328 happ. STREAM-CHANNEL CONTROL [Ch. 18 



lands of the alluvial plain, and a navigation channel 9 feet deep is 

 maintained by regulating works for closing sloughs and secondary 

 channels and narrowing the river, by building new banks where natural 

 width is excessive, by protecting the banks from erosion by revetment, 

 by dredging as required for temporary channel maintenance, and by 

 construction of a lock and canal around the Chain of Rocks, a series 

 of rock ledges in the channel just above St. Louis where swift currents 

 and shallow depths have been a menace to navigation at low stages. 

 Conditions at Chain of Rocks are reported to have been aggravated by 

 effects of dredging and scouring of contracted channel sections below, 

 which are credited with lowering the low-water plane at St. Louis 

 about 8 feet since 1881 (U. S. Congress, 1940, p. 26) . The entire navi- 

 gation project, including Chain of Rocks improvements, was about 

 51 percent complete in 1948. Maintenance operations during the year 

 ending June 30, 1948, included dredging about 4.7 million cubic yards 

 from 41 shoals and repair of dikes and bank revetments (Annual Re- 

 port of the Chief of Engineers, 1948, pp. 1505-1507) . 



Upper Mississippi River 



The upper Mississippi River, above the mouth of the Missouri, is a 

 comparatively stable stream, with low banks that are not generally 

 subject to excessive erosion. It has a much smaller range of flood and 

 low-water stages than the Ohio, Missouri, or lower Mississippi, and 

 flood damage is not a major problem, although alluvial lands below 

 Rock Island are protected by levees. The gradient is low, averaging 

 about 0.35 feet per mile except in short stretches of rock-floored rapids 

 in the vicinity of Rock Island and Keokuk. Early work was con- 

 cerned chiefly with improvements for navigation at the Rock Island 

 and Keokuk rapids, first by rock removal and later by lateral locks, 

 and generally unsuccessful efforts to maintain channels of 4% feet, 

 and later 6 feet, by dredging, regulation, and contraction works. Two 

 low locks and dams were built in the vicinity of St. Paul in 1894 and 

 1905, and a power dam, with supplementary navigation locks, which 

 was constructed at the Keokuk rapids by private interests in 1913, 

 greatly improved navigation conditions in that locality. 



In 1930 a project was adopted for a navigation channel 9 feet deep, 

 by canalization with supplementary dredging and contraction works, 

 and continued operation of several headwater reservoirs constructed 

 under earlier projects for improvement of low-water flows. Slack- 

 water navigation from near the mouth of the Missouri to Minneapolis, 

 a distance of about 658 miles, is now provided by 26 locks and dams, 



