joseph as we know it." Montgomery adds a physical accomplice: an ice gorge at Crum's Point, seven miles below South Bend, where a point of the Michigan moraine juts two and a half miles across the valley, damming the escaping waters and aiding the turn.
The gorge's channel, from this city's edge: The ice dam's overflow left its own earthwork, and its near end is on Mishawaka's ground. East and north of the dam, Montgomery writes, the ponded waters "excavated an interrupted channel or chain of depressions. These depressions are linear, extending from southeast to northwest, being from one-fourth to three-fourths of a mile long, twenty to forty feet deep and from two hundred to four hundred yards wide, with sharp and well-defined banks. They all show evidences of having been filled with water for a long period of time. All have become dry except the lower two, the Notre Dame lakes… This channel or chain of depressions extends from near Mishawaka, northwesterly, to a point on the St. Joseph river one mile north of South Bend, a distance of four miles and a half." When the dam failed, the waters abandoned the circuitous route and divided between the old and new channels until the new carried all. Two smaller deposits of the same rearrangement stand on the record: the gravel bed down the St. Joseph's west bank below South Bend, forty to fifty feet deep, laid where the Dowagiac's checked current dropped its load — and, at this city's own edge, the coarse deposit of the checked Kankakee itself, "stored away in the form of acres of river gravel at Twin Branch, just east of Mishawaka," where the slowed trunk stream dropped its heavier material against the tributary's inflow. Since the capture, Montgomery concludes, "the new river has eroded its valleys from fifty to sixty feet into the old river deposits, but has not yet reached their base level"; and the Kankakee Marsh itself is the capture's monument — had the river kept its old course and cut the same fifty or sixty feet down the silted valley, "there would have been no 'Kankakee Marsh.'"
The sluiceway above: The upper river that feeds this reach was itself a meltwater work. A Western Michigan University master's study of the meltwater deposits of the two lobes (Erber, 2016) names the modern river's channel in south-central Michigan the St. Joseph sluiceway: three cores taken in the sluiceway, tested against cores in the Saginaw lobe's drumlin uplands by sand-grain lithology, grain size and longitudinal profile, showed increased shale in the channel deposits derived from the Paleozoic shales in the Huron-Erie lobe's flow path — the Coldwater, Antrim, Sunbury and Bedford shales — and a gentle east-to-west gradient, together indicating that the sluiceway "carried meltwater from the Huron-Erie lobe." The boundary is printed: the sluiceway is Michigan ground, the headwater reach of the same river whose capture reach lies at this city's western door.
The basin, sequentially built: The modern basin study — Kincare's doctoral dissertation on the late Wisconsin and Holocene development of the St. Joseph River drainage basin (Western Michigan University, 2010) — rebuilds the whole sequence. The basin covers some 11,137 square kilometres; the river runs 316 kilometres with its head 158.8 metres above Lake Michigan at Benton Harbor; its long profile carries four nickpoints dividing five sections — a base-level section at the mouth, a stream-capture section, a central section where the proglacial lakes overflowed to the south, a convex-upward section downstream of flow lost to capture, and a former tributary that became the main stem through capture: Montgomery's river-robbing, restated as profile geometry. The basin, Kincare finds, "was sequentially constructed from the distal areas to the proximal areas" as the ice margin retreated: "Early deglacial drainage was initially south toward the Wabash River and later west to the Kankakee River before attaining its final configuration draining to Lake Michigan," the glaciers ponding proglacial lakes that overflowed south as they went — and, the finding that moves the date, "The St. Joseph River did not drain into the Lake Michigan basin until the Calumet phase of glacial Lake Chicago, 1500 years after previously thought." The phase names carry the lake's current dating frame: the radiocarbon-based chronology of Curry, Bruegger and Conroy (Geology, 2018) puts glacial Lake Chicago's Glenwood highstand at about 17,000 to 15,000 calibrated years ago and its Calumet highstand at about 14,000 to 12,400.
The last twelve thousand years: Kincare's earlier study of the river's response to lake level (Journal of Paleolimnology, 2007) follows the made river through the lake's falls. The floodplain below Niles is inset into an upper terrace and delta graded to the Calumet level of Lake Chicago, 189 metres; in the lower part of that terrace stratigraphy, a bed of clast-supported gravel one and a half to two metres thick "marks the entry of the main St. Joseph River drainage above South Bend, Indiana, into the Lake Michigan basin," the consolidation of drainage occurring "during final melting out of ice-marginal kettle chains allowing stream piracy to proceed between Niles and South Bend" — the capture reach pinned, in the modern study, to the ground between this city and Niles. And the river "is unlikely… palimpsest upon a bedrock valley": the landform it cuts is glaciofluvial and deltaic, not a draped pre-glacial valley. The same study measures the violence of the lake's great lowstand — about 9,800 radiocarbon years ago the Lake Michigan basin fell to its Chippewa low, some 70 metres above sea level, and the rejuvenated river incised up to 43 metres in a valley no more than 1.6 kilometres wide, the incision seen 25 kilometres upstream of the present shoreline, removing some 300 million cubic metres of sediment before the rising lake drowned the work and the valley partly refilled. Those incision figures belong to the lower river, the Michigan reach below Niles, and are printed at that boundary: what they establish for this ground is the mechanism's scale, not a measurement at Mishawaka.
Sources: Howard (1907), Chapter I, §VII, sec. 3 — the Dowagiac reversal, the two falls and the three-and-a-half ratio, Crum's Point, the chain of depressions from near Mishawaka, the Bluff sandbar, the Twin Branch gravel, the fifty-to-sixty-foot incision and the marsh counterfactual; Erber, "Differentiating Saginaw from Huron-Erie Lobe Meltwater Deposits in South Central Michigan…" (Western Michigan University master's thesis, 2016) — the St. Joseph sluiceway and its Huron-Erie source; Kincare, "The late Wisconsin and Holocene development of the St. Joseph River drainage basin" (WMU dissertation, 2010), abstract — basin figures, the five profile sections, the sequential construction, the Wabash–Kankakee–Lake Michigan drainage order and the Calumet-phase finding; Curry, Bruegger & Conroy, "Highstands and overflow history of glacial Lake Chicago…," Geology 46 (2018) — the Glenwood and Calumet dating frame; Kincare, "Response of the St. Joseph River to lake level changes during the last 12,000 years," Journal of Paleolimnology 37 (2007) — the Calumet-graded terrace, the piracy gravel bed, the bedrock-valley judgment, and the Chippewa lowstand incision at its stated reach.