Locks 27 moves more cargo than any other navigation structure on the Mississippi River. The structure is situated near the southern end of the 8.4-mile long, man-made Chain of Rocks Canal. After 1940, only a single impediment prevent the maintenance of a safe and reliable 9-foot navigation channel on the Mississippi River from St. Paul, Minn., to New Orleans. This impediment, known as the Chain of Rocks Reach, was a 17 mile series of rock ledges that began just north of St. Louis and was extremely difficult and dangerous to navigate.

The canal was designed to allow vessels to bypass the treacherous reach. To ensure adequate depths in the pool below the old Lock and Dam 26, a non-moveable, low water dam was constructed across the river, near the old Chain of Rocks bridge. This dam is known as Dam No. 27 and the Chain of Rocks Dam.

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