ENVIRONMENTAL SETTING OF THE
UPPER ILLINOIS RIVER BASIN AND IMPLICATIONS FOR WATER QUALITY
By Terri L. Arnold, Daniel J. Sullivan, Mitchell A. Harris, Faith A. Fitzpatrick, Barbara C. Scudder, Peter M. Ruhl, Dorothea W. Hanchar, and Jana S. Stewart 
ABSTRACT
The upper Illinois River Basin (UIRB) is the 10,949 square mile drainage area upstream of Ottawa, Illinois on the Illinois River. The UIRB is one of 13 studies that began in 1996 as part of the U.S. Geological Surveys National Water-Quality Assessment Program. A compilation of environmental data from Federal, State, and local agencies provides a description of the environmental setting of the UIRB. Environmental data include natural factors such as bedrock geology, physiography and surficial geology, soils, vegetation, climate, and ecoregions; and human factors such as land use, urbanization trends, and population change. Characterization of the environmental setting is useful for understanding the physical, chemical, and biological characteristics of surface and ground water in the UIRB and the possible implications of that environmental setting for water quality. Some of the possible implications identified include depletion of dissolved oxygen because of high concentrations of organic matter in wastewater, increased flooding because of suburbanization, elevated arsenic concentrations in ground water because of weathering of shale bedrock, and decreasing ground-water levels because of heavy pumping of water from the bedrock aquifers. 
 INTRODUCTION
The National Water-Quality Assessment (NAWQA) Program of the U.S. Geological Survey (USGS) was designed to provide a national view of the status and trends of the Nations water resources (Hirsch and others, 1988). On this national scale, consistent monitoring of water-quality conditions over time, and a nationally comparable description of water resources are provided. Concerns for the unique hydrologic systems of individual drainage basins are balanced with nationally consistent design criteria. The program utilizes an interdisciplinary approach to study basin ecosystems by incorporating ground- and surface-water hydrology, and biology. Results from NAWQA studies will help in understanding how natural and human factors affect water quality.
The building blocks of the NAWQA program are approximately 60 major surface-water drainage basins, called study units (fig. 1). The number of study units varies with changes in priority and funding for the NAWQA program. These drainage basins include about one-half of the area of the conterminous United States; supply water to approximately 65 percent of the population that uses public water supplies; and represent a variety of natural and man-made settings, and water-quality issues. The study units are divided into three groups that operate on a 10-year rotational schedule. This schedule is designed to provide periods of initial planning and analysis of available data, intensive data collection and interpretation, report writing, and low-level monitoring activities (fig. 2). (Gilliom and others, 1995).
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In 1986, the upper Illinois River Basin (UIRB) was selected as one of seven study units to participate in the pilot phase of the National Water-Quality Assessment Program. The UIRB pilot study focused on surface water (Mades, 1987), but the study that began in 1996 includes surface-water, ground-water, and biology studies (Friedel, 1998). Fourteen U.S. Geological Survey reports and several journal articles were published as a result of the pilot study. A bibliography of the USGS reports that were published as part of the UIRB NAWQA pilot study are listed in appendix A of this report. The lower Illinois River Basin (LIRB) study unit of the NAWQA Program is currently assessing the surface- and ground-water quality and biology for the drainage area of the Illinois River downstream of the UIRB (Warner, 1998). 
Purpose and Scope
This report describes the environmental setting of the UIRB study unit and its possible implications for water quality. The environmental setting comprises the natural and human factors that define the hydrologic characteristics of the basin. Natural factors include bedrock geology, physiography and surficial geology, soils, vegetation, climate, and ecoregions. Human factors include land use, urbanization trends, and population change. Each of the natural and human factors affects the physical, chemical, and biological characteristics of surface and ground water. The interrelatedness of these factors and characteristics must be considered when determining the implications for water quality in the UIRB. The information contained in this report is a compilation of data collected from Federal, State, and local agencies that will facilitate selection of monitoring sites and interpretation of water quality in the basin.
Location and Extent of the Study Area
The upper Illinois River Basin includes parts of 16 counties in northeastern Illinois (62 percent of the basin), 13 counties in northwestern Indiana (28 percent of the basin), 7 counties in southeastern Wisconsin (10 percent of the basin), and 1 county in southwestern Michigan (less than 0.1 percent of the basin). Because the part of the basin that is in Michigan is small, it will not be specifically discussed in this report. The drainage area of the UIRB is 10,949 mi2 upstream of Ottawa, Illinois, on the Illinois River, and contains the following major subbasins (hydrologic units): Kankakee River, Iroquois River, Fox River, Des Plaines River, Chicago River, and Illinois River (fig. 3). 
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The Kankakee River Basin (hydrologic unit 07120001) drains the largest part of the UIRB, 27.6 percent. The Kankakee River flows from Indiana towards Illinois in a general northeast to southwest trend, and turns northwestward at the confluence with the Iroquois River about 4.8 miles upstream of Kankakee, Illinois (fig. 3). The Kankakee River joins the Des Plaines River to form the Illinois River near the Grundy and Will County line in Illinois. 
The Iroquois River Basin (hydrologic unit 07120002) lies to the south of the Kankakee River Basin, and makes up 19.6 percent of the UIRB. The Iroquois River flows from Indiana into Illinois from northeast to southwest, turns northwestward at Watseka, Illinois, and joins the Kankakee River near Kankakee, Illinois (fig. 3). Because the Iroquois River is a tributary to the Kankakee River, it is often included in discussions of the Kankakee River Basin.
The upper Fox River Basin (hydrologic unit 07120006) and the lower Fox River Basin (hydrologic unit 07120007) make up the Fox River Basin, which is 24.3 percent of the UIRB. The Fox River flows north to south from Wisconsin into Illinois, turns to the southwest at Aurora, Illinois, and joins the Illinois River at Ottawa, Illinois near the terminus of the UIRB (fig. 3).
The Des Plaines River Basin (hydrologic unit 07120004) is 13.3 percent of the UIRB. The Des Plaines River flows north to south from Wisconsin into Illinois, turns southwest at Lyons, Illinois, follows the Chicago Sanitary and Ship Canal, and joins the Illinois River at the confluence of the Illinois and Kankakee Rivers near the Grundy and Will County line (fig. 3). The Des Plaines River drainage area includes 673 mi2 that originally drained to Lake Michigan through the Chicago and Calumet Rivers.
The Chicago River Basin (hydrologic unit 07120003) is the smallest portion of the UIRB, 6 percent. The Chicago River, which flowed into Lake Michigan prior to the early 1900s, now flows from north to south through Lake and Cook Counties in Illinois and joins the Chicago Sanitary and Ship Canal near Chicago, Illinois. The Canal flows into the Des Plaines River at Joliet, Illinois (fig. 3). As a tributary of the Des Plaines River, the Chicago River is often included in discussions of the Des Plaines River Basin.
The Illinois River (hydrologic unit 07120005) is 9.2 percent of the UIRB. The mainstem flows from the confluence of the Des Plaines and Kankakee Rivers westward towards Ottawa, Illinois (fig. 3). The Illinois River, lower reaches of the Des Plaines River, and two canal systems in the Chicago metropolitan area provide a navigable link between Lake Michigan and the Mississippi River.
Acknowledgments
Assistance and data provided by members of the UIRB NAWQA liaison committee and their affiliated agencies are recognized as an integral part of this report. Committee members represent Federal, State, local, and private agencies with interest in the upper Illinois River Basin. 
ENVIRONMENTAL SETTING
Natural Factors
Natural factors are those natural environmental characteristics such as bedrock geology, physiography and surficial geology, soils, vegetation, climate, and ecoregions. These factors can affect the hydrologic characteristics of the UIRB and descriptions of these factors provide the basis for understanding the environmental setting of the UIRB study area.
Bedrock Geology
The composition of bedrock (consolidated) materials affects the flow, transport properties, and chemistry of both surface and ground water in the UIRB. Structural features such as joints and faults also have important effects on ground-water storage and enhancing or reducing the potential for water movement.
The entire UIRB is underlain by Precambrian granitic rocks at depths ranging from about 1,000 ft below land surface in the northern part of the basin to about 7,000 ft in the southeastern part. The Precambrian rocks are overlain by sedimentary rocks of the Cambrian System. These sedimentary rocks are predominately sandstone, and include three of the most heavily used aquifers in the study area. The Cambrian rocks range in thickness from about 1,000 ft in the northern part of the study area to about 5,000 ft in the southeastern part, and are entirely below land surface in the UIRB. Ordovician-aged rocks overlie the Cambrian rocks and are composed predominately of limestone and dolomite, but also include some sandstone and shale. The Ordovician rocks are less than 1,000 ft thick in the northern and western parts of the UIRB where they are exposed in heavily eroded areas, and about 1,500 ft thick in the southeastern part (Willman and others, 1975). Where present, the Maquoketa Shale confines the Cambrian-Ordovician bedrock aquifer.
The uppermost bedrock units of the Kankakee River Basin are predominately Silurian-Devonian dolomite and limestone, and shale (fig. 4a). In Indiana, the limestone and dolomite are part of the Muscatatuck Group and Wabash Formation (Gray and others, 1987). The shale units in Indiana are the Ellsworth, Antrim, and New Albany shales (table 1). In Illinois, the limestone and dolomite are undifferentiated, and the shale is the Maquoketa Shale (Willman and others, 1975) (table 1).
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The uppermost bedrock units of the Iroquois River Basin are predominately Silurian-Devonian dolomite and limestone, and shale, Mississippian siltstone, and Pennsylvanian sandstone. In Indiana, the limestone and dolomite compose the Muscatatuck Group, Wabash Formation, and Louisville Limestone; the shale is the New Albany shale; the siltstone is part of the Borden Group, and the sandstone is part of the Raccoon Creek Group (Gray and others, 1987). In Illinois, the limestone and dolomite is undifferentiated, the shale is part of the Upper Devonian and the Mississippian Kinderhookian Series, the siltstone is part of the Valmeyeran Series, and the sandstone is the Spoon Formation (fig. 4; table 1). The Kentland Dome, a small structural feature (approximately 5 mi2), is located in southern Newton County, Indiana (fig. 4b). This feature is represented by dipping Ordovician and Silurian rocks that have been uplifted approximately 2,000 ft (Fenelon and others, 1994).
The uppermost bedrock units of the upper Fox River Basin are predominately undifferentiated Silurian-Devonian dolomite and limestone; and Ordovician shale of the Maquoketa Shale Group in Illinois, and the Maquoketa Shale and Sinnipee Groups in Wisconsin (fig. 4; table 1). The uppermost bedrock units of the lower Fox River Basin are predominately undifferentiated Silurian-Devonian dolomite and limestone, Ordovician limestone of the Galena-Platteville Group, Ordovician dolomite of the Prairie du Chien Group, Ordovician shale of the Maquoketa Shale Group, and Cambrian-Ordovician sandstone. The Prairie du Chien Group is exposed along the Fox River Valley (Willman and others, 1975) (fig. 4). The Sandwich Fault is evident in southern De Kalb County in Illinois where Cambrian rocks are adjacent to Ordovician rocks (fig. 4).
The uppermost bedrock units of the Chicago and Des Plaines River Basins are predominately undifferentiated Silurian-Devonian dolomite and limestone, and Ordovician shale of the Maquoketa Shale Group (fig. 4). The Des Plaines Disturbance fault, unexposed at the surface, is located in northern Cook County, Illinois near the Des Plaines and Chicago River Basin drainage divide (fig. 4). The origin of the fault has been explained as being from either volcanic activity or from meteoric impact (Willman and others, 1975).
The uppermost bedrock units of the Illinois River Basin are predominately Pennsylvanian shale of the Carbondale Formation, Ordovician shale of the Maquoketa Shale Group, and Ordovician limestone and dolomite of the Galena-Platteville Formation (fig. 4) (Willman and others, 1975). The Carbondale Formation contains coal seams and is a source for mining in the UIRB (fig. 5).
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Physiography and Surficial Geology
Studying the physiography provides a generalized description of the natural characteristics of an area by aggregating areas that were formed under similar conditions and have similar surficial geology. A description of these factors is an important part of the environmental setting because the composition and origin of surficial deposits affect soil composition and water chemistry. When water flows through the interstitial spaces of surficial deposits, its chemical composition and/or the minerals in the surficial deposits can be changed by processes like ion exchange. The surficial sand, and sand and gravel deposits form the important unconsolidated aquifers in the basin. Surficial geologic materials are broken down by weathering processes and form the parent material for most soils in the UIRB. Most of the unconsolidated surficial deposits in the basin are of glacial, fluvial, or aeolian origin.
