To achieve a 9-foot channel in the Upper Mississippi River, the construction of a system of navigation locks and dams was authorized in 1930. Dams are built on rivers to hold back water and form deeper navigation "pools." Most pools in the United States are maintained at a constant minimum water depth of 9 feet for safe navigation. Dams allow river vessels to use a series of locks to "step" up or down the river from one water level to another.
The St. Paul District of the Army Corp of Engineers operates and maintains 13 of the 29 Locks and Dams on the Mississippi River, beginning at Upper St. Anthony Falls in downtown Minneapolis and ending at Lock & Dam #10 in Guttenberg, Iowa. Lock & Dam #4 is located on Mississippi River mile 752.8 in Alma, Wisconsin. Lock & Dam #4 was constructed and placed in operation in May, 1935. Last major rehabilitation completed from 1988 to 1994. The dam consists of a concrete structure 367 feet long with six roller gates and 22 tainter gates. The earth embankment is 5,400 feet long. The lock is 110 feet wide by 600 feet long.
Alma has an observation bridge from which tourists can get a good look at the lock and dam. The bridge is made of grated metal and it extends over the railroad tracks creating quite an experience when the train goes buzzing by only a few feet below!