27, is a lock situated at the southern end of Chouteau Island near St. Louis, Missouri on the Upper Mississippi River.

The Chain of Rocks Lock is operated by the St. Louis District of the U.S. Army Corps of Engineers.

A large, rock-filled steel cell—used to help align barges prior to transiting the lock—split causing the channel to be blocked with rock.

[2] The steel cell has armored sections, to protect it from impacts and abrasion from the heavy barges.

But the water had fallen so low that barges were impacting an unarmored portion of the shaft.