Surficial geologic materials were deposited and land-surface features were was formed in the UIRB during five major glacial periods. With each glacial advance and retreat, succeeding glaciers modified the deposits left during previous periods. The most recent period of glaciation, called the Wisconsinan period, occurred about 17,000-12,000 years ago (Willman and Frye, 1970; Gilbert, 1980). The Lake Michigan, Saginaw, and Huron-Erie glacial lobes once covered the UIRB (fig. 6). Typical glacial features such as till and outwash plains, moraines, kettles, kames, and drumlins are found in various parts of the basin, but moraines are the most prevalent glacial feature (fig. 7). Bedrock has been exposed in places along the Illinois, Des Plaines, and Kankakee River valleys by glacial processes such as meltwater floods (Illinois State Geological Survey, 1973). Except for these bedrock exposures in major river valleys, unconsolidated glacial (Quaternary) deposits cover most of the study area (fig. 8). 
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In recent times, glacial deposits and features have been modified by wind and water erosion (Piskin and Bergstrom, 1975). The glacial and erosional processes are the sources of the present-day landforms in the UIRB; these landforms are the basis for the following physiographic definitions. Overall, topography in the UIRB varies from 443 feet above sea level in the Chicago River Basin and the western parts of the Kankakee River Basin to 1,250 feet above sea level in the northern extremes of the upper Fox River Basin (fig. 9).
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The UIRB lies in the Central Lowlands physiographic province and is divided into two physiographic sections: the Great Lake Section and the Till Plains Section (Fenneman, 1938). Leighton and others (1948) divided the Illinois part of these sections into two subsections each. In the Illinois part of the UIRB, the Great Lake Section was divided into the Chicago Lake Plain and the Wheaton Morainal Plain, whereas the Till Plains Section was divided into the Kankakee Till Plain and the Bloomington Ridged Plain. For the Indiana part of the UIRB, Schneider (1966) subdivided the Great Lake and the Till Plains physiographic sections into the Northern Moraine and Lake Region and the Tipton Till Plain. In the area of the UIRB, the Northern Moraine and Lake Region subsection was further divided into the Valparaiso Morainal Area, the Kankakee Outwash and Lacustrine Plain, and the Steuben Morainal Lake Area (Schneider, 1966).
Although the physiography has been delineated and named in a various ways, defining characteristics of the physiographic sections correspond well across State lines. The Valparaiso Morainal Area in Indiana corresponds to the Wheaton Morainal Plain in Illinois. The Kankakee Outwash and Lacustrine Plain in Indiana corresponds to the Kankakee Till Plain in Illinois. The Tipton Till Plain in Indiana corresponds to the Bloomington Ridged Plain in Illinois. For the purposes of this report, the Illinois terms will be used when describing physiography that corresponds across Illinois, Indiana, and Wisconsin State lines. (fig. 10)
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The Chicago Lake Plain (4.7 percent of the basin) is approximately the area that is now metropolitan Chicago in the Chicago River Basin. It is a relatively flat, glacio-lacustrine deposit formed by the slow moving waters of glacial Lake Chicago. Glacial Lake Chicago was formed when meltwater accumulated behind the Wisconsinan glaciers Valparaiso moraine. The meltwater eventually broke through a low spot in the moraine and drained back into the present day Lake Michigan Basin (Horsley, 1986). The Chicago Lake Plain consists of poorly drained lake clay and silt overlying the limestone of the Niagara Formation (Mayer and Wade, 1969) and lake sand and gravel (fig. 8). These deposits are the Carmi and Dolton Members of the Equality/Atherton Formation. Clayey till of the Wedron Formation (Wadsworth Till Member) also is present and is deposited as moraines (table 2). Surficial deposits in the Chicago Lake Plain are generally less than 200 ft thick with areas of less than 50 ft thick near the Des Plaines River, Chicago River, Calumet Sag Channel, and Lake Michigan shore (fig. 11). Local topography typically varies less than 50 ft with a minimum elevation of 580 ft and a maximum elevation of 699 ft above sea level.
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The Wheaton Morainal Plain (32 percent of the basin) is approximately the area of the upper Fox River Basin and the northern portion of the Kankakee River Basin (fig. 10). This area is characterized by gently rolling Wisconsinan-age moraines that are approximately parallel to the Lake Michigan shoreline. The Wheaton Morainal Plain is predominately clayey till of the Wadsworth Till Member/Oak Creek Formation, and sandy loamy till and sand and gravel of the Haeger Till Member/New Berlin Formation (fig. 8, table 2). Other surficial deposits in this area include lake clay and silt, and alluvium. Bedrock is exposed along the downstream reaches of the Des Plaines River and along parts of the Fox River. Surficial deposits in the Wheaton Morainal Plain are generally between 50 and 200 ft thick; however, deposits in some northern areas of the UIRB are less than 50 ft thick, and deposits in many southern areas are greater than 200 ft thick (fig. 11). Relief is generally less than 100 ft with a minimum elevation of 505 ft and a maximum elevation of 1,188 ft above sea level (fig. 9). As the Wisconsinan glacier retreated through the upper Fox River Basin, many lakes (kames) and some swamps were formed by isolated buried ice blocks melting in depressions made by the glacier (Mades, 1987).
The Kankakee Till Plain (37 percent of the basin) is approximately the area covered by the western three-quarters of the Kankakee River Basin, the Illinois River Basin, and the western part of the Iroquois River Basin (fig. 10). The Kankakee Till Plain is an outwash plain formed by the variable gradient, variable volume, and variable velocity of glacial floods (Mickelson and others, 1984). Surficial deposits are predominately outwash sand and gravel of the Henry/Atherton Formation (Mackinaw and Batavia Members in Illinois, undifferentiated in Indiana) (fig. 8, table 2), but also include alluvium and fill materials. Bedrock is exposed along the downstream reaches of the Kankakee River. Surficial deposits are generally less than 200 ft thick; however, some deposits in the Kankakee River lowland are less than 50 ft thick, and some deposits in the upland are greater than 200 ft thick (fig. 11). Local changes in elevation are generally less than 100 ft with a minimum elevation of 482 ft and a maximum elevation of 899 ft above sea level (fig. 9). 
The Bloomington Ridged Plain (22 percent of the basin) is approximately the area of the lower Fox River Basin and the southern part of the Iroquois River Basin (fig. 10). Surficial deposits in this area are mostly loamy till (Malden Till Member) and clayey till (Yorkville Till Member) of the Wedron Formation (fig. 8, table 2) deposited as moraines by the Lake Michigan glacial lobe (fig. 6). The lower Fox River Basin has well developed rolling moraines whereas the southern part of the Iroquois River Basin has lower and more gently rolling moraines. In the Iroquois River Basin, the Bloomington Ridged Plain is mostly loamy till (unnamed in Illinois; Trafalgar Formation in Indiana), and lake clay and silt (Carmi Member of the Equality Formation) deposited by the Huron-Erie glacial lobe (fig. 6). Surficial deposits are generally less than 200 ft thick; however, there are areas in both basins that are more than 400 feet thick (fig. 11). Topography in upland areas of the Bloomington Ridged Plain is flat or gently rolling. Lowland areas, such as along the Fox and Illinois Rivers, are very hilly and rugged with local relief as great as 300 ft where the rivers are deeply incised and bedrock is exposed (Piskin and Bergstrom, 1975). Topography generally varies less than 300 ft with a minimum elevation of 472 ft and a maximum elevation of 981 ft above sea level (fig. 9).
The Steuben Morainal Lake Area (4.3 percent of the basin) is the eastern-most part of the Kankakee River Basin (fig. 10). This area is characterized by hilly knob and kettle glacial terrain. Knobs are formed when a melting glacier deposits hills of sand and gravel into a depression left by the ice. Kettles are formed when ice blocks from a glacier melt in depressions left by the ice. The kettles form the basins for many kames and peat bogs in the area. Surficial deposits are predominately loamy till of the Trafalgar Formation deposited by the Saginaw glacial lobe (fig. 6) and are generally greater than 200 ft thick (fig. 11). Topography is generally less than 100 ft with a minimum elevation of 721 ft and a maximum elevation of 899 ft above sea level (fig. 9) (Schneider, 1966).
Soils
The texture and composition of soil affects the chemistry and infiltration rate of water. Soil texture and composition predominately is determined by climate, parent material, and vegetation. Temperature and precipitation determine the rate of chemical weathering and the development of soil materials. The more humid the climate, the faster soils can develop. During the Wisconsinan period, when most of the soils in the UIRB formed, the climate was more humid and warmer than at present (Fehrenbacher and others, 1984).
In the UIRB, parent materials of soil are generally the underlying surficial deposits in which the soils are developed. The parent materials can be identified from the map showing Quaternary deposits (fig. 8). Some of the soil along the major river valleys, such as the Illinois River valley, is from decomposition of exposed bedrock (Fehrenbacher and others, 1984); but most of the soil in the UIRB is derived from the glacial surficial deposits discussed earlier in this report. 
In addition to glacial materials, alluvium is a prime parent material for younger soils (entisols) around stream banks and flood plains in the UIRB (fig. 12). Entisols generally are light colored and sandy (Fehrenbacher and others, 1984); have poor horizon definition (Muller and Oberlander, 1984); and, on a relative scale, are moderately to highly permeable (fig. 13). Entisols are common in the Kankakee and Illinois River Basins and cover 5 percent of the UIRB.
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Soil development also is affected by vegetation because of the organic matter it contributes to the soil. Soils formed under grasslands tend to be dark and high in organic content (mollisols); in contrast, soils formed under forested land tend to be light colored and low in organic content (alfisols) (Fehrenbacher and others, 1984). Mollisols generally contain greater than 1 percent organic matter in their surficial horizon, whereas alfisols generally contain less than 1 percent organic matter.    Mollisols cover 59 percent of the UIRB and dominate the Iroquois, Illinois, Chicago, and lower Fox River Basins (fig. 12). Relative soil permeability of mollisols in the UIRB is low to very low (fig. 13). Alfisols cover 33 percent of the UIRB, and are common around dry morainal areas and in gently rolling, well drained areas of the Kankakee, Des Plaines, and upper Fox River Basins (fig 12.).   Relative soil permeability of alfisols is low to moderate (fig 13). 
Histosols, soils from mostly organic parent materials, cover 2 percent of the UIRB and are scattered throughout the Fox, Kankakee, and Iroquois River Basins (fig. 12). Generally, histosols have greater than 20 percent organic content in their surficial horizon and are found in low lying areas that remain wet most of the time (Fehrenbacher and others, 1984). In the UIRB, histosols have relatively very low permeability (fig. 13). 
 Inceptisols are soils with poorly developed soil horizons. These soils cover approximately 1 percent of the UIRB, and are found in the Fox and Des Plaines River Basins (fig. 12). On a relative scale, inceptisols are moderately permeable (fig. 13).
Vegetation
Vegetation is an important part of soil formation and the water cycle. The type and quantity of plant material contributes to the organic content of the soil and the activity of root growth affects breakdown of soil components. In addition, vegetation intercepts precipitation and also draws moisture from the soil that could otherwise infiltrate to ground water.
 The central portion of the UIRB lies in the eastern lobe of the grassland region of central North America and the eastern and northern portions lie in the deciduous forest regions. Prior to European settlement of the area, the UIRB vegetation communities included prairies, savannas, woodlands, and wetlands. Today, only fragments of the presettlement vegetation remain; the dominant modern vegetation is agricultural. 
Plant communities that would exist if humans were not present in the basin and a climax plant community were allowed to develop naturally are called potential-natural vegetation. Kchler (1964) classified these potential plant communities based on the one to three (usually two) dominant plant genera present in an area. On a national scale, six potential-natural vegetation types are defined by Kchler (1964) in the UIRB: oak-hickory forest, beech-maple forest, maple-basswood forest, oak savanna, bluestem prairie, and mixed bluestem prairie and oak-hickory forest (fig. 14).
