Surface Water Studies in the Rock River Region

Rock and Mississippi River watersheds of NW Illinois

Surface Waters in Illinois

The major surface waters in the Northwestern Illinois Water Supply Planning Region are the Rock and Mississippi Rivers, and their tributaries. Water levels in tributaries may be bolstered during times of drought using stored water from low-head impoundments or off-channel reservoirs.

Map of Illinois' public waters, with the Northwestern Illinois watersheds outlined in red.

Map of defined public waters throughout the state of Illinois. Illinois public waters, also known as  Illinois Public Bodies of Water  according to Illinois Administrative Code, are generally defined as all lakes, rivers, streams, and waterways that are or were navigable, are open or dedicated to public use and include all bayous, sloughs, backwaters, and submerged lands connected by water to the main channel or body of water during normal flows or stages ( Illinois General Assembly, 1984 ). Data to create this map came from the Illinois Department of Natural Resources.

Above is a map of the official public waters in Illinois. The Rock River Region includes the Illinois portion of the Rock River watershed and the portion of the Mississippi River's watershed that exists in Northwestern Illinois.

The Rock River watershed and a portion of the Mississippi River valley cross the Wisconsin-Illinois state line and meet at these rivers' confluence in Northwestern Illinois. These rivers are the primary surface waters of the Rock River Water Supply Planning Region.

The Rock River watershed covers 12 counties in Wisconsin and 15 counties in Illinois. Major tributaries of the Rock River include: the Pecatonica, Kishwaukee, and Green Rivers (highlighted in the map below).

The Rock River watershed and the portion of the Mississippi River valley in the Northwestern Illinois Water Supply Planning Region. Both catchments span the Wisconsin-Illinois state line.

The Rock River watershed and the portion of the Mississippi River valley in the Northwestern Illinois Water Supply Planning Region.

Other important tributaries of the Mississippi River in this region include the Galena, Apple, and Plum Rivers, as well as Rush and Johnson Creeks.

These watersheds in northwestern Illinois encompass six physiographic regions: the Wheaton Morainal Country, the Bloomington Ridged Plain, the Rock River Hill Country, the Galesburg Plain, the Wisconsin Driftless Section, and the Green River Lowlands.

Physiographic divisions of the Northwestern Illinois Water Supply Planning Region.

Publication by Leighton, Ekblaw, and Horberg (1948) titled, Physiographic Divisions of Illinois. This is Report of Investigations, no. 129 from the Illinois State Geological Survey.

In general, the region's topography was created from different glacial geologic processes. The Driftless Region, confined to Jo Daviess County and parts of Stephenson and Carroll Counties in Illinois, was not covered by glaciers, producing varied and rugged terrain, especially when compared to the rest of Illinois. This includes deeply carved and steep river valleys that discharge into the Mississippi River.

Land Cover and Use

The region's land cover is dominated by agriculture. In the Rock River Region, 81.6% of land cover in the Rock River watershed is agricultural, and 62.5% in the Mississippi River watershed.

Major developed areas in the region include Rockford (Winnebago County) and Rock Island (Rock Island County). However, developed land is spreading. Developed land accounts for 10.2% of the Rock River watershed (as of 2016), an increase from 5.3% in the 1990s.

Interactive Map: Land use / land cover in the Northwestern Illinois Water Supply Planning Region. While Herbaceuous/Hay was included in the map as a separate land cover, it was included in the Agriculture category for this analysis. The region is dominated by agricultural land use (gray); however, developed land extent (red) is increasing. Legend in lower left corner.

Surface Water Withdrawals

Total surface water withdrawals in the Rock River Region were 1,147.3 million gallons per day (mgd) in 2018. The overwhelming majority of withdrawn water (98%) is used by power generation for cooling purposes. Some power generation plants return the withdrawn water back to their source at a slightly increased temperature (such as the Exelon-Quad Cities Station on the Mississippi River). Other power plants use a recirculating cooling system, which consumes much of the withdrawn water via evaporation (such as the Exelon-Byron Station on the Rock River). Future regional surface water demand is greatly impacted by the future plans of these power generation plants.

Public water supply is the second largest use of surface waters. The cities of Rock Island, Moline, and East Moline rely on the Mississippi River for their public supply, and these cities should have sufficient water quantity to meet their future water demands. Most water withdrawn from the Mississippi River in the region is returned as either wastewater or sewage.

Considering the available streamflow in the Mississippi and Rock Rivers in the Rock River Region, surface water supply is sufficient to meet current and future demands.

Interactive Map: Surface water and groundwater withdrawals. Public water supplies and industrial water withdrawals from surface (pink) and groundwater (green) in the Northwestern Illinois Water Supply Planning Region. Legend in lower left corner.

Analyzing Streamflow

There are 23 active streamflow gauges monitored by the United States Geological Survey (USGS) in and adjacent to the Rock River Region, with another 20 where streamflow was previously monitored. Five gauges are along the Rock River and one is along the Mississippi River.  What is a streamflow gauge? The USGS explains .

Interactive Map: Active (green) and inactive (red) streamflow gauges. Streamflow monitoring stations maintained by the United States Geological Survey (USGS) in the Northwestern Illinois Water Supply Planning Region. Legend in lower left corner.

In general, differing watershed characteristics lead to varying hydrologic processes. The Mississippi and Rock Rivers typically experience stable flow levels, although they may exhibit extreme (high or low) river water levels. In general, both rivers provide stable and ample water supplies to meet the region's demands.

Of the region's major tributaries, the Pecatonica River provides a greater quantity and more reliable water supply than the Kishwaukee and Green Rivers.

For the region's smaller tributaries, supplemental water supply may be necessary if these smaller streams are to be used for water supply, due to minimal streamflow during drought periods.

Summary and Recommendations

The Mississippi and Rock Rivers have a large amount of available water even during severe drought conditions and are close to many major population centers such as Rockford and Rock Island. Both rivers also have dam systems that maintain water levels during drought conditions.

The physiographic setting of this region is favorable to groundwater recharge that promotes stable streamflows. Therefore, the Mississippi and Rock Rivers provide sufficient and reliable surface water supply for public water supply and power generation purposes.

The Pecatonica, Kishwaukee, and Green Rivers are other potential surface water sources that could potentially meet future water demands. Smaller tributaries in the region, however, do not provide sufficient water for direct withdrawal during drought conditions unless augmented by impoundments or off-channel storage.

An environmental flow assessment is needed to determine aquatic ecosystem water demand to determine minimum flow requirements in the region. Furthermore, long-term continuous streamflow records are critical to assess surface water supply and calibrate hydrologic models that may be used to assess surface water under changing conditions. Increased reporting of surface water withdrawals to the Illinois Water Inventory Program would aid in understanding public, power generation, and industry water use patterns.



Funding Information

This story map is part of a series about Water Supply Planning throughout the state of Illinois. Water Supply Planning is an ongoing research effort conducted by the Illinois State Water Survey and is funded by the   Illinois Department of Natural Resources Office of Water Resources.  

The Rock River watershed and the portion of the Mississippi River valley in the Northwestern Illinois Water Supply Planning Region.

Physiographic divisions of the Northwestern Illinois Water Supply Planning Region.

Map of defined public waters throughout the state of Illinois. Illinois public waters, also known as  Illinois Public Bodies of Water  according to Illinois Administrative Code, are generally defined as all lakes, rivers, streams, and waterways that are or were navigable, are open or dedicated to public use and include all bayous, sloughs, backwaters, and submerged lands connected by water to the main channel or body of water during normal flows or stages ( Illinois General Assembly, 1984 ). Data to create this map came from the Illinois Department of Natural Resources.