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Locks and Dams
Below the confluence of
the Mississippi and Missouri Rivers, the Mississippi was deep enough and had
minimal impediments for river travel. North of the confluence numerous
snags, sandbars, rapids, and other obstructions made river travel difficult.
Beginning in the 1830s the Federal Government, acting through the U.S. Army
Corps of Engineers began improvements in the interest of navigation.
Projects included removing the worst snags and sandbars and dynamiting
several rapids. In
1878, Congress authorized the Corps to establish a 4.5-foot channel. This
was accomplished primarily by constructing canals with navigation locks to
bypass the Des Moines Rapids near Keokuk, Iowa and the Rock Island Rapids
between Rock Island and Moline, Illinois. Riverboat passage virtually
impossible over these rapids because the river became shallow and the
riverbed consisted of rock. The Mississippi became completely navigable from
New Orleans to St. Paul with the opening of the Moline Lock that bypassed
the Rock Island Rapids in 1907. In 1907
a 6-foot channel was authorized, the increased depth being obtained by
building hundreds of wing and closing dams. Built close together, wing dams
are brush and stone structures extending from the riverbank toward the
channel, usually at a 90-degree angle. Made of similar materials closing
dams were used to block the connections between the main channel and the
backwaters and side channels of the floodplains. These original structures
are still a common feature on the Upper Mississippi River. However with the
rise of the use of railroads, the commercial aspects of river transportation
became uneconomical and the industry began to die off. The
20th Century brought a number of advancements that would bring the river
transportation industry back to life. New ideas in lock and dam
construction, particularly the roller-gate dam, were being tested and
proven. Diesel powered river vessels became capable of pushing large numbers
of heavily laden steel barges. It was determined that a dependable 9-foot
channel was need for the Upper Mississippi River that could accommodate the
new towboats and barges being used on the Ohio and lower Mississippi Rivers.
The addition of this new channel would form an integrated transportation
system for the central United States. In 1930 Congress authorized the 9-foot channel navigation project on the upper Mississippi River from Minneapolis to the confluence with the Missouri River. This legislation provided for a navigation channel with a minimum width of 400 feet to be achieved by the construction of a series of locks and dams. Construction of these structures occurred mostly the 1930s and 1940s and resulted in a total of 27 locks and dams. This system created what is commonly called a “Stairway of Water” as the Mississippi falls 420 feet from the Falls of St. Anthony in Minnesota to Lock and Dam #27 in Granite City, Illinois. Slackwater pools are created behind the dams allowing towboats and other river vessels to be raised and lowered as they proceed from one pool to the next.
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