The Illinois River Basin must be understood as both a natural legacy, to which humans responded, and as a developed landscape, in which hydrological systems have been profoundly altered, sometimes intentionally and sometimes inadvertently. The Basin lies largely within the Prairie Division, a natural classification based on macroclimate and native vegetation that spans portions of 12 states (Bailey 1998), and it is also in the center of the Corn Belt, a classification based on intensive human use of the land for row crop agriculture (Hudson 1994). In addition to these natural and agricultural classifications, the basin is also characterized by a significant urban component which includes the city of Chicago. From a social perspective, IRB represents many others in the developed world, where the consequences of development are now evident and there is rising public interest not only in maintaining beneficial commercial uses, but also in recovering natural goods and services, including attractive environments for outdoor recreation, maintenance of biodiversity, flood attenuation, groundwater protection, and nutrient retention and cycling. Social desires to naturalize or restore rivers and watersheds will test our scientific understanding of these systems and our ability to make useful predictions about alternative approaches to naturalization. The IRB is a good place to address the scientific challenges of understanding the water-related parts of an ecosystem well enough to regain and sustain natural goods and services within a largely developed landscape. The IRBO proposes to understand the legacies of the past to the extent necessary to understand the present, and support sustainable development, through predictive studies, that balances human wants and needs with desirable ecosystem processes and functions.

The Illinois River begins at the confluence of the Des Plaines and Kankakee rivers near Chicago, Illinois, and flows 380 km. southwest to the Mississippi River at Grafton, Illinois. The topography in the basin is relatively flat with surface elevation ranging from 180 to 240 meters above mean sea level. Glacial features, originating in the Pleistocene Epoch are the major landforms in IRB. Several glacial advances covered the region, which have distinct effects on watershed characteristics. The climate of the basin is humid continental with cold and relatively dry winters, and warm, wet summers. The climate is well suited for agricultural production with little need for irrigation. The average annual precipitation in the basin is about 90 cm with an average snowfall of 65 cm. IRB experiences spring flooding during the months of March through May and occasionally in summer and autumn. Spring storm events and snowmelt create flood pulses connecting the stream and floodplain ecosystems. Extreme events such as floods (e.g. 1993) and droughts (e.g. 1988) have a significant human and economic impact on the region. Millions of waterfowl and other migrant birds travel through the Mississippi flyway, in which the basin floodplains provide important resting and foraging sites for them.

The Illinois River, as part of the Upper Mississippi River system, is one of the most important navigable waterways in the world which grew from the need to meet the demand of transporting agricultural products from the region to national and international markets. Commercial barge traffic transports over 44 million tons of commodities with a total value of about $9.5 billion dollars per year. To accommodate this traffic, eight single-chamber lock and dam systems were constructed along the main stem of the Illinois River about 70 years ago. These structures have greatly modified the hydrology and hydraulics of the river. In addition, the main stem has been altered by the reversal of the Chicago River by the Chicago Sanitary and Ship Canal in the late 1800s to provide an outlet to dilute urban waste in this river by flushing it downstream into the Illinois River with water from Lake Michigan. The reversal of the Chicago River breached a continental divide between drainage to the Gulf of Mexico and to the Atlantic Ocean via the Great Lakes system, and subsequently had important consequences, first on the Illinois River, then on the Great Lakes.  The water diverted from Lake Michigan increased summer water levels in the river and thereby drowned bottomland forests along the river.  The pollution adversely affected aquatic life progressively further downstream. At first, the gross pollution in the Chicago River and associated canal system was a barrier to transfer of aquatic organisms between the Great Lakes and the Mississippi basins, but improvements in waste treatment since the Clean Water acts were passed in the 1970s have steadily reduced the "pollution barrier", so that several harmful aquatic species have been able to transit the canal system recently.  These include the zebra mussel and the Russian round goby, which entered the Great Lakes in the ballast water of ocean-going ships and moved through the canal into the Mississippi basin, where they threaten native aquatic species and, in the case of zebra mussels, cause economic damage by blocking water intakes.  Unwanted species also move in the upstream direction, from the Mississippi toward the Great Lakes, and include the Asian bighead and silver carps, now only 50 miles from Lake Michigan. Bighead and silver carp are planktivores that have the potential to compete with established sport fishes, such as yellow perch, for limited plankton resources in Lake Michigan, and with the primary prey base of the important salmon fishery. The Asian black carp may be next in line, as at least thirty were accidentally released into a tributary of the Mississippi River from an Arkansas aquacultural facility during a flood in 1994.  Although restoration of the natural drainage divide has been proposed as a solution to the interbasin transfers of harmful aquatic species, Chicago still depends on the canal for adequate drainage of storm water (the city was originally built in a low-lying wetland); downstream conveyance and dilution of treated wastewater that would otherwise adversely affect Lake Michigan beaches and drinking water supplies; and navigation that includes tour boats, recreational craft, and commercial barges.  Thus the Illinois Waterway, which includes the Illinois River, its tributaries, and the man-made canal system in Chicago is of great socio-economic and ecological importance, and depends upon good understanding and management of main-stem hydrology.

