In the early 2000s, a little-noticed but vital piece of Chicago’s infrastructure was fast eroding and in dire need to replacement.
Despite years of expensive temporary fixes, Chicago’s 68-year-old Harbor lock continued to periodically malfunction resulting in the disruption of recreational and commercial boating and stoking fears about the remote but real potential threat of a downtown flood – when the lock’s gates malfunction, engineers lose their ability to open or close them, a last-resort safety valve that’s been used five times since 1996 during heavy rain to keep the Chicago River from overflowing its banks.
If the lock failed to open under such conditions, hundreds of millions of gallons of river water could spill into Union Station, Lower Wacker Drive and basements of buildings along the river. That’s far more than the roughly 250 million gallons that poured into downtown Chicago during the Loop Flood in April 1992, when a freight tunnel under the river was punctured.
Though the river normally is about a foot lower than Lake Michigan, the narrow channel rises much faster during downpours. Under the right conditions, less than 3 inches of rain over 12 to 24 hours can cause the river to rise 5½ feet. At that level, engineers with the Metropolitan Water Reclamation District of Greater Chicago open the lock to prevent flooding.
A $17-million overhaul of the lock’s four steel gates in 1998 and 1999 was supposed to last at least 10 to 20 years, but the lock needed urgent repairs again two years ago when six of the eight rollers that move the 133-ton gates broke down.
Seeking a more permanent fix, the U.S. Army Corps of Engineers conducted an Environmental Assessment under the National Environmental Policy Act (NEPA) to plan for repairs to existing breakwaters and replace. During the environmental review process, NEPA revealed a better method of repairing and extending the life of the breakwaters at a fraction of the replacement cost by replacing the structural steel sector gate leafs and associated hydraulic operating machinery. As a result, the new lock gates are designed to reduce repair and maintenance costs and prevent service disruptions. The new gates and operating machinery have fewer parts and are more reliable, and are modeled after the proven design of the Thomas J. O’Brien Lock.
Today, the Chicago Lock is fourth in the nation in terms of commercial lock usage and second in the nation in terms of recreational lock usage. On a busy day, 50-100 vessels can be locked at once.
 “Chicago Harbor Lock.” S. Army Corps of Engineers. 2009. Available at:
 “Clock ticks on Chicago Harbor lock.” Crain’s Chicago Business. June 4, 2006. Available at: