Illinois Water Supply Planning



Surface Water

The streams and rivers of Illinois flow abundantly during most time periods; but in cases of drought many streams experience flow rates well below their normal condition. The extent to which flow levels drop during dry periods varies considerably across the State and is dependent in great part upon the surficial geology. In many locations in northern Illinois, a considerable amount of the streamflow originates from shallow groundwater sources that are hydrologically connected to the stream. In these cases, the groundwater sources are often capable of sustaining the flow in streams during short-term periods of precipitation deficit. Even in cases when a long-term precipitation deficit reduces shallow groundwater levels thereby affecting low streamflow, the quantity of flow is typically much greater compared to other streams that lack a strong groundwater connection. In southern Illinois, for example, the lack of shallow groundwater sources can lead many streams to dry up following comparatively short periods without rainfall.

Ironically, the development of surface waters for water supply is least common in areas that have a plentiful and continuous supply of streamflow. Regions that have sustained low flows in streams also typically have plentiful groundwater resources, and in such circumstances groundwater is usually the preferred source because there is less need to treat the water. In areas where groundwater is plentiful, surface water is typically used as a resource only when large quantities of water are needed that exceed the sustainable supply of the groundwater resource.

In locations where low streamflows have been insufficient to provide a continuous supply of water, dams have often been built to impound the stream and create sufficient reservoir storage to sustain water use during periods of drought. Water supply for much of southern Illinois is provided through man-made reservoirs. When smaller volumes of storage are needed, an off-channel reservoir can alternatively be constructed into which river or stream water can be pumped.

Illinois is also surrounded by a number of major bodies of water, including Lake Michigan and the Mississippi, Ohio, and Wabash Rivers. Lake Michigan, in particular, has long provided a source of water for the Chicago metropolitan area, and currently provides nearly 60 percent of all drinking water used in Illinois. The diversion of water from Lake Michigan, much of which is initially used for the Chicago regionís water supply and then returned as treated wastewater into the Illinois Waterway, provides another large source of water that is used for industrial and hydroelectric purposes.

Existing Community Surface Water Supplies

Figure 1 gives the location of the roughly 100 community systems that obtain their water supply from surface water bodies in Illinois. As can be seen, most of these supplies are located in southern and central Illinois in regions where the availability of groundwater supplies are limited. Many of these supply systems serve multiple communities, such that more than 400 communities receive water from a surface source, including all the communities in the Chicago metropolitan area that receive water from Lake Michigan. Also shown are separate maps that the show the locations of systems categorized by three basic source types: Lake Michigan (Figures 2), Rivers and Streams (Figure 3), and Reservoirs (Figure 4). Table 1 shows the number of systems and average annual water use from each source category. Within the rivers and streams category, total water use is almost equally split between the 13 systems that withdraw water from the Mississippi and Ohio Rivers and the 11 systems systems that withdraw water from the intrastrate rivers such as the Fox, Kankakee, Kaskaskia, and Illinois Rivers. Most reservoir systems use stream impoundments as their primary source of supply; however, 11 systems rely on off-channel reservoirs.


Location of Illinois community water supply systems using surface sources.

Figure 1. Location of Illinois community water supply systems using surface sources.


Location of Illinois community water supply systems withdrawing from Lake Michigan.

Figure 2. Location of Illinois community water supply systems withdrawing from Lake Michigan.


Location of Illinois community water supply systems using direct withdrawals from rivers.

Figure 3. Location of Illinois community water supply systems using direct withdrawals from rivers.


 Location of Illinois community water supply systems in which the primary surface source is an impounding or off-channel reservoir.

Figure 4. Location of Illinois community water supply systems in which the primary surface source is an impounding or off-channel reservoir.

Number of supply systems
Total amount of water supplied
(million gallons per day)
Lake Michigan
Rivers and Streams

Table 1. Water sources for community surface water supply systems

Historical Water Supply Droughts

Surface water supplies in Illinois have been impacted by numerous droughts over the past century; among these being the droughts of 1914-1915, 1930-1934, 1940-1941, 1944-45, 1953-1957, 1963-1964, 1976-1977, 1980-1981, 1988-1989, 1999-2000, and 2005. Of these, the two major droughts that have had the greatest overall impact to Illinois water resources have been the droughts of 1930-1934 and 1953-1957. Several of the other drought periods were locally intense, producing water supply concerns for portions of Illinois, but were generally of shorter duration and did not cause the broad statewide impact experienced during the 1930-1934 and 1953-1957 droughts. Table 2 lists the number of community surface water supplies that experienced shortages during selected drought periods. Because this table includes estimates from different sources, there is likely some variation in the criteria used to identify a drought shortage. Although Illinois communities as a whole are better prepared to face droughts than they were in the 1930s and 1950s, a study of the adequacy of surface water supplies in Illinois (McConkey and Singh, 1989) indicated that 27 communities would be at risk of experiencing shortages during a 50-year drought. The most recent 2010 assessment of drought vulnerability ( in surface water supplies indicates that indicates that 21 communities are now considered inadequate or at risk .

Drought years
Number of communities impacted by the drought
Total number of surface water systems susceptible to drought

Table 2. Community Surface Water Supplies Experiencing Drought Shortages

Lake Michigan Diversion

Since the flow of the Chicago River was reversed in the early 20th century, water from Lake Michigan that is used in Illinois has been diverted to the Mississippi River watershed. That diversion is governed by a 1967 U.S. Supreme Court decree and a 1980 amendment. The 1967 decree limits the amount diverted to 3,200 cubic feet per second, averaged over 5 years. The 1980 amendment made technical changes to permit Illinois' effective use and management of the permitted diversion, increasing the averaging period to 40 years and adding a goal to reduce withdrawals from the Cambrian-Ordovician (deep bedrock) aquifer.

U.S. Supreme Court Decree (1967) (pdf ~25kb)

U.S. Supreme Court Amendment (1980) (pdf ~27kb)

Compliance Report: Court Decree & Allocation Program (pdf ~35kb)

Global Warming Here At Home: How is Lake Michigan Faring? (pdf ~1.1mb) -- Presented by Chief Derek Winstanley, Illinois State Water Survey, at the League of Women Voters "Climate Matters" Symposium in Glenview on October 20, 2007.

U.S. Army Corps of Engineers Lake Michigan Diversion Accounting Reports

Illinois State Water Survey

2204 Griffith Dr
Champaign, IL 61820-7463

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