The watershed of the Illinois River is representative of low-relief glaciated landscapes of the Midwestern United States that is among the “most productive agricultural regions” in the world. Land use in this geologically young landscape is dominated by modern industrial agriculture, which encompasses about 80-85% of the basin area Prior to settlement much of the flat landscape was poorly drained and required artificial drainage to make it productive. In the process, over 80% of the natural wetlands and most prairie vegetation were destroyed, and seasonal hydrologic patterns became “flashier”, with more frequent high and low streamflow and less frequent periods of moderate streamflow. Today, the hydrology of the watershed is influenced strongly by the land-drainage system, including subsurface drains and surficial drainage ditches. Subsurface drains consist of a network of perforated pipes installed at a prescribed depth below the surface. When the shallow water table rises to this level, the water drains away through these pipes. This results in significant alteration in the quantity of water stored in the vadose zone. These elements of the land-drainage system connect directly with natural rivers and streams, many of which have been channelized to enhance their hydraulic efficacy.

These changes have had a pronounced influence on the hydrology of the watershed. From a water-quality perspective, massive inputs of chemicals to support industrial agriculture are an important consideration in this landscape. Each year farmers apply about 4 million tons of fertilizer on Illinois farm lands including 962,000 tons of nitrogen and 391,000 tons of phosphorus. The phosphorus loading in the stream system due to agricultural and sewage contributions are comparable. The downstream flux of nutrients, such as nitrogen and phosphorus, has been linked to pressing environmental concerns, such as eutrophication and hypoxia in the Gulf of Mexico. Nitrates have also been implicated in a number of health-related issues, including methemoglobinemia, non-Hodgkins lymphoma and stomach cancer. The nitrate issue, and other water quality issues related to agricultural activities, are a widespread problem throughout a large, populated, and economically critical area extending from Indiana and Illinois to Minnesota. Moreover, the exposure of soils once covered by thick prairie sod, along with the channelization of many streams and rivers, has led to large downstream fluxes of sediment through the watershed system. This sediment is delivered to the Illinois River waterway where they have accumulated within backwater lakes on the floodplain and within the main stem of the river. Sediment dynamics within this watershed have been greatly altered by landscape-scale disturbance of uplands, modification of the hydrologic regime and stream channel characteristics, and the regulation of the main stem hydrologic properties through the construction of locks and dams. As an agricultural watershed representative of the Midwestern United States, the establishment of the Illinois River basin as an observatory testbed will provide insight into the hydrology, water quality and sediment dynamics of many similar watersheds throughout the Midwestern United States.

The Illinois River system, prior to extensive human influence and modification, was a highly diverse natural ecosystem, but is now the focus of a massive integrated-management initiative that seeks to improve ecological conditions while maintaining the needs of human communities in the watershed. This initiative includes federal, state and local government agencies, citizens groups and private companies. Current general management plans have been formulated based on focused scientific efforts, including several multidisciplinary studies in small tributary watersheds and in the floodplain along the main river, funded by NSF, USEPA, and others.  Links to these projects and others are provided by the web-based Illinois River Decision Support System, maintained by the Illinois State Water Survey. Further management decisions will have a continued need for comprehensive study of complex interconnections among components of the watershed system at various temporal and spatial scales. Since over 90% of the population of Illinois (12.5 Million) lives in the IRB, the challenges for environmental management in the face of human needs are great, but typical of IMLs throughout the Midwest.

Management of the river involves controlling water levels to maintain sufficient depth for barge traffic, while attempting to meet the hydrological needs of riparian ecosystems. Often these two needs are in conflict and historically the preference given to meeting the needs of barge traffic has resulted in severe degradation of natural ecosystems. Some discussion has emerged about “controlled” experiments to meet hydrologic requirements of riparian ecosystems, ala controlled releases conducted recently for the Colorado River system, but thus far no decisions have been made about this type of action. Also, it is unlikely that this type of hydrologic experimentation will solve the problem of massive sediment accumulation within floodplain and main-stem habitats.

Estimates of water fluxes to and from the groundwater system are essential to complete our examination of the elements of the hydrologic cycle. The IRB includes several significant surficial and deep aquifers. These aquifers are the major water source for cities in east-central and northeastern Illinois, and the long-term sustainability of these systems will play a major role in the economic development of the state. Hence it is crucial to understand recharge and discharge fluxes for these aquifers, including the impacts of urbanization upon groundwater recharge, and conversely, the effects of increasing groundwater development on surface waters.

From a watershed perspective, the hydrology of the landscape is now primarily controlled by the surface and sub-surface drainage systems, either in agricultural settings (subsurface drains and channelization) or urban/suburban settings (sewers). Management centers on the need to control better the downstream flux of nutrients, sediment and runoff from agricultural lands into streams, rivers and major waterways. Stream naturalization/restoration to mitigate the adverse effects of human modifications is an important consideration. However, such management should be based on sound scientific understanding of the temporal and spatial variability and trend of these fluxes, through integrated and sustained studies of a type that currently does not exist. The establishment of an Observatory testbed in the Illinois River basin would help to generate such understanding and the general principles will be transferable to many other watersheds throughout the Midwest.