Ch. 18] OHIO RIVER 329 



including the Keokuk power dam. All navigation dams are of over- 

 flow type, with unusually large gates to pass heavy ice flows, and sills 

 approximately at river bed in order not to interfere seriously with 

 flood discharge. 



In 1940 no serious trouble was anticipated from sedimentation in 

 the pools formed by the navigation dams, as no unusual difficulty had 

 been encountered in maintenance of the navigation channels at the 

 head of Lake Pepin and in the pool formed by Keokuk power dam, 

 although the latter, which had an original length of 60 miles, has been 

 reported to have lost about 23 percent of capacity in 15 years, most of 

 the sediment being soft mud deposited in the shallower areas and along 

 sides of the main channel without seriously interfering with the navi- 

 gation channel (U. S. Congress, 1940, pp. 66-67). There has been con- 

 siderable deposition on the flood plain and in the lower parts of some 

 tributary valleys, however, and it is apparent that maintenance 

 dredging will have to be continued in the pools (Hathaway, 1948). 

 Large quantities of sand are brought in by the tributary Chippewa 

 and Wisconsin rivers, and the delta or fan of the Chippewa long ago 

 blocked the Mississippi and formed Lake Pepin, a ponded section of 

 the main river about 21 miles long with maximum water depth of 

 about 50 feet. Continued sand inflow requires dredging 250,000 to 

 300,000 yards annually to maintain the navigation channel immediately 

 below the mounth of the Chippewa (U. S. Congress, 1940, p. 66) . 



The navigation project as a whole, including proposed extension 

 above St. Anthony's Falls at Minneapolis, was about 75 percent com- 

 plete in 1948. During the year ending June 30, 1948, maintenance 

 dredging necessary at 116 localities amounted to a total of more than 

 5 million cubic yards. 



Ohio River 



The Ohio River was a major route of travel and commerce in the 

 days of shallow-draft steamboats, and it is today an outstanding ex- 

 ample of a major river canalized for barge transportation. Its present 

 commercial importance is due to the heavy and bulky freight traffic 

 required by the coal, iron, and steel industries concentrated in the 

 upper Ohio Basin. A canal around the Louisville rapids was opened 

 in 1830, and construction of locks and dams to improve navigation in 

 the Pittsburgh area was initiated in 1879, but it is reported that 

 automobiles were driven across the river bed, at places 45 to 75 miles 

 below Pittsburgh, during low water in 1908 before canalization was 

 complete (Duis, 1944) . A series of 46 locks and dams now provides a 

 channel of 9 feet depth, at low stage, in a series of slack-water pools 



