Chain of Rocks Lock and Dam, also known as Locks No. 27, is a lock situated at the southern end of Chouteau Island near St. Louis, Missouri, on the Upper Mississippi River. Its associated dam is just downstream of the Chain of Rocks Bridge, and the lock is located over 3 miles (4.8 km) southeast on the Chain of Rocks canal. The canal and locks allow river traffic to bypass a portion of the river that is unnavigable in low water due to an anticlinal exposure of bedrock in the river—a "chain of rocks".