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The potential-natural vegetation of the central part of the UIRB was mostly a mixture of tallgrass bluestem prairie and oak-hickory forest prior to European settlement. In the western part of the basin, tallgrass prairie typically grew as broad expanses on the flat uplands and forests grew on protected slopes, in ravines, and in river valleys. The prairie ecosystem was formed and maintained by a harsh climate, grazing by free-ranging large animals, and periodic fires. Tallgrass prairie plants typically have extensive deep root systems that build soil fertility. During the mid-nineteenth century, natural prairie land was converted to agricultural land when the metal plow was introduced and settlers discovered that prairie soils were more productive than forest soils (Illinois Department of Energy and Natural Resources, 1994). As a result, only small fragments of prairie remain in the UIRB.
A national prairie restoration effort is on-going at the Midewin National Tallgrass Prairie in Will County, Illinois. This 24-mi2 area was established in 1996 on the grounds of the former Joliet Army Ammunition Plant (1940-96) near the confluence of the Des Plaines and Kankakee Rivers. This is the Nations first Federally designated tallgrass prairie, and, in conjunction with nearby natural areas, forms the 62-mi2 Prairie Parklands Macrosite, which is a mix of public, private, and corporate lands managed to protect diversity (Midewin National Tallgrass Prairie, 1998).
Further east, in the upper reaches of the Kankakee River Basin, prairie vegetation characteristically would have decreased and been replaced by oak-hickory and beech-maple forest vegetation. In the upper Fox and Des Plaines River Basins, the tallgrass prairie would have graded northward into oak savanna, and eventually into a maple-basswood forest. Oak savanna is tallgrass prairie with deciduous trees, characteristically Bur oak (Quercus macrocarpa), scattered singly or in groves (Kchler, 1964). Similarly, Finley (1976) depicted the original vegetation of the Wisconsin portion of the UIRB as a mosaic of oak forest, oak savanna, prairie, maple-basswood-oak forest, and herbaceous and shrub wetlands.
Climate
The climate of the UIRB is classified as humid continental because of the cool dry winters and warm humid summers. Both seasons are usually dominated by cool dry air from Canada (continental polar), warm moist air from the Gulf of Mexico (maritime tropical), and occasionally warm dry air from the Pacific Ocean (maritime polar). The combinations of cool dry and warm moist air are the sources of most of the precipitation in the basin. However, any combination of these three air masses can be over the basin at any time. Large daily fluctuations in temperature and precipitation can result from this combination (Muller and Oberlander, 1984). 
The average annual temperature for the UIRB (1961-90) ranged from 46o F in the north of the basin to 51o F in the south of the basin. The coolest temperatures occurred during the winter months of December, January, and February; the minimum average monthly temperature occurred in January. During the period of record through 1995, minimum average January temperatures ranged from less than 4o F in the northwestern Wisconsin portion of the basin to more than 11o F in the northeastern Indiana and southern Illinois portions of the basin. The average basin-wide January temperature ranged from 18o F to 25o F during the same period (National Oceanic and Atmospheric Administration, 1995).
The warmest temperatures in the UIRB (1961-90) occurred during the months of June, July, and August; the maximum average monthly temperature occurred in July.   Maximum average July temperatures during the period of record through 1995 ranged from less than 77o F in the northern portions of the basin to greater than 82o F in the southern portion of the basin. The average basin-wide July temperature ranged from 72o F to 75o F during the same period (National Oceanic and Atmospheric Administration, 1995). Lake Michigan has a moderating effect on temperature near the shoreline because the water in Lake Michigan heats up and cools down more slowly than the surrounding land.
Average annual precipitation, including the liquid equivalent of snowfall, for the period 1961-90 ranged from less than 32 in. in the northern Wisconsin portion of the basin to more than 38 in. near the southern and eastern Lake Michigan shoreline in the Indiana portion of the basin (fig. 15). During the same period, the minimum mean monthly precipitation occurred in February (1.54 in.) and the maximum mean monthly precipitation occurred in June (3.61 in.) (National Oceanic and Atmospheric Administration, 1995). About 50 percent of the average annual precipitation, approximately 16 to 18 in., generally occurs during the growing season from May through mid-October. 
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On average, for the period of record through 1995, annual snowfall (including snow, ice, sleet, and hail) varied from less than 30 in. a year in the southwestern portion of the basin in Illinois to greater than 65 in. a year east of Lake Michigan in the extreme northeastern portion of the basin in Indiana. West of Lake Michigan, in Wisconsin, average annual snowfall for the same period was less than 50 inches per year.   Lake Michigan produces seasonal effects on precipitation in the basin for areas near the shoreline (Changnon, 1968). 
 Evapotranspiration (moisture released from plants) returns an estimated 70 percent of the average annual precipitation to the atmosphere. Based on this percentage, average annual evapotranspiration ranges from about 22 in. in the northern portion of the UIRB to about 27 in. in the eastern portion. Approximately 75 percent of the average annual evapotranspiration usually occurs during the growing season (U.S. Geological Survey, 1970). Evapotranspiration normally exceeds precipitation during the growing season and, thereby, depletes available soil moisture. During the nongrowing season, precipitation generally exceeds evapotranspiration and, thereby, replenishes soil moisture and recharges ground-water sources.
Ecoregions
Ecoregions are areas of relatively homogeneous ecological systems or relations between organisms and their environments (Omernik, 1987). Ecoregion classification builds on single-purpose geographical classifications (such as physiography, climate, or soils) to create a framework for understanding regional patterns, and management of aquatic and terrestrial resources. Classification of ecoregions can assist environmental planners and decision makers in addressing environmental problems on a regional scale. Based on this regionalization, chemical or biological standards within a region can be established in order to evaluate environmental conditions. 
Omerniks (1987) ecoregion classification, which divided the conterminous United States into 76 level-3 ecoregions, is used by the NAWQA program to evaluate regional water-quality patterns. Omernik primarily used maps of land use, potential-natural vegetation, surficial geology, physiography, and climate to develop his classification which resulted in areas that were relatively homogenous in their characteristics. Omernik and Gallant (1988) defined the most typical areas of each level-3 ecoregion for the upper Midwest States. A most typical area of an ecoregion has all of the characteristics that typify the level-3 ecoregion.
Over 80 percent of the UIRB is classified as the Central Corn Belt Plains ecoregion. Aside from the Chicago metropolitan area, land use in the Central Corn Belt Plains is mostly corn and soybean cultivation for livestock feed crops and some livestock production. The soils are typically mollisols. Historically, vegetation in the Central Corn Belt Plains was a mosaic of bluestem prairie and oak-hickory forest. The prairie typically covered the flat uplands, and the forest typically occupied stream valleys and moraines. High stream turbidity and sedimentation are a problem in the streams of this ecoregion (Omernik and Gallant, 1988).
Parts of the upper Fox and upper reaches of the Des Plaines River Basins, about 10.5 percent of the UIRB, are classified in the Southeastern Wisconsin Till Plains ecoregion. In contrast to the feed-grain cultivation in the Central Corn Belt Plains, dairy and livestock production are the predominant land use in this ecoregion. Other land uses include cultivation for livestock feed-grain, canning and truck crops, pasture, and woodlots. Watersheds are generally smaller in the Southeastern Wisconsin Till Plains than in the Central Corn Belt Plains, typically covering from about 386 to 956 mi2. Approximately 25 to 30 percent of the streams are intermittent in this ecoregion. The Wisconsin Till Plains ecoregion lies within the Wheaton Morainal Plain physiographic subsection (fig. 10). The soils are typically alfisols and mollisols. The historical vegetation in the Southeastern Wisconsin Till Plains was a mixture of tallgrass prairie to the south, oak savannas to the west, and hardwood forests to the north (Omernik and Gallant, 1988).
The upper reaches of the Kankakee River Basin, 8.5 percent of the UIRB, are classified as less typical areas of the Southern Michigan/Northern Indiana Till Plains ecoregion. This is a transitional region between the hillier regions in northern Michigan and the flatter central and eastern corn belt plains to the south, and overlaps the Steuben Morainal Lake Area physiographic subsection (fig. 10). Crop and livestock production are the most extensive land use in the UIRB portion of this ecoregion, and gravel quarries are common. Streams tend to be sluggish, often bordered with swampy tracts. Stream channelization is widespread (Omernik and Gallant, 1988).
Human Factors
Human factors that affect the hydrologic characteristics of the UIRB include land use, urbanization, and population change. These human factors have had an appreciable effect on the basin because people reshape the landscape to fit their needs. One of the earliest human activities was the development of transportation corridors. These corridors opened a passageway between the Great Lakes and the Mississippi River, first by using natural waterways and man-made canal systems and later by building railroads. With the ease of transportation came rapid settlement. Population in the basin grew steadily, and created urban and industrial growth areas along the Lake Michigan shoreline and along major rivers such as the Illinois and Des Plaines. Because of rapid urban growth in these areas, wastewater disposal became a serious issue (Mayor and Wade, 1969). Railroads such as the Illinois Central; Chicago, Burlington, and Quincy; and Chicago, Rock Island, and Pacific railroads soon supplemented and nearly replaced waterway transport (Conzen and Morales, 1989). Agriculture also developed as farmers moved into the basin to supply the growing urban areas with food and goods. Some of the earliest traded goods were lumber from Chicago, and coal and stone from the Illinois Valley (Conzen and Morales, 1989).
Man-made features in the UIRB can be grouped into two broad categories that relate to agricultural and urban land use (Mades, 1987). Today, the UIRB receives numerous inputs of contaminants and nutrients from man-made sources that include municipal and industrial releases, urban and agricultural runoff, and atmospheric deposition. Data collected during the UIRB NAWQA pilot study (1986-92) indicated that differences in fish community structure and water quality could be correlated with differences in urban and agricultural land use (Fitzpatrick and others, 1995; Ruhl, 1995; Sullivan and others, 1998).

Land Use
Agriculture accounts for about 75 percent of the land use in the basin (table 3). Corn is the principle row crop harvested in the basin, followed by soybeans. In 1985, approximately 1.8 million acres of corn were harvested for seed, grain, silage, or sweet corn; and approximately 1.4 million acres of soybeans were harvested for beans (Battaglin and Goolsby, 1994). Urban areas account for about 17 percent of the land use in the basin and are mainly concentrated in the metropolitan areas in and around Chicago. Of this 17 percent, 11 percent is used for low to high density residential purposes and 6 percent is used for commercial and industrial purposes (Hitt, 1992). The land drained by the Calumet and lower Des Plaines Rivers is some of the most heavily industrialized in the Nation. Along these rivers, petroleum refining and steel manufacturing are the dominant industries (City of Chicago and Illinois Environmental Protection Agency, 1989). Forests cover about 5 percent of the UIRB and are concentrated in city forest preserves and along large-stream riparian areas (fig. 16). 
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Since the 1970s, agricultural areas have decreased by 2 percent while urban areas have increased by 3 percent (table 3). Most of the recent urbanization is the result of development of new suburban and residential areas   The effects from converting previously agricultural land to new residential land are not fully known. Some of the possible implications of urbanization on streams include accelerated erosion and channel instability; loss of aquatic habitat; increase in peak flow, duration of peaks, and flood volumes; and loss of base flow from ground-water pumping and change in surface water drainage networks. 
Urbanization
Urbanization began in the Chicago area soon after European settlement and was concentrated in the Des Plaines River Basin. In the last few decades, westward urban development of both the Chicago and Milwaukee metropolitan areas has affected the Fox and lower reaches of the Kankakee River Basins (fig. 17). Specific areas of urban expansion are indicated by U.S. Geological Survey digital land-use data from approximately 1975-82 (Fegeas and others, 1983; Fitzpatrick-Lins, 1980; Anderson and others, 1976) which was updated using 1990 Census data (Hitt, 1995). This urban expansion land-use category accounts for about 3 percent of the basin and indicates areas with more than 1,000 people per square mile. The updated land-use data indicate that low-density residential land use has increased from 8 percent to 11 percent in approximately a 10-year period between 1980 and 1990. The Northeastern Planning Commission estimates that between 1970 and 1990, the population of Chicago grew by 4 percent while the amount of urban land expanded by 51 percent - more than 360,000 acres (Illinois Department of Energy and Natural Resources, 1994). There are presently no land-use/land-cover data available for the entire basin to quantify the exact amount of expansion, but expansion of suburbs has continued since 1990. Subbasins within the UIRB that are mostly affected by urban expansion include parts of the Des Plaines, Du Page, and Fox River Basins. 
