The Chicago Harbor Lock is a pound lock located in Chicago, Illinois, separating Lake Michigan from the Chicago River. Operated by the U.S. Army Corps of Engineers, the lock was designed and built between 1936 and 1938 by the Sanitary District of Chicago as a component of the project to reverse the flow of the Chicago River and is one of two entrances from the Great Lakes to the Chicago Area Waterway System - the other entrance being the T.J. O'Brien Lock and Dam on the Calumet River.