River (Wiebe, 1927). During August and September, 1926, the dissolved 

 oxygen content of the Mississippi ranged from 60 to 90 percent saturation 

 above the Twin Cities, but was greatly reduced (4 to 50 percent) for 

 several miles below that metropolitan area (Wiebe, 1927: 143). 



The first extensive limnological survey of the upper Mississippi 

 River was made by the U.S. Bureau of Fisheries in 1921 (Galtsoff, 1924). 

 The survey was conducted on a 455-mile segment of the Mississippi from 

 Hastings, Minnesota (Mississippi River mile 814.0) to Alexandria, 

 Missouri (Mississippi River mile 359.0). Primarily concerned with 

 the composition, amount, and distribution of plankton in the river, the 

 survey party also collected data on discharge, suspended sediment, 

 water clarity, and water temperature. Unfortunately, dissolved oxygen 

 levels were not determined, leaving a void of information on this 

 parameter for the period just prior to construction of the nine-foot 

 navigation system. 



In 1944, after construction of the nine-foot navigation system, 

 the U.S. Fish and Wildlife Service conducted water quality studies of 

 the Mississippi River from Hastings, Minnesota to Caruthersville, Mis- 

 souri (Platner, 1946). The mean oxygen content of the Mississippi 

 during a period of low water was 5.0 mg/1; in high water it averaged 

 7.6 mg/1; and in midwinter, 13.6 mg/1 (Platner, 1946: 74). In summing 

 up the general water quality of the Mississippi in 1944, Platner (1946: 

 72) stated: "Comparing the water quality of the Mississippi River with 

 waters producing good fish fauna, it would be rated as good." 



Dorris et al. (1963: 85) also reported seasonal differences in 

 dissolved oxygen levels in their 1955 investigations on the Mississippi 

 River near Quincy, Illinois (Mississippi River mile 325.0). They re- 

 ported a mean winter dissolved oxygen of 11.2 mg/1 and a mean summer 

 dissolved oxygen of 6.8 mg/1 (Dorris et al. , 1963: 85). 



Recent (1974) dissolved oxygen data for Pools 24, 25, and 26 were 

 obtained from Colbert et al. (1975). These investigators also found 

 seasonal fluctuations in dissolved oxygen concentrations. In a variety 

 of aquatic habitats which included the main channel, side channels, 

 dikes, and river border areas, Colbert et al. (1975: Table 6) reported 



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