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Physiography
The majority of the Lower Illinois River Basin (LIRB) is in the Tills Plains Section of the Central Lowland physiographic province. A small area near the confluence of the Illinois and Mississippi Rivers is in the Lincoln Hills Section, which is part of the Ozark Plateau Province.

The Till Plains Section is composed of four subsections; the Bloomington Ridged Plain (38%), the Galesburg Plain (26%), the Springfield Plain (35%), and the Kankakee Plain (.5%). Landforms in each of the subsections are the result of glaciation. The site selection process was narrowed to the first three subsections as they make up 99% of the Tills Plains Section, which is the majority of the basin (98%). The Bloomington Ridged Plain Subsection includes the Wisconsinan glacial moraines and the associated glacial topography. The Galesburg Plain Subsection and the Springfield Plain Subsection were grouped together because they are both distinguished by the older Illinoian drift.

Land Use
Agriculture is the predominant land use (87%) in the LIRB. The major crops grown are corn and soybeans. The rest of the land use in the LIRB is forest (8%), urban (2%), water and wetlands (2%), and a remainder of miscellaneous land-use areas (1%). The category of agriculture is of greatest significance because of its role in water-quality issues. Urban land use also greatly impacts water-quality, but because of its meager representation within the LIRB, it was best suited to locate sites of study to areas representative of agriculture.

Soil Permeability
For our needs, the permeability of the soil is categorized into four groups (high, moderate, low, and impermeable). Small portions of the area are classified as impermeable (2%) and highly permeable (2%). The impermeable soils are found mostly in Livingston county in the northeast part of the basin. The highly permeable soils are located mostly in Mason county in the center of the LIRB. Close to and above these highly permeable soils are the cities of Peoria and Pekin. 22% of the soils in the basin are of moderate permeability. Most of these soils are located in the river valleys. The majority of the LIRB is categorized as low permeability (73%). As each drainage basin is composed mostly of the low permeable soil, each of the sites is categorized as having low soil permeability. The remaining 1% of the basin is categorized as open water and has no permeability classification.

Drainage Basin Size
One station with a small drainage basin size was chosen in both the Galesburg/Springfield Plain and the Bloomington Ridged Plain. Similarily, stations with medium and large drainage basins sizes were selected. In addition, two stations were selected to measure the inflow and outflow of the LIRB. This makes a total of eight stations of which four are at the output of large watersheds, two are at the output of medium sized watersheds, and two are at the output of small basins. These sites are split evenly between the physiographic subregions. The stations draining the large basins in the Galesburg/Springfield Plain are the Sangamon River near Oakford (5,093 square miles), and the Illinois River at Valley City (27,000 square miles). The station at Valley City records the outflow for all of the other selected stations. The stations draining the medium and small sized basins in the Galesburg/Springfield Plain are on the La Moine River at Colmar (655 square miles), and Indian Creek near Wyoming (63 square miles) respectively. The stations draining the large basins in the Bloomington Ridged Plain are the Mackinaw River near Green Valley (1,073 square miles), and the Illinois River at Ottawa. The station at Ottawa records the inflow of surface water into the Lower Illinois River Basin. Ottawa's station records drainage of the Upper Illinois River Basin, an area approximately 11,000 square miles. The stations draining the medium and small sized basins in the Bloomington Ridged Plain are on the Sangamon River at Monticello (550 square miles), and Panther Creek near El Paso (94 square miles), respectively.