Streams and Rivers

forested stream

From The Changing Illinois Environment: Critical Trends, 1994, Summary Report of the Critical Trends Assessment Project

By any measure, Illinois is a water-rich state, and most of those riches flow through its streams and rivers. Illinois' ten river basins are drained by more than 26,000 miles of flowing waters. This system of streams, rivers, and creeks supplies humans with drinking water, recreation, transportation, industrial process, and cooling water, and it supplies habitat to Illinois' other living creatures.


Water moves plentifully through Illinois, but not everywhere all of the time. Previous studies of Illinois' large watersheds show increases in average annual flow for the years 1941-1985, with corresponding increases in peak flows and a doubling of the days in peak flow, all of which correspond to higher rainfall in the watersheds. However, natural streamflows vary considerably, making it difficult to distinguish the ups and downs in streamflow caused by local factors and those that signal longer trends of climate change. Much of northern Illinois has seen a significant increase in average flow and low flow. These changes are strongly correlated to regional fluctuations in average precipitation that attend normal climatic variability. However, most streamflow data was collected after major land-use changes--such as field tiling and the channelizing of streams--in the 1800s.

Low streamflows compromise virtually all the manifold economic and ecological functions that streams serve. From 80% to 90% of the water that is withdrawn for municipal and individual use in Illinois is returned to streams as effluent from wastewater treatment plants; indeed, during droughts the water moving through some streams may be almost entirely effluent, as is the case in many streams in Illinois' urban northeast. (Regulations mandate a required level of wastewater treatment that will not exceed the dilution provided by a given stream even during low flow.)

Of the water drawn from Illinois surface sources, 4% comes directly from border rivers and another 2% comes from its major in-state rivers. With groundwater sources insufficient in some parts of the state, and with high costs making the construction of new surface reservoirs less likely, streams may be tapped more heavily for public drinking water in the future. In the past ten years, public water supply withdrawals from the Fox River on Chicago's western suburban fringe have grown to the point where further withdrawals may require management and/or regulation.


Since 1977, angling days statewide have gone up 21%; in 1989 the two million angling days spent on the Illinois River alone are thought to have earned local economies some $40 million. Four Illinois rivers--the Mississippi, the Illinois, the Kaskaskia, and the Wabash--also support significant commercial fisheries. Catches tend to vary from year to year; the average dollar value of the catch in recent years has been $1.26 million, of which 94% is provided by the Illinois and Mississippi.

Illinois' commercial fishery is but a shadow of what it was at the turn of the century. The abundant mussel beds of the Mississippi and Illinois rivers supplied the turn-of-the-century pearl button industry, until over-exploitation depleted the resource. In the 1960s, revived populations of mussels supplied a new market as a raw material for Japan's cultured pearl industry. Demand pushed up prices, and the harvest of all mussel species increased dramatically between 1987 and 1990; the value of the catch from the Mississippi and Illinois rivers rose to nearly $3 million by 1990. (Figure 3-1) Harvests of threeridge mussels from the Illinois River more than doubled to more than 900,000 pounds, and 60% more live washboard mussels were hauled in during that time.

Figure 3-1. Commercial Harvest of Miscellaneous Mussel Species in the
Illinois and Mississippi Rivers 1920-1943, 1975-1991

Figure 3-1. Commercial Harvest of Miscellaneous Mussel Species in the Illinois and Missisippi Rivers 1920-1943, 1975-1991

Source: Ecological Resources, Illinois Natural History Survey, 1994


The general quality of the water in Illinois streams has improved over the past 20 years. Measurements of metals, for example, show a trend toward decreasing concentrations, while concentrations of common pesticide compounds showed no significant trends over that time. Research also indicates decreasing trends in chemical oxygen demand, another measure of conventional water pollution. However, a highly significant trend is toward increased concentrations of substances in common agricultural use such as phosphorous and nitrate nitrogen, the latter showing up in the Rock, Illinois, and Kaskaskia river basins.

During the first 60 to 70 years of this century, the discharge of sewage and factory wastes into Illinois streams was the major determinant of their water quality. From 1957 to 1992, the fish species that dominated the polluted, oxygen-poor water of the Illinois River downstream from Chicago's sewage pipes were five species that are more tolerant of pollution than most native species. (The introduced carp and goldfish led the list.) The domination of stream environments by such species is made easier when predators like the largemouth bass, which ordinarily would control their numbers, fail to thrive because of poor quality water.

