The upper Illinois River is warmer than the lower River, as a result of warm municipal and industrial effiuents.
The upper river is less turbid, because the bottom is generally rocky, whereas Peoria, La Grange, and Alton Pools contain 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 pleasurecraft.
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, 65-122 percent of saturation 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 two areas that provided good physical habitat for largemouth bass, Lower Bath Chute, La Grange Pool (mile 107) and Chillicothe Island Chute, Peoria Pool (mile 180), midsummer oxygen levels were at 35 percent saturation or below for 4-5 years out of the 8-year period 1963---1970. Laboratory experiments have shown that oxygen levels below 35 percent saturation reduce the survival of larval largemouth bass and levels below 70 percent retard their growth.
The number of fish species taken by electrofishing in the Dresden Pool, Des Plaines River portion of the Illinois Waterway during the period 1959-1974 was consistently low (Tables 29 and 30). Only carp and goldfish and hybrids 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 occurring 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.
The following species were most abundant in one or both of the two middle pools of the river, La Grange and Peoria Pools, which have the most connecting lake area: gizzard shad, carp, river carpsucker, smallmouth buffalo, bigmouth buffalo, black buffalo, yellow bullhead, green sunfish, bluegill, largemouth bass, white crappie, black crappie, and freshwater drum.
Gizzard shad and carp were generally abundant throughout the river.
Black bullheads were abundant at one atypical station, Ballard Island Chute, Marseilles Pool (mile 247.8- 248.2), which apparently provides preferred habitat for this species.
Gamefish populations declined during the low water years 1962-1964, and recovered following the high water years 1971- 1973. Largemouth bass populations did not recover to 1959- 1962 levels. The recovery appears attributable to improved oxygen levels in the river, and perhaps to increased dilution of toxic materials, and demonstrates how rapidly fish populations respond to improved conditions in the river.
The commercial and sport fisheries in the Illinois River have generally declined from levels around the turn of the century. The decline is attributable to a loss of habitat and increasing pollution. Habitat was lost due to leveeing and draining of bottomland areas in the period 1903-1926 and due to sedimentation in the remaining areas. Sedimentation has resulted in undesirable habitat modification, as well as habitat reduction.
Northern pike, yellow perch, and walleye (Stizostedion vitreum vitreum) were once abundant in the river but are now rare or limited in their distribution. Yellow perch populations have declined probably as the result of the disappearance of beds of aquatic plants and disappearance of clean sand or pebble substrates perch use for spawning.
In the past the bottomland lakes and backwater areas offered havens for fish and fish food organisms, as the river became increasingly polluted. Now dissolved oxygen levels in the river seem to have improved, while the lakes have filled with sediment that apparently exerts an oxygen demand, keeps aquatic plants from growing, and does not support an abundance of food organisms.
More and better waste treatment facilities are being constructed by industries and municipalities in the drainage basin of the Illinois River. However, the production of fish and wildlife in the Illinois River and its bottomland lakes is not likely to improve unless sediment pollution is also brought under control.
The consequences of future uses of land in the drainage basin and the consequences of future uses of the river must be predicted, so that a wise selection of alternatives can be made. If the river is to be managed in the future for a variety of beneficial uses, then the various state, federal, and private agencies charged with managing land and water within the drainage basin must work in a coordinated fashion, rather than at cross purposes.
Growth of Spiny Lobster Panulirus homarus Fed with Moist Diet