Illinois has an abundance of water resources, including surface water, floodplains and groundwater. There are approximately 119,000 miles of streams, including major rivers such as the Des Plaines, Fox, Kankakee, Illinois, Rock, Sangamon, Kaskaskia, Little Wabash, and Big Muddy Rivers. The Mississippi, Wabash, and Ohio Rivers form the State’s western, eastern, and southern borders. Lake Michigan borders northeastern Illinois in the Chicago area. There are more than 91,000 inland lakes and ponds in Illinois. About three-fourths of Illinois’ inland lakes are man-made, including dammed streams, strip-mine lakes, borrow pits and other excavated lakes. Natural lakes include glacial lakes in the northeastern counties, sinkhole ponds in the southwest and oxbow and backwater lakes along major rivers.

The Department maintains approximately 60,000 bridges over streams and rivers. During the construction or rehabilitation of bridge structures, IDOT must comply with the Clean Water Act, the National Environmental Policy Act (NEPA), the Fish and Wildlife Coordination Act, and other related laws and regulations. To comply with these regulations the Department conducts environmental surveys. These field surveys range from the identification of Federal and State listed endangered and threatened species to the characterization of the physical (substrate), biological (fish, mussels, aquatic macroinvertebrates), and chemical (concentrations of heavy metals, chloride, dissolved oxygen) components of streams. The results of these surveys are used to obtain permits (section 404 of the Clean Water Act) from the Corps of Engineers and/or to meet the requirements of NEPA.  

The Department relies on information provided by other State and Federal agencies to determine potential impacts to water quality and other resources. The Illinois Environmental Protection Agency’s (IEPA) Illinois Integrated Water Quality Report and Section 303(d) List (http://www.epa.state.il.us/water/tmdl/303d-list.html) is used to determine the water quality standards, the designated uses of these water bodies, and the IEPA assessment of these uses. The Illinois Department of Natural Resources (IDNR) Natural Heritage database is used to identify aquatic endangered and threatened species locations, Natural Areas, and Nature Preserves. The IDNR Biological Stream Rating System is also used to identify those streams that are considered high quality based on the streams’ diversity of fish, mussels and aquatic macroinvertebrates. (http://www.dnr.state.il.us/orc/BioStrmRatings/).

The 1987 Illinois Groundwater Protection Act was implemented to protect groundwater resources form degradation. The Act allows for the establishment of both regulated groundwater recharge areas and groundwater protection areas. For transportation projects groundwater is assessed in terms of recharge areas to identify the potential for proposed highway corridors to contaminate shallow aquifers. Groundwater studies using monitoring wells are often performed in areas that are considered sensitive because they support plant communities containing endangered or threatened species or land areas that have been designated as Natural Areas or Nature Preserves. Potential impacts to these resources can involve interruption and/or change in groundwater flow patterns. Areas featuring karst topography are especially subject to groundwater contamination because these areas do not contain the usual filtering layers provided by soil so that any surface water contamination flows directly into the groundwater. An aquifer is an area containing saturated (with groundwater) soils and geologic materials that are sufficiently permeable to readily yield economically useful quantities of water to wells, springs, or streams under ordinary hydraulic gradients. A sole-source aquifer is one which supplies at least 50% of the drinking water consumed in the overlying area with no practicable alternative drinking water source. While Illinois has no sole-source aquifer, it has many shallow (sand and gravel) and deep (bedrock) aquifers. Public well water supplies are regulated through set back zones established by IEPA.

Every two years the IEPA issues an “Illinois Integrated Water Quality Report and Section 303(d) List”. Information on groundwater standards and groundwater quality can be found in this report, which can be accessed at http://www.epa.state.il.us/water/tmdl/303d-list.html.

Floodplains are areas of land that could be inundated by floodwaters from any source. Floodplains usually occur adjacent to streams, rivers and lakes. Transportation projects are assessed for the potential for impacts to floodplains, more specifically encroachments into the 100-year floodplain. The 100-year floodplain is defined as an area that has a 1% or 1 chance in 100 of experiencing a flood condition that exceeds any previous 1 year period. Federal regulatory requirements limit longitudinal impacts to floodplains as fill material in these areas would reduce the capacity of the area to accommodate flood waters. Transverse encroachments such as those that are involved in bridge crossings are acceptable as they do not result in long term modification of the waterway. Bridge structures located over major rivers such as the Mississippi must be coordinated with the U.S. Coast Guard to ensure that pier placement does not affect shipping channels. Coordination with the U.S. Army Corp of Engineers and the Department of Natural Resources Office of Water Resources (IDNR-OWR) is required to obtain the necessary permits for work within floodplains.

A regulated floodway is the channel of the stream plus any adjacent floodplain areas. These areas must be kept free of encroachment so that the 1% chance annual flood can occur without substantial increases in flood heights. Depending upon the location of the regulated floodplain, the regulatory authority may be the community within which the floodway is located, FEMA or the Illinois Department of Natural Resources (IDNR) Office of Water Resources. Floodplains are delineated by the Federal Emergency Management Agency (FEMA). The IDNR and FEMA are currently in the process of updating the Illinois floodplain maps by county. These maps are in digital format and can be accessed at http://www.illinoisfloodmaps.org/.