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 1265 ft long and consists of three roller gates and ten tainter gates. A 1400 ft long submersible dike continues to the Missouri shore. The main lock is 110 ft wide by 600 ft 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. 21 Historic District, #04000181 covering 1075 acre, 1 building, 4 structures, 4 objects. Currently there is a study being conducted by the city of Quincy to convert Lock & Dam No. 21, 20, and 22 into being hydroelectric by 2016.