Analysis of discharge monitoring reports filed by major facilities discharging wastes under the federal Clean Water Act of 1972 shows a trend toward reduced effluent loads. Fifty-five major municipal sewage treatment facilities showed reductions of 72% in loadings of biological/carbonaceous oxygen demand from 1987 to 1992. Sixty-one facilities reported loadings of ammonia that were 65% lower in 1992 than in 1987. Seventy-one facilities showed a 42% decline in discharges of total suspended solids. Sixty-one facilities reported that from 1987 to 1992 loading of residual chlorine from chlorination had dropped by almost 25%. (Figure 3-2)

Figure 3-2. Estimates of Statewide Pollutant Discharge Loading for Analyzed Major Municipalities

Figure 3-2. Estimates of Statewide Pollutant Discharge Loading for Analyzed Major Municipalities.

Source: Sources of Environmental Stress, ENR Office of Research and Planning, 1994

Trends in loadings of chromium, copper, cyanide, and phenols from major manufacturing and utility facilities also showed declines between 1987 and 1992, with reductions ranging variously from 37% to 53%.

In spite of these improvements, the number of fish kills has increased since 1965. Although numbers fluctuate from year to year, the annual number of fish killed by pollution (rather than by drought or other natural causes) has been rising. The proportion of kills attributable to industrial point sources has declined in the last 30 years and now stands at roughly 10%. Similarly, fish kills attributable to acid runoff from coal-mine waste are becoming rarer as abandoned mine sites are cleaned up. Kills attributable to agriculture, often from unregulated nonpoint sources, have risen steadily.

Analysis of data from 28 permanent fish census stations on the Illinois River for the years 1963 and 1992 reveal what appears to be a general trend toward recovery in terms of certain measures of stream ecosystems. Native species such as minnows and green sunfish have returned to the upper Illinois, relegating carp to seventh place among the more populous fishes. (Figure 3-3 ) Similar turnabouts have been recorded in other reaches of the river. The bluegill supplanted the carp as most populous species in the middle and lower Illinois. The incidence of external abnormalities in fish (mainly sores and eroded fins) taken from the Illinois River also declined markedly between 1963 and 1992.

Using biological criteria, however, one study found that as of 1988 the proportion of stream-miles in Illinois in fair to very poor condition (based on a sample of approximately 29% of the total) was 66%, while those rated good to excellent comprised 34%. (Figure 3-4)

Figure 3-4. Percentage of Illinois Stream Miles Surveyed* Rated by the Biological Stream Characterization

Figure 3-4. percentage of Illinois Stream Miles Surveyed.

Source: Ecological Resources, Illinois Natural History Survey, 1994

Surveys of Champaign County streams dating back to 1892 showed that the numbers of species sampled dropped by one-fourth from 1928 to 1959, when modern agriculture systems came into wide use and industrial expansion accelerated. Since 1959, however, Champaign County streams have seen revived populations of a few fish species such as the black basses and channel catfish, but poor physical habitat and increased urbanization limit full recovery. (Figure 3-5)

Figure 3-5. Mean Numbers of Indigenous Fish Species Per Sample in Champaign County Streams

Figure 3-5. mean Numbers of Indigenous Fish Species Per Sample in Champaign County Streams.

Source: Ecological Resources, Illinois Natural History Survey, 1994

As pollution has been reduced in the open water of Illinois streams, attention has shifted to bottom sediments, where metals and other lingering industrial toxics tend to accumulate. The condition of sediments in the Illinois River appears to have improved over the past 30 years, but fish consumption advisories remain in effect for the Illinois River downstream as far as Peoria for bottom-feeding fish such as the freshwater drum and channel catfish. The physical abnormalities that still appear on fish are more common in species such as carp and catfish that feed among the bottom sediments.


Physical changes remain a perturbing force in Illinois stream ecology. As long ago as the 1940s, researchers suggested that the poor quality of Illinois streams resulted as much from the manipulation of the landscape in their watersheds as from pollution. The remaining riparian forest that acts as an erosion buffer for streams continues to shrink. Along parts of the Little Vermilion, Embarras, and Kaskaskia rivers, forest cover shrank 40% to 80% from 1958 to 1988, as urban uses along Champaign County streams increased more than 37% during those years, much of that occurring within 100 feet of the water.

In Illinois' larger rivers, physical perturbations to the stream environment also come from within their banks. Propellers churn up bottom sediments, waves erode banks, dredging (to keep channels clear) and spoil-dumping physically reshape river channels and floodplain habitats.

