In the 1930's, Congress directed the Army Corps of Engineers to develop and maintain a 9-foot deep, 400-foot wide navigation channel on the Upper Mississippi between St. Louis, Missouri, and St. Paul Minnesota. To accomplish this, a system of 20 locks and dams was built, most between 1930 and 1940. Each dam creates a lake-sized pool of water above it to maintain the 9-foot channel. Locks adjacent to the dams raise river traffic to the up river pool or lower it to the downstream pool. The lift in the locks varies form 5.5 feet to 49.2 feet. The lock and dam system acts as a stairway along the upper river, lowering the river about 420 feet over the 699 miles between St. Paul and St. Louis. Nine dams are located along Wisconsin's Great River Road, with six locks located on the Wisconsin side of the river at Alma, Fountain City, Trempealeau, La Crosse, Genoa, and Lynxville. Observation areas at each lock & dam provide an excellent vantage point from which to watch the fascinating locking procedure.