The drainage divide: this ground runs to the Gulf

The headline answer

Union Township drains south to the Mississippi basin — not north to Lake Michigan. The chain, link by link. (1) The state's potentiometric map of the county (IDNR Division of Water, 2011) splits the county between the St. Joseph River basin in the northeast and the Kankakee River basin in the western and southern areas, groundwater flowing southwest in the Kankakee basin, the potentiometric high — 830 feet — standing in the central-southeast county, on this township's side, with water moving away from it southward. (2) Pleasant Lake's own bathymetry fixes it in watershed HUC 071200010304, whose parent basin, 07120001, is the Kankakee. (3) The Headwaters Yellow River watershed plan names Lakeville inside it and lists both lakes among its waters. (4) The Yellow River — whose present course begins at a confluence of farm ditches in southeastern St. Joseph County — joins the Kankakee in southwestern Starke County; the Kankakee reaches the Illinois, and the Illinois the Mississippi. The full statement: Riddles Lake, through the quarter-mile channel to Pleasant Lake, southward through the township's drainage to the Yellow River headwaters, the Yellow River, the Kankakee, the Illinois, the Mississippi, the Gulf of Mexico. Every drop that leaves these lakes by surface flow ends in the Gulf. The town's finer grain agrees: the town stands "completely within the Stock Ditch–Headwaters Watershed" on its own comprehensive plan, its terrain dropping some 30 feet north to south, about 80 per cent of it Brookston and similar soils, "poor to drain" — the till-plain clay still governing what the ground does with water. One caveat prints with the answer: the township's far northeastern sections were not separately watershed-mapped in the sources reached; no source found shows any part of Union Township draining to the St. Joseph River, whose basin the state map confines to the county's northeast.