The humid, temperate climate of Illinois is conducive to the weathering of soil, formation of
clay, and movement of leached chemical constituents downward in the soil profile. Tills commonly
weather to depths of 5 ft and sometimes to a maximum depth of 15 ft along soil discontinuities
(Panno and others, 1994). Most soils in the basin are mollisols, which are dark-colored soils formed
under grass vegetation (Fehrenbacher and others, 1967). Mollisols average more than 1 percent
organic matter. The native prairie vegetation under which soils form contributed to the high accu-
mulation of organic materials, which is valuable to agriculture because of the capacity to store
water and nutrients. The areas that are not mollisols are along stream valleys where the light-col-
ored alfisols formed under forest vegetation. A small area in Mason County has entisol, which is a
sandy, well-drained soil (Fehrenbacher and others, 1967).
Soil type and distribution are two factors that affect the amount of soil erosion that results in
sedimentation of lakes and reservoirs in the LIRB. Sedimentation is one of the most important
water- quality problems in the LIRB. Rates of soil erosion of up to 2 percent per year of farmland
soil has been measured (Stout and Korab, 1993). Sedimentation from soil erosion is particularly
serious in Illinois for three reasons: (1) The loess materials blanketing a large part of the State are
highly erodible by water, even on the gently sloping land that covers most of the State; (2) Under
conventional tillage practices for corn and soybeans, the primary crops leave little residue on the
surface for much of the year; and (3) Rainfall in Illinois is fairly high in the spring when little veg-
etative cover is present on cropland (Neely and Heister, 1987, p. 22).
The climate in the LIRB is humid continental, with cold, relatively dry winters and warm,
wetsummers. Three air masses affect the climate in Illinois. The coldest, driest air mass is from
Canada and most frequently covers Illinois in the winter. The warmest, most humid air mass and
source of most of the precipitation originates from the Gulf of Mexico and is most frequent in the
summer. The third air mass originates over the Pacific Ocean and tends to bring mild, dry air to
Illinois. Any of the three air masses can be found over Illinois during a given season; thus, account-
ing for large day-to-day temperature and humidity variations (Neely and Heister, 1987, p. 120).
The mean annual temperature in the LIRB ranges from 50°F in the north to 55°F in the south. Vari-
ations inprecipitation and temperature may occur in any year because the basin is far from large
physical features, such as oceans or mountain ranges that modify regional weather patterns.
Precipitation is normally 35–38 in/yr (fig. 9).
In 1993, many States in the Midwest set records
for annual precipitation, which resulted in massive flooding. Heavy, sustained precipitation in early
spring through October contributed to the wettest year on record for Illinois with 50 in. as the state-
wide average precipitation for the year. The previous record was 49.5 in., which was set in 1927.
Precipitation ranged from 50 to 70 percent above the long-term average across the State (Wendland
and Dennison, 1993).
An understanding of the geology is critical to understanding the location, movement, and nat-
ural quality of surface and ground water in the LIRB. The drainage pattern of the LIRB has been
shaped by many geologic processes. The bedrock distribution and topography affected subsequent
glacial depositional processes. Glacial processes have strongly affected thehydrology of the
