Lakes and Reservoirs

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

The uses of Illinois lakes are varied, from water supply and recreation to flood control and cooling water, with most large reservoirs providing at least three of the four.


In the recent geologic past, Illinois was a land of lakes. Today, the largest survivor of that era is Lake Michigan, the sixth largest lake in the world. Most of Illinois' few other true lakes have glacial origins, being depressions or "kettles" formed in the northeast part of the state when blocks of ice buried in glacial till melted. A handful of other natural lakes remain in the few natural floodplains of major rivers that created them. Most Illinois lakes, however, are manmade. They range in scale from huge flood control reservoirs like Lake Shelbyville to worked-out stone quarries, gravel pits, and farm ponds.

In 1991, 70% of the water withdrawn by Illinois public water supplies (PWSs) came from lakes. Together, the City of Chicago and a number of its suburbs withdraw 1.1 billion gallons each year from Lake Michigan--more than half of all the water used by PWSs in the state and 75% of all the water drawn from surface sources.

Where it is available, groundwater is usually preferred over surface water as a source of public drinking water in Illinois because it needs less treatment and because construction of expensive above-ground storage reservoirs is unnecessary. But where groundwater is scant, as in the parts of southern and central Illinois, surface water is the more viable alternative. Eight percent of the state's current supply comes from these smaller manmade lakes.

In general, demand on Illinois' drinking water reservoirs has flattened in recent years after rising steeply since about 1930. Drought is more likely to stress Illinois' manmade lakes than is demand. Towns dependent on smaller manmade lakes (mainly in southern Illinois) are delicately poised between demand and supply. Recent estimates suggest that 25% of the state's PWSs would be inadequate in a 50-year drought without considerable water conservation--the result in many cases not only of increased demand but also the loss of storage capacity to siltation.

Building reservoirs has been a traditional response to drought in Illinois. Construction peaked in the early 1930s and again in the 1960s. (Figure 9-1) The most significant increase in reservoir acreage occurred in the late 1960s and early 1970s. Most of this acreage came in the form of large flood control reservoirs such as Lake Shelbyville and Carlyle Lake, although the Rend Lake Intercities Water System, established in 1972, also eased the threat of water shortages in a part of the state prone to them. However, lack of suitable dam sites, shortages of federal funding, and controversies over flooding of farmland and wildlife habitat have narrowed prospects for construction of major new manmade lakes. This is true even in communities like Bloomington-Normal that are threatened by drinking water shortages, although current studies seem likely to reveal additional groundwater resources in that area.

Figure 9-1. Amount of Lake Surface Area Impounded During 5-year Time Periods Since 1880

Source: Ecological Resources, Illinois Natural History Survey, 1994


Building a lake usually creates new habitat for some fish and wildlife (often in the form of shallows at the points where feeder streams enter lake basins proper.) Fish populations of Illinois lakes have proven sufficient to sustain catches by recreational and a few commercial fishermen. But sluggish backwater lakes are usually less rich habitats than the free flowing streams they replace. Manmade lakes permanently flood stream valleys whose forests and associated wetlands are usually adapted to seasonal wet and dry cycles.

Manipulation (inadvertent and otherwise) of the biological component of lakes also can upset natural systems. On Lake Michigan, overfishing and the introduction of exotic species have combined to unsettle that ecosystem for prey and predator species. The lake's stocks of commercially desirable chub, lake trout, and whitefish began to decline as early as the mid-1800s; a century later, one in seven native fish species in Lake Michigan was either extirpated or had suffered severe population crashes.


Lakes tend to collect not only sediment but most of the pollutants that are washed into them, and thus they function, in part, as environmental sinks. Eroded soil that both muddies water and fills in lake bottoms has significantly degraded lake ecosystems across Illinois. Lake Michigan is an exception. (Whereas siltation from farmed land is the major pollution source in other Illinois lakes, none of the Illinois portion of the Lake Michigan watershed is farmed.) Another factor degrading lakes is excessive algal and macrophyte growth caused by plant nutrients washed into them from farm fields and septic fields. Oxygen levels in some lakes are so chronically low that the composition of fish and other aquatic populations has shifted toward species more tolerant of such conditions.