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Population Change
Out of the approximately 7.6 million people currently living in the UIRB, about 6 million live in the Chicago metropolitan area (3 million people live within the city limits of Chicago). Total population for the UIRB has increased from 7,211,669 in 1970 to 7,602,203 in 1990 (U.S. Bureau of the Census, 1991, Lanfear, U.S. Geological Survey, unpublished data, 1993; Hitt, U.S. Geological Survey, unpublished digital data, 1992). County population data for the years 1970, 1980, and 1990 indicate a similar trend for urban expansion in the counties surrounding Chicago (table 4). Counties with the greatest increases in population from 1970 to 1990 are on the western and southern edge of the Chicago area and include McHenry (39 percent), Du Page (38 percent), Kendall (31 percent), Will (31 percent), Lake, (26 percent), Kane (21 percent), and Grundy (18 percent) Counties in Illinois. Other counties in the basin with population growth greater than 10 percent are near smaller urban areas that also are undergoing urban expansion, such as the Counties of Waukesha, Wisconsin and Porter, Indiana. In contrast, many counties that predominately consist of either agricultural or densely-populated urban land have had declines in population from 1970 to 1990 (table 4). 
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Hydrologic Characteristics
Surface Water
The quantity, distribution, and variability of streamflow has an appreciable effect on the quality of surface water in the UIRB. The quantity of water in a stream can affect the streams ability to support aquatic life, to assimilate or dilute waste discharges, and to carry suspended sediment. The temporal variability of streamflow is an important cause of the temporal variability of water quality, and knowledge of streamflow is important to understanding the water and ecological dynamics of a watershed.
Human factors have caused four major changes in the UIRB and have significantly affected the quality of surface waters. These changes are the construction of navigable waterways, diversion of Lake Michigan water, construction of wastewater-treatment plants, and agricultural activities. Mades (1987) discusses these changes in detail; selected changes are discussed here.
A number of canals were built to enable navigation between Lake Michigan and the Mississippi River. The 96 mi long Illinois and Michigan Canal was opened to traffic in 1848 (Illinois Department of Public Works and Buildings, 1969). This was a canal that connected the Illinois River 14 mi downstream from Ottawa with Lake Michigan at Chicago. Subsequent competition from railroads led to further improvements in the Illinois River for navigational purposes; as a result, Chicago replaced St. Louis as the major center of commerce for the Illinois River Valley trade. The present navigable waterway was completed in 1939.
The city of Chicago has relied on Lake Michigan as a major source of freshwater from the time of its incorporation in 1837 (Schodek, 1987). Waste disposal was accomplished through a sewer system that fed into the Chicago River, and then into Lake Michigan. Untreated sewage often would flow into the water-supply intake cribs during floods and create health hazards. During the latter half of the 19th century, periodic outbreaks of typhoid fever, amoebic dysentery, and cholera were attributed to contaminated lake water. In an effort to protect their Lake Michigan water supply, the city of Chicago redirected the flow of industrial and domestic wastes to the upper Illinois River system by way of the Chicago Sanitary and Ship Canal (Forbes and Richardson, 1913, 1919; Richardson, 1928). The Chicago Sanitary and Ship Canal links the Chicago River to the Des Plaines River. A second canal, the Calumet Sag Channel, links the Chicago Sanitary and Ship Canal to the Calumet River (Mades, 1987).
Diversion of Lake Michigan water was begun in 1900 at a rate of about 3,000 ft3/s. Of this amount, about 1,200 ft3/s was used for water supply and the remainder was used to dilute domestic and industrial wastes. The allowable rate of diversion varied over the years with maximum diversions of about 8,470 ft3/s occurring during the late 1920s. Guidelines, outlined in a 1980 amendment to the 1967 Supreme Court Decree, limit the diversion, including withdrawals for water supply, to an average of 3,200 ft3/s over a 40-year running accounting period (Espey and others, 1981).
Water-quality conditions in the UIRB were dramatically affected by these diversions, but water quality has improved since wastewater-treatment practices were begun. The Metropolitan Sanitary District of Greater Chicago (now called the Metropolitan Water Reclamation District of Greater Chicago, or MWRDGC) began operation of the first large-scale treatment plant in 1922 (Metropolitan Sanitary District of Greater Chicago, 1982). As a result of the Clean Water Act, the U.S. Environmental Protection Agency (USEPA) helped to finance improvements for wastewater treatment in order to reduce municipal and industrial pollution (U.S. Environmental Protection Agency, 1979). In 1982, MWRDGC treatment plants served an area of 870 mi2, including Chicago and 124 communities in Cook County. These plants treated a domestic wastewater load of 5.1 million persons and an industrial load equivalent to 4.5 million persons (Terrio, 1994). The return flow from these treatment operations averaged 2,200 ft3/s.   In 1983-84, MWRDGC ceased chlorination of wastewater at three treatment plants because chlorination provided limited benefits to the streams receiving the treated wastewater and possibly affected aquatic life. (Terrio, 1994). In addition, implementation of Chicagos Tunnel and Reservoir Plan (TARP), a series of tunnels and reservoirs that capture overflow from combined sewers and store the water until it can be treated, resulted in improved quality and controlled quantity of effluent discharged from Chicagos wastewater-treatment plants (Terrio, 1994).
Despite improvements in the effectiveness of wastewater-treatment plants since 1900 (U.S. Environmental Protection Agency, 1979; 1989), point sources in the Chicago area continue to be major sources of nutrients and contaminants in the UIRB. Approximately 196 wastewater-treatment plants discharge wastewater to streams in the UIRB, the majority of which are located in the greater Chicago area (U.S. Environmental Protection Agency, 1997b). 
Agricultural practices in the lower Fox, Des Plaines and Kankakee River Basins changed appreciably during the 1940s and 50s, nearly a century after these basins were first developed for agricultural purposes. Increasing demands for corn and soybeans in domestic and international markets caused an intensification in the cultivation of these row crops. Commercial fertilizers and pesticides were used in larger amounts to increase crop yields and profits. The practice of crop rotation decreased, and fall plowing increased. Irrigation, especially in the Indiana portion of the Kankakee River Basin, has steadily increased. A nationwide study of water-quality trends concluded that trends in concentrations of total phosphorus and total nitrate observed from 1974 to 1981 show strong associations with measures of agricultural activity such as fertilizer application rates (Smith and others, 1987). Because these intensified agricultural practices were negatively impacting water quality at the same time that wastewater-treatment improvements were positively impacting it, expected improvements in water quality have not been as dramatic as anticipated. 
Streamflow Characteristics
Streamflow in the study area consists of overland flow, ground-water discharge, and point-source return flow. Overland flow is rainwater or snowmelt that flows over the land surface toward stream channels. Ground-water discharge enters stream channels from springs or as seepage. Typically, point-source return flow is previously used water that is discharged to streams from industrial and municipal wastewater-treatment facilities. In the UIRB, the original source of return-flow water is often ground water or Lake Michigan water, and, thus, represents water introduced to the surface-water system that may not have been present under natural conditions.
The mean annual flow from the upper Illinois River Basin is estimated at 12,600 ft3/s, based on streamflow records from gaging stations near the terminus of the basin (fig. 18 and table 5). These stations, the Fox River at Dayton, Ill. (05552500) and the Illinois River at Marseilles, Ill. (05543500), have been in operation since 1915 and 1920, respectively. In 1997, there were 78 active streamflow gaging stations operated by the USGS in the study area (fig. 19, table 5).
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Effects of geology and natural land-surface features on streamflow are indicated by the differences in flow-duration characteristics for rivers that receive little return flow such as the upper reaches of the Kankakee River, the Iroquois River, and the upper Fox River (table 6). The drainage area size and land use above gaging stations on the Kankakee and Iroquois Rivers are similar, but the clayey soils in the Iroquois River Basin result in slower infiltration of precipitation and less ground-water discharge and, thus, greater variations in streamflow. In fact, the index of variability for the Iroquois River station is almost 10 times greater than that for the Kankakee River station (table 6). In the upper Fox River Basin, the large percentage of low, poorly drained land and numerous lakes provide natural storage that affects streamflow in a manner similar to that of the sandy soils of the upper Kankakee River Basin.
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Seasonal streamflow characteristics are consistent throughout the study area. The highest monthly streamflow of all the streams and rivers generally occurs during June or July, and the lowest monthly streamflow generally occurs during December or January (fig. 20).
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Streamflow data analyzed during the UIRB NAWQA pilot study indicated that flow trends during 1978-86 were above the long-term average at many gaging stations (Schmidt and Blanchard, 1997). These trends are presently continuing. Regression analyses indicate significant increases in streamflow at a majority of stations for the period 1950-97 (table 7). Analysis of precipitation records for the 1950-90 period showed no significant trend (National Oceanic and Atmospheric Administration, 1995); therefore, increased precipitation is probably not the source of the increased streamflow. For streamflow analysis, three flow regimes were analyzed: 7-day low flow, mean annual flow, and maximum annual flow. Mean annual streamflows increased during the 1950-97 period at all seven stations shown in table 7, which include streamflow in all the major subbasins of the upper Illinois River Basin. The 7-day low flow also increased at five of seven stations analyzed, while maximum flow increased at three of seven stations. The most dramatic increases occurred at stations draining urbanized land, although there also were increases at stations draining agricultural land.
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The largest increase in mean annual streamflow from 1950 to 1997 was 84 percent at the Des Plaines River at Riverside, Ill. (05532500). However, further analysis of mean annual streamflow data indicate a step increase around 1980. The two major treatment plants upstream of this station began operations in 1975 and 1980 and contributed an average of about 32 and 53 ft3/s, respectively, to the streamflow at this station during 1978-88 (Terrio, 1994). During the period 1975-97, mean annual streamflow was 682 ft3/s, compared to 565 ft3/s for the period 1950-97. Even though there was an increase above the additional discharge from the treatment plants, most of the increase in streamflow at this station is due to effluent return flow. Prior to 1975, effluent from this part of the Chicago area was treated at treatment plants that discharged to the Chicago Sanitary and Ship Canal.
Streamflow data at the other stations shown in table 7 are normally distributed, and seem to indicate a more gradual increase in flows. These trends are probably the result of three factors: land-use changes that have resulted in less infiltration and faster runoff of precipitation from the basins; gradually increasing ground-water pumpage and/or use of Lake Michigan waters; and the associated increase in wastewater-treatment plant effluent return flows. In urban areas, less infiltration and faster runoff result from an increase in impervious surfaces including roads, parking lots, and buildings. In agricultural areas, less infiltration and faster runoff result from the trend toward more intensive row-crop production and practices designed to speed drainage, including tile drains and the drainage of wetlands by ditching. In agricultural areas of the basin, an increase in the acreage of irrigated cropland probably also has contributed to increased streamflows as ground water from aquifers is brought to the surface and channeled toward streams through agricultural drainage systems.
Floods and Droughts
Floods and droughts are caused by climatic conditions and affect surface runoff, soil moisture, stream discharge, and ground water. Local flooding is generally caused by isolated thunderstorms, whereas widespread flooding is caused by more extensive thunderstorms that cover a wide area, by rapid snowmelt in the spring, or by a combination of these factors (U.S. Geological Survey, 1991). Some flooding generally occurs in the UIRB every year. In some years the floods are minor; in other years, floods are extensive and cause loss of life and property. The most widespread flooding to affect the UIRB was during 1943. 
Floods of greater than 100-year recurrence intervals have been experienced in the UIRB a number of times since records have been kept. On July 13, 1957 local rainfall of greater than 6 in. caused 9 deaths and extensive damage in northeastern Illinois. During February and March of 1985 rain fell on snow-covered areas in northeastern Illinois and was followed by a second intense rainfall period. Subsequent flooding caused 26 counties to be declared disaster areas with $10 million damage. During a 2-week period from September 20 to October 3, 1986, local rains caused flooding that resulted in 4 deaths and $50 million in damages.
Heavy rainfall on August 13 and 14, 1987, caused severe flooding of urban areas by streams in Cook and Du Page Counties in Illinois. An all-time 24-hour rainfall record was set during the storm, with 9.35 in. falling at OHare International Airport (Chicago, Ill.). This was more than 3 in. greater than the previous 24-hour record. The worst flooding occurred along the Des Plaines River and its tributary, Salt Creek. Record maximum peak streamflows were recorded at 10 gaging stations on 8 streams.
