English: Aerial view of T.J. O’Brien Lock and Dam on the Calumet River, which is part of the Illinois Waterway. The dam is situated at river mile 326.5 on the Waterway (326.5 river miles up from the Mississippi River). T.J. O’Brien lock is the uppermost lock on the waterway, and there is not another barrier between the lock and Lake Michigan. There is very little elevation change in the lock, which serves mainly to regulate water flow from Lake Michigan into the waterway. The dam is located in Chicago, Illinois.