The impact on lakes of atmospheric deposition of acids, as well as of toxic pollutants and nutrients, has not been well documented in Illinois. While the surface water in most parts of Illinois has a very high buffering capacity, this is not true in extreme southern Illinois. Given the amounts of such substances known to be at large in the atmosphere--total sulfur deposition downstate through the 1980s amounted to nearly 20 kilograms per hectare--significant negative impacts are probable in that part of the state.

As noted, many of Illinois' manmade lakes were built in the 1960s. Three decades may be too brief a time for all meaningful trends to emerge, even if data were systematically gathered, which they are not. Data collected at 659 lake water quality stations were analyzed for the years 1971 to 1991. Data that might reveal the presence in lake water of phenolics and the pesticides chlordane, dieldrin, and DDT were too sparse to be analyzed. The data show no significant trends for the common industrial metals such as cadmium, chromium, and lead; fecal coliform bacteria; pH; phosphorous; and two common compounds of nitrogen. Some lakes showed some deviations from that profile (mainly increased nitrogen and decreased heavy metals); that may reflect recent relative declines in industrial activity in otherwise agricultural watersheds.

The Illinois portion of Lake Michigan's drainage is not large, but it contains Illinois' heaviest concentrations of industry. Prior to 1890 the lake was used as a sink in which to dump sewage and factory wastes, and it has been classed the second most degraded of the Great Lakes. Levels of sulfate, cadmium, magnesium, sodium, potassium and chloride increased in the lake waters since the 1800s, although new protocols adopted in 1978 banned the discharge into the lake of some chemicals and reduced the discharge of others.

Unfortunately, many of the toxics already dumped there, such as PCBs, are long-lived. Large predator fish, such as the lake trout, feed at the top of the lake food chain and so accumulate in their fatty tissues organic chemical poisons originally ingested by their prey from bottom sediments; as a result, people are advised to limit the amount they eat of these otherwise commercially desirable fish. Lake Michigan water also carries heavy loads of phosphorous and nitrogen compounds as a result of runoff and fertilizer use in the watershed; these nutrients have led to increases in phytoplankton populations.

Tighter regulation has reduced direct discharges of industrial wastes into Lake Michigan, as have reductions in industrial activity. (Much of the more than 15% decline in manufacturing in Cook County between 1969 and 1989 occurred within the Lake Michigan drainage, and total cargo shipments in and out of Chicago deep-draft ports dropped 74% between 1974 and 1988.) Changes in indirect discharges of pollutants into the lake are harder to calculate. The extent of acid deposition over Lake Michigan from distant, inland sources via either wet or dry processes can be estimated but seldom has been measured on the lake surface itself; nor has the impact of atmospheric deposition of toxic pollutants and nutrients to lakes been documented.


Agriculture is the biggest threat to Illinois lakes. Many chemicals in common agricultural use have a strong affinity for fine soil particles. When the latter erode, these chemicals are carried with them into surface waters. The soil itself is a problem when it accumulates in quantity in lakes. Lake Pittsfield lost nearly a quarter of its volume to sedimentation in only 24 years. (Figure 9-2)

Figure 9-2. Storage Capacity Loss for Eight Illinois Reservoir Lakes

Source: Ecological Resources, Illinois Natural History Survey, 1994

While methods to remove silt and rejuvenate lakes have been successful in some places, replacing lost water storage capacity is expensive. To recover the 13% of capacity lost since the mid-1930s, the City of Springfield removed sediments from a small arm of Lake Springfield in 1988 at a cost of $10 million. Less obviously, sedimentation buries once-varied underwater topography under a blanket of mud, degrading it as a feeding and fish-spawning site.


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