Droughts are the result of more persistent climate patterns than those of floods, produce less precipitation than normal, and usually have a duration measured in years (U.S. Geological Survey, 1991). Four droughts have been severe in all or part of the basin: 1930-36, 1952-57,1962-67, and 1975-78. The most severe drought basin-wide was around 1957 when precipitation was 64 percent of the average, but less severe droughts have been observed during every decade from 1920 to 1980. The most severe drought in the Midwest, one that resulted in dustbowl conditions, occurred in the 1930s. The drought of 1952-57 was the most severe in terms of deficient streamflow in Illinois, and was followed only 5 years later (1962-67) by another drought of almost equal severity. The last major drought observed in all or part of the basin was during 1975-78, and it was the least severe of the four. 
Lakes and Wetlands
Few natural lakes are present in the study area, with the exception of glacial lakes in the upper Fox River Basin in Wisconsin and McHenry County, Illinois. A few small remnant glacial lakes also exist in Lake County, Illinois. There are a number of man-made lakes throughout the basin.
Many wetland types occur in the basin including marshes and shrub swamps, sedge meadows, fens, and bogs (Sullivan, undated). These wetland types are differentiated by their time of inundation, chemical properties, water depth, place in the landscape, and the plants and animals that occupy them. Wetlands are often located between terrestrial and aquatic systems, and function in hydrological, chemical, and biological roles (Mitsch and Gosselink, 1993). Even though wetlands make up a relatively small amount (1 percent) of land cover in the UIRB, a great proportion of species, including endangered and threatened species, are dependent on wetlands (Illinois Department of Energy and Natural Resources, 1994). Alteration of wetlands changes the character of streams and can affect stream water quality (Mitsch and Gosselink, 1993). Wetlands were once a major feature of the basin, but most of the wetlands in the upper Illinois River Basin were drained prior to the 1850s. The effects of such extensive wetland drainage on the hydrology and water quality of the UIRB cannot be quantified because most of the drainage was completed before water-data collection programs were established. 
Development of farmland in the Kankakee Till Plain (fig. 10) was made possible by the drainage of wetland areas such as the Grand Marsh. The Grand Marsh of the Kankakee River Basin was one of the largest wetlands in the interior United States until the early twentieth century (Mitsch and Gosselink, 1993). This marsh and swamp complex covered an area of about 400,000 acres in a corridor 3-5 mi wide on each side of the Kankakee River (Bellrose, 1976; Meyer, 1936). The wetland was a mosaic of various types of grassy wetlands, hardwood swamps, and sandy islands; and supported a rich and diverse variety of fish, wildlife, and plants (Meyer, 1936). During the middle nineteenth century, tributaries to the Kankakee River were channelized and land was cleared and drained for agriculture. Prior to these changes, the area was a prime hunting and fishing region used by Indians, European traders and settlers, and American sportsmen. Channelization of the mainstem Kankakee River began in 1906 and the entire river in Indiana was channelized by 1918. Because of channelization, the length of the Kankakee River in Indiana was reduced from about 250 mi to about 80 mi. Riparian forest wetlands between the Indiana border and Momence, Illinois are remnants of the Grand Marsh (Ivens and others, 1981; Mitsch and Gosselink, 1993). Presently (1998), the U.S. Fish and Wildlife Service is proposing the re-creation of a 30,000-acre Grand Kankakee Marsh National Wildlife Refuge in the Kankakee River Basin. 
Remaining wetlands in the UIRB are mainly in riparian areas, with the largest areas occurring along the Kankakee River on both sides of the Illinois-Indiana border, and along the Des Plaines River in Wisconsin. Small wetlands are scattered about the Fox River Basin in Wisconsin and in McHenry County, Illinois, and in the upper reaches of the Kankakee River Basin in Indiana.
Water Chemistry
Information on water-quality conditions in the UIRB is widely available. The NAWQA pilot study (1986-92), produced numerous reports about changes in wastewater-treatment practices and effects on water quality (Terrio, 1994); trace elements in water, sediment, and biota (Fitzpatrick and others, 1995); relations between fish communities and environmental factors (Ruhl, 1995); nutrients, dissolved oxygen, and fecal-indicator bacteria (Terrio, 1995); organic compounds in water, sediment, and biota (Sullivan and others, 1998); and a summary report (Schmidt and Blanchard, 1997). In addition, the Illinois Environmental Protection Agency (IEPA) produces a biannual report that summarizes results from their monitoring network (Illinois Environmental Protection Agency, 1996a, 1996b, 1996c).
Results of analyses of historical data indicate that the primary factor affecting water quality in the UIRB is land use (Schmidt and Blanchard, 1997). Distinct chemical signatures are seen in streams that drain urban land compared to streams that drain agricultural land. For trace inorganic constituents, loads were 2-13 times greater from the Chicago metropolitan area, mainly from the Des Plaines River Basin, and continuing to the Illinois River (Schmidt and Blanchard, 1997). Non-agricultural synthetic organic compounds were similarly enriched in urban areas compared to agricultural areas (Sullivan and others, 1998). Although the specific pesticides differed among land use, pesticides were found in both urban and agricultural areas with a few exceptions. The most commonly used herbicide in the Midwest in the 1980s and 90s, atrazine, was found in concentrations that exceeded USEPA standards in both urban and agricultural streams (Sullivan and others, 1998).
As of 1996, IEPA samples 39 stream sites in the UIRB (Illinois Environmental Protection Agency, 1996b). At most sites, samples are analyzed for nutrients and major ions. A number of other constituents may be monitored, depending on the location of the stream and important water-quality issues.
The IEPA reports that overall water quality in the State has steadily improved over the last 26 years (Illinois Environmental Protection Agency, 1996c). The report notes declining trends in many conventional pollutants, as well as increased species diversity in the Illinois River. Much of the mainstem rivers in the upper Illinois River Basin were assessed as having poor water quality in 1972; in 1996 most had been upgraded to fair and a few were classified as good. Most of the non-mainstem streams were assessed as having good water quality in 1996; most were not assessed in 1972.
Constituent loads were calculated for selected sites in the UIRB (fig. 21) with IEPA data collected during 1978-97 using the Estimator program (Cohn and others, 1989). In general, nutrient loads, as illustrated by nitrogen and phosphorus, were greatest from the urban center of the Chicago metropolitan area, reflecting the effect of wastewater return flows to the Des Plaines River and Chicago Sanitary and Ship Canal. Chloride loads also indicated a strong urban effect. Total-suspended solids loads were greatest from agricultural areas.
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Aquatic Biological Characteristics
Water-quality assessments commonly utilize information about aquatic biology for determining the status of water resources. Perhaps the simplest approach is to use the presence of species with restrictive ecological requirements to indicate particular environmental conditions. For example, because trout require cool water, their presence can indicate that water temperature does not exceed a given threshold. A more complicated approach is to use information about the relative abundance of aquatic species to calculate various numerical indices. The Index of Biotic Integrity (IBI), which is based on the relative abundance of fish species, is one example of a widely-used numerical index (Karr and others, 1986). 
Although each of these approaches varies in complexity, all of them require an understanding of biogeography, which is the study of the geographic distribution of plants and animals and the factors that determine those distributions. These factors fall into two broad categories: (1) ecological factors, and (2) historical factors. Ecological factors include structural habitat features, water-chemistry conditions, and biological interactions such as competition and predation. Historical factors include geologic and climatic conditions acting over geologic time scales and recent influences such as introduction of constituents associated with human activities (Gilbert, 1980).
Water-resource assessments usually focus on ecological factors and recent historical influences. Assessments based solely on ecological factors and recent historical influences, however, can be inaccurate or misleading if long-term geologic and climatic factors also are not considered. This result is particularly true at regional scales, where long-term geologic and climatic antecedent conditions can be important factors affecting species distributions (Gilbert, 1980; Jackson and Harvey, 1989).
Early Biogeographic Setting
The dominant long-term geologic and climatic factors affecting pre-settlement species distributions in the UIRB were the glacial periods. As the Wisconsinan glaciers advanced, aquatic species were either eliminated or displaced to areas where they could exist until the ice retreated. When the glaciers retreated, species reinvaded the glaciated regions. The majority of the present-day aquatic fauna of the UIRB reinvaded by moving north from the lower Mississippi River Basin. Interconnected meltwater lakes at the southern end of the retreating glaciers enhanced this reinvasion by providing avenues for rapid dispersal. The unglaciated (driftless) region of southwest Wisconsin and northwestern Illinois also was a source for reinvasion of the UIRB. Fishes that may have reinvaded from the driftless region include northern brook lamprey (Ichthyomyzon fossor), central mudminnow (Umbra limi), brassy minnow (Hybognathus hankinsoni), pugnose shiner (Notropis anogenus), blackchin shiner (Notropis heterodon), and common shiner (Luxilus cornutis) (Burr and Page, 1986). An outlet between the Great Lakes and the Susquehanna River on the Atlantic Slope provided an invasion pathway for spottail shiner (Notropis hudsonius) and banded killifish (Fundulus diaphanus). Red shiner (Cyprinella lutrensis) and suckermouth minnow (Phenacobius mirabilis) probably came from the west through aquatic connections to the Missouri River Basin (Burr and Page, 1986). The red shiner is of particular interest because the range of this plains-adapted species is expanding eastward and it is displacing other fishes (Burr, 1991). Even though the red shiner is not historically widespread in the UIRB, it may be expanding its range in the region. In 1990, it was captured in the Iroquois River Basin at stations in Illinois and in Indiana (Simon, 1992). 
One of the likely consequences of the Wisconsinan glaciation is that fishes that prefer small, clear streams were favored during the southern dispersal. Because water became tied up in glacial ice, rainfall declined, and the Mississippi River and its tributaries became less turbid, upland and clear-water species were able to utilize the tributaries for dispersal and refuge (Burr and Page, 1986). The pool of species available for reinvading the glaciated regions thus included many fishes that were adapted to small, clear streams. This suggests that the pre-settlement fish assemblage of the UIRB may have been particularly sensitive to increases in turbidity and sedimentation.
Another consequence of the Pleistocene glaciation is that the aquatic fauna of the upper midwestern United States, including the UIRB, consists of relatively few endemic species compared to other, more stable regions such as the Tennessee Uplands and the Ozark Plateau. Endemic species are those that are confined to a certain region or area. In general, areas with stable geologic and climatic history will have more endemic species than unstable regions. For example, the Great Lakes area, which also was covered by glaciers during the Pleistocene, has nine endemic fish species, whereas the climatically more stable Tennessee Uplands area has about 50 endemic fish species (Gilbert, 1980).
One of the most notable features of the aquatic biota of the UIRB is the presence of relic populations of species which have current-day distributions centered in the southern coastal plain of the Mississippi River Basin and Gulf Coast. These include populations of the ironcolor shiner (Notropis chalybaeus) and the weed shiner (Notropis texanus), which are characteristic of southern lowland swamps. Both species are found in the northeastern portion of the Kankakee River Basin (Burr and Page, 1986). Other relic populations found in or near the UIRB include the nonparasitic lamprey Ichthyomyzon cf. gagei, and several kinds of plants. Ichthyomyzon cf. gagei, which is found in the southeastern United States, also may be found in Wisconsin (Lyons and others, 1997). Several southern Coastal Plain plants are found in the sandplains near Lake Michigan in southwestern Michigan and northern Indiana (Reznicek, 1994).These relic populations probably became established during an unusually warm post-glacial climatic period between 4,000 and 8,000 years ago. Subsequent climatic cooling has reduced their northern range until they are found in only a few locations in the upper Midwest (Burr and Page, 1986).
Early Aquatic Communities
Landforms created by the Pleistocene glaciations, post-glacial faunal reinvasion, and subsequent climatic variation are important factors for the development of pre-settlement aquatic communities in the UIRB. Although the productivity and diversity of pre-settlement aquatic communities were not formally cataloged, accounts of the early explorers and investigations of the late nineteenth and early twentieth century indicate that these communities were large and diverse. When the first European explorers entered the Illinois River Basin they described the area as teeming with fish and wildlife. Fish and waterfowl were plentiful, the mainstem of the Illinois River ran clear, and the associated backwaters and bottomland lakes had firm bottoms and luxurious stands of submerged and emergent aquatic macrophytes (Steffeck and Striegl, 1989). In 1900, prior to the opening of the Chicago Sanitary and Shipping Canal, the economic value of the commercial fishery of the Illinois River was ranked third nationally behind the salmon fishery of the Pacific coast and the Great Lakes fishery (Steffeck and Striegl, 1989). Between 1870 and 1900 at least 38 mussel species were collected from the Illinois River (Starrett, 1971). Smith (1979) recorded 97 fish species that were captured before 1908 in the Illinois portion of the UIRB.
