Ch. 18] MIDDLE MISSISSIPPI RIVER 327 



credited with lowering overbank flood stages by as much as 13 to 14 

 feet at Arkansas City, 9 to 10 feet at Vicksburg, and lesser amounts 

 elsewhere, and thereby increasing channel capacities by 100,000 to 

 700,000 or 800,000 cubic feet per second (Annual Report of the Chief 

 of Engineers, 1943). Other benefits included elimination of need for 

 an auxiliary floodway between the Arkansas and Red rivers or equiva- 

 lent increase in levee heights, and increased low-water depths which 

 made feasible an increase from 9 to 12 feet in the navigation project 

 depth in 1944 (Matthes, 1948) . 



It has been estimated that caving banks add about 800,000,000 cubic 

 yards of sediment to the lower Mississippi annually, and that most of 

 this material comes to rest on bars immediately downstream (Senour, 

 1948). This is believed to be the principal source of the sand that 

 must be dredged each year in order to maintain the navigation chan- 

 nel. Maintenance of the navigation channel 9 feet deep and 300 feet 

 wide required dredging about 43 million cubic yards from 64 locations 

 during the year ending June 30, 1948, in comparison with an average 

 of 30 million cubic yards annually since 1928, although as much as 75 

 million cubic yards has been required in years of extreme low water 

 of long duration (U. S. Dept. of Army, 1948, p. 3013). It is reported 

 that hydrographic surveys show no general tendency toward aggrada- 

 tion of the river bed within the period of records (Senour, 1947). 



Dredging for navigation improvement was first undertaken in 

 Southwest Pass, one of the river mouths, in 1839, and has been contin- 

 ued intermittently but with generally increasing volume. Jetties to 

 narrow and deepen the mouth of South Pass were begun in 1875, and 

 similar jetties were later built at the mouth of Southwest Pass. These 

 jetties, with repairs and extensions, are still maintained, and they have 

 been outstandingly successful. Below the mouth of Red River the 

 Mississippi maintains a relatively deep and narrow channel in thick 

 clay deposits, and the banks have been generally quite stable during 

 the period of records. A navigation channel 35 feet deep is maintained 

 up to Baton Rouge, 233 miles above the Head of Passes, and levees 

 have been built close to the banks without excessive caving. The dif- 

 ference between this section and the variable channel widths and rapid 

 bank caving farther upstream is ascribed to the greater resistance of 

 the clay banks to erosion, in contrast to the more sandy banks up- 

 stream (Fisk, 1944, p. 53). 



Middle Mississippi River 



In the middle section of the Mississippi River, between the Missouri 

 and the Ohio rivers, levees provide flood protection for agricultural 