Since 1950 freight-carrying barge traffic statewide has increased fivefold (measured in tons), although the rate of increase overall has leveled off since 1980. The exact impact of barge traffic on river ecosystems is not understood but may be assumed to be substantial.

While most soil erosion in Illinois occurs from open fields, most of the estimated 26,000 miles of Illinois streams also experience some enhanced erosion of their banks, especially where banks have been denuded of vegetation. The straightening or channelization of streams also increases their vulnerability to erosion by speeding the flow of water. How much of the sediment load of Illinois streams comes from their own banks varies with local conditions; estimates range from 20% to 80%.

Streams also collect soil particles eroded from elsewhere in their watersheds. The Illinois River basin contains more than 60% of the agricultural acreage in the state. On average, 8.2 million tons of sediment are delivered by tributary streams and deposited in the Illinois valley each year. (This number does not include soil eroded from the river's own banks and from valley bluffs.) Peoria Lake, the largest and deepest of the bottomland lakes on the Illinois River, lost 68% of its capacity between 1903 and 1985. The lake's average depth shrank from 8 feet to 2.6 feet, and if present trends continue it will become a mudflat split by a narrow stream. (Figure 3-6 )

In the mid-1970s, an estimated 15.4 million tons of sediments were deposited in the valley each year. The rate at which sediment is being flushed into the Illinois River has been slowing since that time, as acreage planted in row-crops declined from record high levels. But while erosion slowed, the sediments stayed; fish species such as the northern pike and the black buffalo that are dependent on the now-buried shallow backwaters are virtually gone from the lower Illinois.

Changes in the physical shape or morphology of streams profoundly affect their function. Water in channelized streams moves faster--usually at a cost of reduced habitat and increased erosion. More than 25% of the total length of sizeable streams in the Rock, Sangamon, Fox/Des Plaines, and Kankakee/ Vermilion/Mackinaw basins has been straightened. (Figure 3-7 ) Many of the state's minor streams are wholly artificial, having been built mainly for drainage.

Dams are another major manipulation of stream systems. Almost every sizeable stream in Illinois is dammed in at least one spot, creating a total inventory of nearly 1,200 dams of all sizes. (Figure 3-8) Most dam building occurred in the 1930s, creating small water supply lakes on lesser streams and, on the major rivers, creating a series of deepwater pools to improve navigation. In the 1960s and 1970s, another spate of construction dammed downstate rivers for recreation, flood control, and public water supply. The costs of such benefits have been significant. In large rivers, navigation dams combined with high artificial levees have prevented the natural flooding and drying cycle in the floodplains that formerly maintained a highly productive and diverse biota.

Figure 3-8. Number of Dams Constructed in Illinois in Each Decade and Mean Normal Storage 1900-1990

Fig. 3-8. Number of Dams Constructed in Illinois in Each Decade and Mean Normal Storage 1900-1990.

Source: Ecological Resources, Illinois Natural History Survey, 1994


Of the larger animal species present in Illinois streams at the turn of the century, about one in five of the fish, one in three of amphibians and reptiles, more than half the freshwater mussels, and one in five of the crayfish have been extirpated or are threatened by extinction. Information is not available to gauge the survival status of insects and aquatic plants.

Forty-four of the 100 species of amphibians and reptiles known in Illinois have a stream-dependent larval stage lasting from several months to a few years, and nearly all of them also deposit their eggs in water. These complex life cycles are especially dependent on high-quality, varied stream habitat. For example, the hellbender salamander requires fast-running clear water and gravelly streambeds of the sort that have been buried by silt throughout much of Illinois; the animal has been seen only once in Illinois since the 1950s.

Sixteen exotic fish species are reproducing in Illinois streams and rivers. Most of them were introduced by accident. Some native species, such as the red shiner, that are tolerant of poor water conditions--wide fluctuations in water pH, low dissolved oxygen, high water temperature--have expanded their ranges.


Aquatic plants are essential to stream ecosystems, providing food and cover and recycling nutrients. Sedimentation, complicated by pollution, is thought to have caused massive declines in plant life along the Illinois River. Many submerged plants anchor themselves on the stream bottom but need light; sedimentation blocks light, and soft, silty bottoms provide poor root anchorage against currents. Artificially high water levels also leave bottom-rooted plants too far from light. When a two-year drought lowered sediment loads and dropped water levels in the Mississippi River's Pool 26 near Alton, beds of submerged plants reappeared that had not been seen there for years; a return to more typical--if not more natural--water levels caused them to disappear again.

The Illinois River | Land

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