Pre-settlement aquatic communities encompassed warm- to cool-water assemblages adapted to low- to moderate-gradient streams and rivers. Although level-3 ecoregion classification suggests a rather uniform environment, the glacial landforms provided diverse habitats and ecological niches. The Kankakee, upper Fox, and Des Plaines River Basins contained many small lakes, ponds, and wetlands. Some cool-water streams persist in the Little Kankakee River Basin in Indiana and in the northern Fox River Basin in Wisconsin, some of which are capable of supporting stocked trout (Fago, 1984; Robertson, 1979). 
Recent Conditions and Trends
During the last 150 years, aquatic communities of the UIRB have been affected by human activities. In the State of Illinois, 7 percent of the fish species have disappeared since the turn of the century, and many more species are in decline (Illinois Department of Energy and Natural Resources, 1994). Smith (1971) listed eight conditions responsible for the extirpation or decimation of many fish populations in Illinois streams. Some of these conditions can be found in the UIRB, including: siltation; drainage of natural lakes, swamps, and prairie marshes; desiccation during drought; species interactions, including competition from nonindigenous fish; industrial, domestic and agricultural pollution other than siltation; dams and impoundments; and increased water temperature.
One of the most notable human impacts occurred between 1900 and 1920 in the Illinois River downstream of Chicago, after the Chicago Sanitary and Shipping Canal opened. The biological repercussions of industrial and domestic wastes being diverted to the upper Illinois River peaked about 1920, when plants, benthic organisms, and fish were practically eliminated from the Illinois River for about 160 miles downstream from Chicago (Richardson, 1928). Since the 1920s there has been a gradual improvement in this reach of the Illinois River because of the improved wastewater-treatment practices upstream. Monitoring by the Illinois Natural History Survey (INHS) has documented partial recovery of fish communities. Between 1957 and 1992, common carp and goldfish dominated fish communities in the mainstem of the upper Illinois River. By 1993, however, native species had returned to the mainstem of the river and the relative abundance of common carp and goldfish had declined significantly (Sparks and Lerczak, 1993). In contrast, fish life in the bottomland lakes along the lower Illinois River has not increased to the degree anticipated from improvement in dissolved oxygen conditions (Mills and others, 1966; Sparks and Starrett, 1975).
Urban streams in the UIRB tend to have degraded aquatic communities relative to streams in agricultural areas. Studies have consistently shown that fish communities in urbanized areas generally have lower IBI scores, relatively low species richness, and are often dominated by tolerant fish species such as green sunfish (Lepomis cyanellus) and common carp (Smith, 1971; Bertrand, 1984; Illinois Environmental Protection Agency, 1987, 1988a, 1988b, Ruhl, 1995). During the 1970s, some streams in Chicago were so degraded that no fish could be captured despite repeated sampling attempts (Dennison and others, 1997).
In contrast to urban streams, streams in predominantly agricultural areas generally have higher IBI scores and more diverse fish communities that often include several intolerant fish species (Smith, 1971; Bertrand, 1984; Illinois Environmental Protection Agency, 1987, 1988a, 1988b, Ruhl, 1995). Ruhl (1995) analyzed IEPA fish, habitat, and water-chemistry data and found that differences in fish community structure in the Fox, Des Plaines, and Du Page River Basins were strongly related to water-quality gradients associated with differences between agricultural and urban land uses. Differences were only moderately related to habitat structure. Bertrand (1984) also concluded that fish communities in the Calumet, Des Plaines, and Du Page River Basins were probably limited by water quality rather than by habitat.
The contrast between urban and agricultural streams also is reflected in the IEPAs Biological Stream Characterization (BSC) of UIRB streams (Bertrand and others, 1993). The BSC is a system for classifying streams according to the status of the their aquatic biota (table 8). The BSC is based primarily on IBI scores but also is based on game fish and macroinvertebrate information as needed. In the primarily agricultural Kankakee River Basin, 54 percent of the total stream miles were classified as highly valued aquatic resources and none were classified as limited aquatic resources. In contrast, in the more heavily urbanized Des Plaines River Basin (including the Chicago Waterways and Calumet Basins) only 7 percent of the river miles were classified as highly valued resources and 56 percent were classified as limited aquatic resources (table 9).
There is some evidence that aquatic communities in the Chicago Waterways have improved recently in response to changes in wastewater treatment and implementation of the TARP. Dennison and others (1997) have documented increased fish abundance (from an average of 43 fish per unit effort to 111 per unit effort) and fish species richness (from 41 species to 61 species) at sampling stations downstream from affected areas. They also documented an increase in the proportion of game fish (from 16 percent to 36 percent) found in the MWRDGC monitoring collections.
<<<< table 8 near here - partial page>>>>
<<<< table 9 near here - partial page>>>>
Nonindigenous Species
Most nonindigenous (also known as exotic, alien, or non-native) species are regarded as negative indicators of water quality (Karr, 1995) because they often have direct negative effects on the landscape. Many nonindigenous species are able to colonize and become abundant only where water quality has already been negatively affected. Nonindigenous species may alter habitat, prey on native species, compete with native species for food and habitat, or transmit diseases or parasites (Florida Caribbean Science Center, 1998). Nonindigenous species may be introduced by intentional and accidental stocking, release from bait buckets, release of unwanted aquarium fish, escape from aquaculture facilities, and discharge of ballast water.
Nonindigenous fish species in the UIRB include the common carp (Cyprinus carpio), goldfish (Carassius auratus), rainbow trout (Oncorhynchus mykiss), brown trout (Salmo trutta), white perch (Morone americana), rudd (Scardinius erythrophthalmus), oriental weatherfish (Misgurnus anguillicaudatus), and round goby (Smith, 1979; Fago, 1984; Page and others, 1992; Sparks and Lerczak, 1993; Steingraeber and others, 1997). The common carp was introduced into the United States over 100 years ago. Today it is one of the most abundant fish in many streams. Common carp is considered a pest because it disturbs habitat by stirring up the stream bottom, uprooting aquatic vegetation, and increasing turbidity. Goldfish, similar to common carp, are extremely tolerant and can be the most abundant fish species where habitat and water-chemistry conditions are severely degraded.
Rainbow trout and brown trout are intentionally stocked in streams in the upper Fox River Basin in Wisconsin and in the Little Kankakee River Basin in Indiana (Robertson, 1979; Fago, 1984). These basins contain streams that are able to support stocked trout because water temperatures remain relatively cool during summer.
Several additional nonindigenous fish have either recently arrived in the UIRB or have the potential to enter the basin. The round goby (Neogobius melanostomus), tubenose goby (Proter-orhinus marmoratus), and the ruffe (Gymnocephalus cernuus) have become established in the Great Lakes and have access to the UIRB through the Chicago Waterways. The round goby, a native of Eurasia, was collected from the Little Calumet River in 1996 (Steingraeber and others, 1997). It has decimated populations of mottled sculpin (Cottus bairdi) in the St. Clair River near Detroit, Michigan (Jude and Crawford, 1995). Breeding populations of the grass carp (Ctenopharyngodon idella) and bighead carp (Hypophthal-michthys nobilis) are found downstream from the UIRB but may invade UIRB rivers and eventually the Great Lakes (Florida Caribbean Science Center, 1998).
The zebra mussel (Dreissena polymorpha) invaded the UIRB from Lake Michigan in 1991 and has spread throughout the Mississippi River Basin (Neves and others, 1996). Zebra mussels can attach to almost any hard surface, and populations can reach densities of greater than 50,000 per square meter, threatening native mussel species and fouling water pipes.
Other nonindigenous species are of concern in the UIRB. The zooplankter Daphnia lumholtzi, a type of water flea, moved upriver from the Mississippi Basin and has become established in the Illinois River. The behaviorally aggressive rusty crayfish (Orconectes rusticus), which first appeared in Illinois in 1973, is now found in all UIRB drainages and has displaced native crayfish species (Taylor and Redmer, 1996). In many UIRB wetlands, purple loosestrife and glossy buckthorn, two plant species originally from northern Europe, have spread rapidly and also have displaced native plants (Illinois Department of Energy and Natural Resources, 1994).
Threatened and Endangered Species
A number of aquatic species of the UIRB are listed by the individual States or by the Federal government as endangered, threatened, of special concern, or extirpated. These include fish such as the river redhorse (Moxostoma carinatum) and greater redhorse (Moxostoma valenciennesi), mollusks such as the snuffbox (Epioblasma triquetra) and sheepnose (Plethobasus cyphyus), and a Federally endangered insect - the Hines emerald dragonfly (Somatochlora hineana) (Illinois Department of Energy and Natural Resources, 1994). Nineteen fish species that historically were found in the UIRB are listed as threatened or endangered by Illinois, Indiana, or Wisconsin (table 10). The river redhorse and greater redhorse are listed by all three States as threatened or endangered (table 10). Lists of threatened and endangered species can change as a result of new distributional and life history information. The most up-to-date information can be accessed using the internet (Illinois Natural History Survey, 1998; Indiana Department of Natural Resources, 1998; Wisconsin Department of Natural Resources, 1998; U.S. Fish and Wildlife Service, 1998). 
<<<table 10 near here>>>>
Ground Water
Aquifers in the UIRB are associated with the Quaternary, Silurian-Devonian, and Cambrian-Ordovician stratigraphic systems. Surficial or unconsolidated aquifers are in the Quaternary system, whereas bedrock or consolidated aquifers are in the Silurian-Devonian or Cambrian-Ordovician systems (Friedel, 1998). Regional ground-water flow is affected by geology, and, in the UIRB, is generally from west to east; intermediate and local flow is spatially variable throughout the basin (Friedel, 1998). 
Surficial Aquifers
Surficial aquifers are composed of the unconsolidated Quaternary glacial deposits that overlie the bedrock in the basin (fig. 8). These deposits form one of the most productive aquifers in the UIRB and consist mainly of sand and sand and gravel. Before water suppliers began using Lake Michigan water, the surficial aquifers supplied approximately 10 percent of the water for the urban Chicago area and almost 85 percent of the water for the rural areas in the southeastern part of the basin.   Since that time, the importance of the surficial aquifers has increased. Some Chicago suburbs are growing into areas that are too far away to efficiently utilize Lake Michigan water; therefore, the ground water in surficial aquifers is under ever increasing demand (Fitzpatrick and others, 1992). Trilinear plots of water-chemistry data from Illinois indicate that the chemical composition of water from the unconsolidated aquifers is predominately calcium bicarbonate, and, where unconsolidated material may be in contact with underlying shale, the chemical composition is predominately calcium sulfate.
Bedrock Aquifers
 The Cambrian-Ordovician aquifer is the oldest and deepest aquifer underlying the UIRB (fig. 22). This aquifer consists of numerous alternating layers of sandstone, limestone, and dolomite. On a regional scale, each of the layers of Cambrian and Ordovician strata are hydraulically interconnected and act as a single aquifer (Visocky and others, 1985).
<<<< figure 22 near here - whole page >>>>
The Mount Simon Sandstone, Elmhurst Sandstone Member of the Eau Claire Formation, and Ironton and Galesville Sandstones (Willman and others, 1975) of the Cambrian system are important sources of freshwater north of the Kankakee River. The Mount Simon aquifer is composed of the Mount Simon Sandstone and the Elmhurst Member of the Eau Claire Formation. Ground water in this aquifer occurs under leaky artesian conditions because of the overlying relatively confining siltstone and shale beds of the Eau Claire Formation (Walton and Csallany, 1962).
 The Prairie du Chien Series and Galena Platteville Group of Ordovician age overlie the Cambrian system and yield small to moderate quantities of water. Where directly underlying drift, solution activity of water has enlarged openings in the dolomite of the Galena-Platteville Group allowing water to flow more freely (Walton and Csallany, 1962).
The Maquoketa Shale is a confining unit of the Cambrian-Ordovician aquifer. Localized areas of recharge from the shallow surficial aquifers to the deeper bedrock aquifers suggest that the Maquoketa Shale may not completely confine the Cambrian-Ordovician aquifer (Michael Friedel, U.S. Geological Survey, written, commun., 1998).
 Dolomites and limestones of Silurian and Devonian age constitute one of the principal shallow bedrock aquifers in the UIRB (fig. 22). The Silurian-Devonian aquifer is the uppermost bedrock aquifer and is overlain primarily by unconsolidated glacial deposits. Although the Devonian is absent throughout most of Illinois and Wisconsin, the name Silurian-Devonian will be used for comparisons and consistency. 
