Lock and Dam No. 21 is a lock and dam located at Quincy, Illinois on the Upper Mississippi River around river mile 324.9. The movable portion of the dam is 1,265 feet (385.6 m) long and consists of three roller gates and ten tainter gates. A 1,400 feet (426.7 m) long submersible dike continues to the Missouri shore. The main lock is 110 feet (33.5 m) wide by 600 feet (182.9 m) long and there is also an incomplete auxiliary lock. In 2004, the facility was listed in the National Register of Historic Places as Lock and Dam No.