The Upper Illinois River is warmer than the lower river, as a 

 result of warm municipal and industrial effluents. 



The upper river is less turbid because the bottom is generally 

 rocky, whereas the lower portion, including Peoria, La Grange, and Alton 

 Pools contains flocculent muds that have entered the river and are kept 

 in suspension by the river current and by wave action resulting from 

 wind, towboats, and pleasurecraf t . 



Dissolved oxygen levels at the surface and the bottom of the river 

 were virtually the same in the fall of 1974, and dissolved oxygen levels 

 were 77-97 percent of saturation in Alton Pool, 56-122 percent of satura- 

 tion in la Grange and Peoria Pools, and 47-104 percent of saturation in 

 the upper pools of Starved Rock, Marseilles, and Dresden. Local areas 

 of super-saturation occurred where plankton blooms appeared to be in 

 progress. In an area that provided good physical habitat for largemouth 

 bass, Chillicothe Island Chute, Peoria Pool (mile 180), midsummer oxy- 

 gen levels were at 35-percent saturation or below for 4 years out of the 

 8-year period 1963-1970. During this period, the number of largemouth 

 bass taken by electrof ishing in Chillicothe Island Chute decreased con- 

 siderably. Laboratory experiments have shown that oxygen levels below 

 35-percent saturation reduce the survival of larval largemouth bass and 

 that levels below 70 percent retard their growth. 



The number of fish species taken by electrof ishing in the Dresden 

 Pool, Des Plaines River portion of the Illinois Waterway during the 

 period 1959-1974 was consistently low. Only carp and goldfish and hy- 

 brids of these two pollution- tolerant species were commonly taken. 



The following species showed a trend of increasing abundance in the 

 downstream direction, away from Chicago, with the largest number occur- 

 ring in Alton Pool: shortnose gar, bowfin, goldeye, mooneye, channel 

 catfish, flathead catfish, and white bass. 



Goldfish showed a trend of increasing abundance in the upstream 

 direction, toward Chicago. 



74 