Historically, the Silurian-Devonian aquifer has been heavily pumped in the basin. Pumping of the aquifer has resulted in slowly degrading water quality because heavily mineralized water has migrated upward from the deeper parts of the Elmhurst and Mount Simon sandstones (Visocky, 1997) and water has locally migrated downward from unconsolidated aquifers (Mike Friedel, U.S. Geological Survey, written commun., 1998). Trilinear plots of water chemistry data from Illinois indicate that the chemical composition of water from the Silurian-Devonian aquifers is variable and ranges from calcium bicarbonate to sodium chloride.
Over 35 percent of the ground water pumped in the Chicago area is from the Silurian-Devonian aquifer (Kirk, 1987). In 1986, western suburban Chicago heavily pumped the Silurian-Devonian aquifer for domestic, municipal and industrial use, but a recent trend in suburban Chicago has been to switch to other water supplies including the surficial aquifers and Lake Michigan. Some of the decline in pumpage from the deep aquifer is due to communities complying with the Safe Drinking Water Standards (Visocky, 1997). 
Surface-Water and Ground-Water Interactions
 As precipitation infiltrates the soil and subsoil, the surficial aquifers are recharged. In areas where sand and gravel surficial aquifers directly overlie principal shallow bedrock aquifers, water can flow downward through fissures and solution channels to recharge the bedrock aquifers. In these areas, there is good hydraulic connection between the surficial and shallow bedrock aquifers and recharge is local and rapid (Hughes and others, 1966). Downward leakage of water through the shallow bedrock system also can recharge deeper bedrock aquifers. The permeability of the soil and surficial deposits partially determines the rate at which infiltration occurs. If precipitation is too heavy and falls too quickly for the soil to absorb the water, runoff results. In this case, runoff directly recharges streams by overland flow. 
Streams can recharge aquifers, particularly in areas where ground water is heavily pumped from wells nearby the stream. Heavy pumping can cause a decrease in hydraulic head in and near the well. In these areas, surface water can slowly flow through aquifers toward the source of the pumping to equalize the difference in hydraulic head (Friedel, 1998). In areas such as the lower Fox River Basin where streams are incised into the productive shallow Galena-Platteville and Ancell bedrock aquifers, ground water discharges to the stream and provides base flow to the river (Visocky and others, 1985). Additional withdrawals from these aquifers near the Fox River may reduce ground-water discharges and base flow (Fitzpatrick and others, 1992).
Water Use
During 1995, an estimated 1,375 Mgal/d were used for public water supply in the upper Illinois River Basin (table 11). Many communities in the UIRB supplement their water supply with Lake Michigan water, which is a source outside of the basin. In 1995, there were 1,179 Mgal/d withdrawn from Lake Michigan for public supply (13 percent of the total water used), up from an average of 1,070 Mgal/d percent during 1978-86. Excluding withdrawals from Lake Michigan, ground water supplies about 82 percent and surface water supplies about 18 percent of the total public supply for the UIRB. Increasing restrictions on the use of Lake Michigan water indicates that more ground water may be utilized in the future.
<<<< table 11 near here - partial page >>>>
Thermoelectric power generation is the single largest use of water in the basin. In 1995, water used to cool electric power generating equipment accounted for 94 percent of the water withdrawn from all sources inside the UIRB. Surface water from inside the UIRB provided almost 100 percent of the water used for this purpose in 1995; less than 1 percent of was supplied by ground water resources (U.S. Geological Survey, 1995). 
When all water-supply sources, including Lake Michigan, are considered, ground water accounted for 19 percent and surface water accounted for 81 percent of all water used in 1995 for public and domestic supply, commercial, industrial, mining, livestock, and irrigation in the UIRB. Excluding Lake Michigan water, the percentages increase to 64 percent for ground water and decrease to 36 percent for surface water (U.S. Geological Survey, 1995). 
In 1995, an estimated total of 329 Mgal/d, including water used for thermoelectric, was withdrawn from ground-water sources inside the basin (53 percent from surficial aquifers and 47 percent from bedrock aquifers). These percentages indicate that the amount of ground water used from these two aquifer types is similar and substantiates the decrease in water used from bedrock aquifers and the consequent increase in water used from the surficial aquifers. McHenry County, west of Chicago, relies totally on the surficial sand and gravel aquifer for its water supply. Continued urban expansion of Chicago is likely to increase the total urban use of the surficial aquifer in the near future. In general, there has been a major decline in water use from bedrock aquifers in the UIRB since 1986, but there has been an increase in withdrawals from surficial aquifers. 
Forty-one percent of the surface water supplied from within the basin is used for industry; in contrast, only 14 percent of ground water is used for industry. Forty-nine percent of ground water supplied from within the basin is used for public drinking water; in contrast, only 19 percent of the surface water is used for this purpose. A large proportion of both surface and ground water supply is used for irrigation, 18 percent and 16 percent, respectively (fig. 23).
<<<< figure 23 near here - partial page >>>>
IMPLICATIONS OF ENVIRONMENTAL SETTING FOR
WATER QUALITY
Study Area Stratification by Natural and Human Factors
The quantity and quality of surface waters in the UIRB prior to settlement were undoubtedly much different than at present. Vast areas of wetlands have been drained for agriculture and urban development; Chicago, one of the Nations largest metropolitan areas, has developed and continues to grow rapidly; the flow of the Chicago River has been reversed; and water diversions from Lake Michigan have been added.
The effect of natural and human factors on surface- and ground-water quantity and quality is one of the many topics studied by scientists in the NAWQA program (Hirsch and others, 1988). Previous sections of this report describe the natural and human factors that make up the general environmental setting of the upper Illinois River Basin. Important natural factors in the UIRB include bedrock geology, physiography and surficial geology, soils, vegetation, and climate. Important human factors in the UIRB include land use, urbanization, and hydrologic modifications.
In an attempt to isolate the effects of the natural and human factors that are thought to be most important in affecting water quantity and quality, an environmental framework called stratification was established for the UIRB. This stratification was accomplished by overlaying thematic maps of human and natural environmental factors. A similar approach has been used by other NAWQA study units. Two stratification schemes were developedone for surface-water and one for ground-water sampling designs. Using the resulting stratification schemes, the study area can be divided into subunits with similar natural characteristics and land use.
Surface-water stratification for the UIRB was based primarily on physiography, with further subdivisions based on surficial geology and associated soil permeability, and land use/land cover and associated population density. This stratification, termed level-3 because of the three basic factors used, resulted in 12 subunits of the UIRB: four in the Wheaton Morainal Plain; one in the Chicago Lake Plain; three in the Bloomington Ridged Plain; and four in the Kankakee Till Plain (fig. 24). Major surface-water issues, mainly related to differences in urban and agricultural land use, in the UIRB will be examined within the stratification scheme. Surface-water issues related to urbanization include point and nonpoint sources of sediment, nutrients, trace elements, and organic compounds; streamflow alterations; and the health and community structure of aquatic biota. Surface-water issues related to agriculture include nonpoint sources of sediment, nutrients, trace elements, and pesticides; drainage modifications; and the health and community structure of aquatic biota.
Ground-water stratification was similar to the surface-water stratification, but also included bedrock geology. The ground-water stratification, termed level-4 because of the four basic factors used, was preliminary and resulted in 21 subunits: six in the Wheaton Morainal Plain, one in the Chicago Lake Plain, seven in the Bloomington Ridged Plain, and seven in the Kankakee Till Plain. Major ground-water issues in the UIRB will be examined within the stratification scheme (fig. 25). Ground-water-quality assessments will be conducted where surficial sand, and sand and gravel aquifers overlie principal bedrock aquifers. The surficial aquifers being assessed are those aquifers that underlie areas transitioning from agricultural to urban land use and that overlie the carbonate Silurian-Devonian aquifer in the Great Lakes physiographic section (Wheaton Morainal Plain); and those aquifers that underlie agricultural land use and overlie the carbonate Silurian-Devonian aquifer in the Till Plains physiographic section (Kankakee Till Plain).  (fig. 25).
<<<< figure 24 near hear - whole page >>>>
<<<< figure 25 near here - whole page >>>>
Surface Water and Aquatic Biota
Historical and ongoing urban land-use practices have resulted in a continuing problem of nonpoint and point source contamination of streambed sediment and aquatic biota. In addition to point sources, constituents from urban nonpoint runoff may be derived from residues from gasoline, motor oil, tires, brake linings, galvanized metal, nails, building materials, painted surfaces, pesticides, and fertilizers (Moore and Ramamoorthy, 1984; Streigl and Cowan, 1987; Leed and Belanger, 1981; Kelly and Hite, 1981, 1984). For example, streambed sediments from several urban streams of the UIRB exceeded USEPA National Sediment Inventory reference levels for trace elements (Cd, Cr, Hg, Ni, Pb, Zn) and synthetic organic compounds (heptachlor epoxide, PCBs, chlordane, DDT, DDD, and DDE) (Fitzpatrick and others, 1995; Sullivan and others, 1998; U.S. Environmental Protection Agency, 1994). These reference levels were selected to represent concentrations above which there might be an appreciable threat to either aquatic life or human health. The USEPA also identified four urban subbasins in the UIRB (out of 96 nationwide) as Areas of Probable Concern (APCs), based on the National Sediment Inventory data (U.S. Environmental Protection Agency, 1997a). The UIRB subbasins identified as APCs are the Little Calumet, Chicago, Des Plaines, and upper Fox Rivers. Synergistic effects of these constituents on aquatic biota also are of concern because correlations were found between trace organic compounds and Cr, Cu, Hg, and Zn in streambed sediment. Concentrations of p,p-DDE, chlordane, and dieldrin in whole fish from some sites in the UIRB exceeded various guidelines for protection of fish and fish-consuming wildlife and humans (Sullivan and others, 1998). Concentrations in whole fish from sites on the Des Plaines and Illinois Rivers in and downstream from Chicago exceeded U.S. Food and Drug Administration fish flesh consumption advisory levels for PCBs (Sullivan and others, 1998).
The effects of the continued expansion of Chicago suburbs into the Fox and upper Des Plaines River Basins (fig. 17) has raised several issues, including the potential for accelerated erosion and channel instability, increased flooding, increased nonpoint sources of contaminants, and further loss of aquatic habitat and biological integrity. Physical changes in streamflow and channel characteristics are primarily the result of increases in the amount of impervious surface in the drainage basin, which also has a direct effect on biological communities (Schueler, 1995). 
Large navigational and hydroelectric dams are located on the Illinois, Fox, Des Plaines, and Kankakee Rivers. These dams cause unnatural variations in streamflow characteristics and accumulations of fine-grained sediment upstream of the dam (U.S. Army Corps of Engineers, Chicago District, 1987). In addition, fish migrations are restricted by dams, and mussels are affected by sedimentation and by decreases in fish hosts critical to their reproduction. 
The effects of agricultural activities on water quality in the UIRB has included changes to hydrologic conditions caused by drainage of wetlands, stream channelization, and reduction of riparian vegetation. These changes have resulted in increased loads of suspended sediment to streams and associated turbidity caused by runoff from farm fields (fig. 21). Increased sedimentation and water turbidity reduces the food supply for fish (Bellrose and others, 1977), decreases the visibility needed by sight predators to find food, and interferes with reproductive activities (Sparks and Lerczak, 1993). In addition, disruption of the natural hydrologic regime may exacerbate these problems by affecting spawning and rearing habitats associated with backwater lakes along the lower Illinois River. The Illinois Natural History Survey is currently conducting research that focuses on characterizing relations between hydrologic regime and fish-community composition in the UIRB (Sparks, 1984; Illinois Natural History Survey, oral commun., 1998).
Elevated arsenic concentrations in sediment, fish, and clams in the Kankakee River Basin above the confluence of the Iroquois River (Coleman and Sanzolone, 1991; Fitzpatrick and others, 1995) may be related to multiple and interacting natural and human factors. Agricultural use of arsenical-based chemicals (for example, cacodylic acid) is a potential source; however, elevated arsenic concentrations were not found in the Iroquois River Basin, which is more intensively farmed than the Kankakee River Basin. Highly permeable soils (fig. 13) in the Kankakee River Basin generally require more irrigation than the fine-grained soils of the Iroquois River Basin. Shallow ground water used for irrigation in the Kankakee River Basin is generally enriched in arsenic and may contribute arsenic to streams in that basin (John Winters, Indiana Department of Environmental Management, oral communication, 1992). Weathering of shale bedrock in the region is a potential source of arsenic to ground water (Yarling, 1992; Fenelon and others, 1995).
