Chicago River Flow: Engineered Reversal Away from Lake Michigan

The Chicago River does NOT flow into Lake Michigan. It flows away from the lake — a feat of 19th/20th-century engineering that reversed its natural course.

Natural History

• Pre-1800: Slow prairie stream (Portage Creek) draining into Lake Michigan
• Chicago Portage: Critical low divide between Great Lakes and Mississippi basins
• 1830s: Channel cut to improve harbor; river becomes open sewer

The Great Reversal (1887–1900)

• Problem: Sewage + drinking water intake in same lake → cholera, typhoid epidemics
• Solution: Sanitary & Ship Canal (28 mi, 24 ft deep, 160 ft wide) connects South Branch to Des Plaines River → Illinois River → Mississippi
• Mechanism: Gravity flow + lock at Lockport controls level
• Result: River flows west; lake intake stays clean

System Expansion

• North Shore Channel (1910) — reverses North Branch
• Cal-Sag Channel (1922) — reverses Calumet system
• TARP / Deep Tunnel (1975–) — captures combined sewer overflows
• Disinfection (2016) — UV at Calumet & O'Brien plants

Modern Flow & Ecology

• 70% treated wastewater effluent (cleaner than many rivers)
• Fish species: 70+ (up from 10 in 1970s)
• Recreation: Kayaking, architecture tours, riverwalk
• Asian carp threat: Electric barriers in CSSC

Legal Limit

Supreme Court (1930, Wisconsin v. Illinois) capped diversion at 3,200 cfs (2.1B gal/day). Climate change pressures may force renegotiation.