The effect of channelization on aquatic ecosystems was discussed earlier in this report. Loss of habitat because of channelization, draining of wetlands, and clearing of riparian vegetation has altered the characteristics of the streams in agricultural areas. The extent of these changes is unknown because agricultural activities began before water-quality conditions were documented.
Ground Water
Ground-water issues in the UIRB include water levels and water quality. Heavy withdrawals in the Chicago area beginning about 1950 caused water levels in the Cambrian-Ordovician aquifer to decline by more than 1,000 ft (Sasman and others, 1982). The effect of water-level declines were noticed as far away as Wisconsin. In 1980, the western Chicago suburbs began switching their water source to Lake Michigan or more shallow bedrock and surficial aquifers, which reduced ground-water withdrawals closer to natural recharge rates (Visocky, 1997). At the current pumpage rate, water levels are expected to recover as much as 650 ft by 2010 (Illinois Department of Energy and Natural Resources, 1994).
The water quality of the Silurian-Devonian aquifer is more likely to be affected by increased urbanization than the Cambrian-Ordovician aquifer. The Cambrian-Ordovician aquifer is confined by the Maquoketa Shale and is deeper than the Silurian-Devonian aquifer. Because of this relation, urbanization probably would not affect the Cambrian-Ordovician aquifer as much as it might the Silurian-Devonian aquifer. Volatile organic compounds (VOCs) have been detected in water supplies of communities that pump water from the Silurian-Devonian aquifer. In 1994, there was an increase in the amount of VOC and other synthetic organics detected even though the number of wells with VOC detections did not increase (Illinois Environmental Protection Agency, 1994). In addition, water-quality data indicate that the Cambrian-Ordovician aquifer contains concentrations of naturally occurring radium and barium in excess of standards set by USEPA (U.S. Environmental Protection Agency, 1975).
 Arsenic in ground water in the UIRB may be considered a problem at a regional scale. Yarling (1992) found arsenic concentrations in ground water near Wakarusa, Indiana (Elkhart County) that were above the USEPA maximum contaminant level (50 ug/L). These high arsenic concentrations were detected in ground water from the deep lower Mahomet aquifer, which is composed of unconsolidated quaternary deposits. Yarling (1992) concluded that the high arsenic concentrations were probably related to chemical weathering of black shales in the area. Extensive areas of these same shale bedrock types are present in portions of the Kankakee River Basin and in east-central Illinois. These include the Antrim, New Albany, and Ellsworth Shales (fig. 4A), and they are generally black, enriched in metal content, and were deposited in strongly reducing conditions. Glaciers deposited black shale fragments in proglacial outwash and tills in northern Indiana. A study of aquifers in the St. Joseph River Basin near South Bend, Indiana (immediately adjacent to the headwaters of the Kankakee Basin) showed that arsenic concentrations in the deep aquifer were significantly higher than in the shallow aquifer (Fenelon and others,1995). These higher concentrations were attributed to chemical weathering of shale in the aquifer. In Illinois, Holm and Curtiss (1988) also found high arsenic concentrations in ground water. A study conducted by Warner (U.S. Geological Survey, written commun.,1998) in the LIRB NAWQA study unit found arsenic concentrations above the USEPA maximum contaminant level in some wells in the deep Mahomet aquifer, a major water resource in east-central Illinois. Elevated arsenic concentrations are present in streambed sediment and ground-water discharge areas.
SUMMARY
The U.S. Geological Surveys National Water-Quality Assessment (NAWQA) Program was established, in part, to assess the status and trends of the Nations water quality. The program objectives are accomplished through study units that encompass areas that represent a large portion of the Nations surface- and ground-water resources. The upper Illinois River Basin (UIRB) study of the NAWQA Program that began in 1996 is an assessment of surface- and ground-water quality and biology. An environmental framework of human and natural factors is used to describe the similarities and differences in the hydrologic characteristics of the UIRB. 
The UIRB drains a 10,949 mi2 area upstream of Ottawa, Illinois in northeastern Illinois, northeastern Indiana, and southeastern Wisconsin. The following major subbasins are within the UIRB: Kankakee River, Iroquois River, Fox River, Des Plaines River, Chicago River, and Illinois River. 
The Silurian-Devonian limestones and dolomites form the principle shallow bedrock aquifer. The underlying Cambrian-Ordovician sandstone forms the dominant deep bedrock aquifer. Ground-water levels in the deep bedrock aquifer have changed over time. In 1986, western suburban Chicago heavily pumped the aquifer for domestic, municipal and industrial use, but a recent trend of suburban Chicago has been to switch to other water supplies such as those of shallow aquifers and Lake Michigan.
The UIRB lies in the Central Lowlands physiographic province which is subdivided into two physiographic sections: the Great Lake Section and the Till Plains Section. The subdivisions of these two sections are based on the geomorphology of the landforms in the basin, which are mostly a result of Wisconsinan glaciation. The surficial unconsolidated sand and gravel deposits form the major surficial aquifers in the UIRB. Surficial deposits are the parent material of most of the soils in the basin. 
The climate of the UIRB is humid continental and large daily fluctuations in temperature and precipitation can result throughout the basin. The normalized average annual temperature for the UIRB (1961-90) ranged from 46 o F in the northern part of the basin to 51o F in the southern part of the basin. Average annual precipitation for the period 1961-90 ranged from 32 in. to 38 in. 
Most of the UIRB is in the Central Corn Belt Plains ecoregion. Parts of the upper Fox and upper reaches of the Des Plaines River Basins are classified in the Southeastern Wisconsin Till Plains ecoregion. The upper reaches of the Kankakee River Basin are classified as Southern Michigan/Northern Indiana Till Plains ecoregion. 
The UIRB contains Chicago, Illinois, one of the largest metropolitan areas in the United States. Urban areas around Chicago have been expanding into previously agricultural and forested areas. Subbasins within the UIRB that are mainly affected by urban expansion from the Chicago area include the Des Plaines, Du Page, and Fox River Basins. The primary crops grown in the agricultural areas of the basin are corn and soybeans. 
Five major changes in the upper Illinois River Basin have altered the quality of surface waters. These changes are construction of navigable waterways, diversion of Lake Michigan water, construction of wastewater-treatment plants, drainage of wetlands, and agricultural activities. Results of historical water-quality data analysis indicate that the primary factor affecting water quality in the UIRB is land use. Distinct chemical signatures are found in streams that drain urban land compared to streams that drain agricultural land. 
Aquatic biota is affected by the glacial history of the UIRB, land use, man-made modifications to water bodies and streams, and invasion of nonidigenous species. One of the consequences of past glaciation is that aquatic fauna of the UIRB has relatively few endemic species. The original aquatic communities of the UIRB were diverse and productive. During the last 150 years, aquatic communities of the UIRB have been significantly affected by human activities.
In an attempt to isolate the effects of the natural and human factors that are thought to have the most effect on water quantity and quality, an environmental framework called stratification was established for the UIRB. The stratification scheme divides the study area into subunits with similar natural characteristics and land use; and will be used during the UIRB study to design a framework for where to collect water and biology samples.
Surface-water issues related to urbanization include point and nonpoint sources of sediment, nutrients, trace elements, and organic compounds; streamflow alterations; and the health and community structure of aquatic biota. Surface-water issues related to agriculture include nonpoint sources of sediment, nutrients, trace elements, and pesticides; drainage modifications; and the health and community structure of aquatic biota. Ground-water issues include declining water levels and degraded water quality, and elevated arsenic concentrations in ground water.
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APPENDIX A. BIBLIOGRAPHY OF U.S. GEOLOGICAL SURVEY REPORTS FROM THE UPPER ILLINOIS RIVER BASIN PILOT STUDY OF THE NATIONAL WATER-QUALITY ASSESSMENT PROGRAM
Colman, J.A., and Sanzalone, R.F., 1991, Surface-water-quality assessment of the upper Illinois River Basin in Illinois, Indiana, and Wisconsin: Geochemical data for fine-fraction streambed sediment from high- and low-order streams, 1987: U.S. Geological Survey Open-File Report 90-571, 108 p.
Fitzpatrick, F.A., and Colman, J.A., 1993, Surface-water-quality assessment of the upper Illinois River Basin in Illinois, Indiana, and Wisconsin: Data on manmade nonagricultural volatile and semivolatile organic chemicals in water, May 1988 through March 1990: U.S. Geological Survey Open-File Report 92-467, 70 p.
Fitzpatrick, F.A., Scudder, B.C., Crawford, J.K., Schmidt, A.R., Sieverling, J.B., and others, 1995, Water-quality assessment of the upper Illinois River Basin in Illinois, Indiana, and Wisconsin: Major and trace elements in water, sediment, and biota, 1978-90: U.S. Geological Survey Water-Resources Investigations Report 95-4045, 254 p.
Mades, D.M., 1987, Surface-water-quality assessment of the upper Illinois River Basin in Illinois, Indiana, and Wisconsin: Project description: U.S. Geological Survey Open-File Report 87-473, 35 p.
Marron, D.C., and Blanchard, S.F., 1995, Surface-water-quality assessment of the upper Illinois River Basin in Illinois, Indiana, and Wisconsin: Cross-sectional and depth variation of water-quality constituents and properties in the upper Illinois River Basin, 1987-88: U.S. Geological Survey Water-Resources Investigations Report 95-4021, 19 p.
Ruhl, P.M., 1995, Surface-water-quality assessment of the upper Illinois River Basin in Illinois, Indiana, and Wisconsin: Analysis of relations between fish-community structure and environmental conditions in the Fox, Des Plaines, and Du Page River Basins in Illinois 1982-84: U.S. Geological Survey Water-Resources Investigations Report 94-4094, 50 p.
Schmidt, A.R., and Blanchard, S.F., 1997, Surface-water-quality assessment of the upper Illinois River Basin in Illinois, Indiana, and WisconsinResults of investigations through April 1992: U.S. Geological Survey Water-Resources Investigations Report 96-4223, 63 p.
Steffeck, D.W., and Striegl, 1989, An inventory and evaluation of biological investigations that relate to stream-water quality in the upper Illinois River Basin of Illinois, Indiana, and Wisconsin: U.S. Geological Survey Water-Resources Investigations Report 89-4041, 54 p.
Sullivan, D.J., and Blanchard, S.F., 1994, Surface-water-quality assessment of the upper Illinois River Basin in Illinois, Indiana, and Wisconsin: Fixed-station network and selected water-quality data for April 1987-August 1990: U.S. Geological Survey Open-File Report 91-175, 213 p.
Sullivan, D.J., and Terrio, P.J., 1994, Surface-water-quality assessment of the upper Illinois River Basin in Illinois, Indiana, and Wisconsin: Data on agricultural organic compounds, nutrients, and sediment in water, 1988-90: U.S. Geological Survey Open-File Report 93-421, 61 p.
Sullivan, D.J., Stinson, T.W., Crawford, J.K., and Schmidt, A.R., 1998, Surface-water-quality assessment of the upper Illinois River Basin in Illinois, Indiana, and WisconsinPesticides and other synthetic organic compounds in water, sediment, and biota, 1975-90 with a section on Historical conditions by John A. Colman.: U.S. Geological Survey Water-Resources Investigations Report 96-4135, 131 p.
Terrio, P.J., 1994, Relations of changes in wastewater-treatment practices to changes in stream-water quality during 1978-88 in the Chicago area, Illinois, and implications for regional and national water-quality assessments: U.S. Geological Survey Open-File Report 93-4188, 56 p.
Terrio, P.J., 1995, Water-quality assessment of the upper Illinois River Basin in Illinois, Indiana, and Wisconsin: Nutrients, dissolved oxygen, and fecal-indicator bacteria in surface water, April 1987 through August 1990: U.S. Geological Survey Water-Resources Investigations Report 95-4005, 79 p.
Zogorski, J.S., Blanchard, S.F., Romack, R.D., and Fitzpatrick, F.A., 1990, Availability and suitability of municipal wastewater information for use in a national water-quality assessment: A case study of the upper Illinois River Basin in Illinois, Indiana, and Wisconsin: U.S. Geological Survey Open-File Report 90-375, 68 p.