Illinois Department of Conservation, credits improved water quality for partial restoration of gamefish populations in the upper 100 miles of the river, including the appearance of a sauger Stizostedion canadense (Smith) population that has supported a nationally ranked annual fishing tournament (Conlin, 1987).

Although restoration of water quality is often a necessary component of river and stream restoration, improving water quality alone may not be sufficient to restore streams and rivers, as described next in the case of the Illinois River.

Stress Thresholds: Sedimentation in the Illinois River

Some ecosystems have a degree of biotic control or compensation that may mask a gradual, detrimental change in some physical-chemical factor until a threshold is reached. Once the threshold has been crossed, the ecosystem may degrade rapidly into a stable condition that is very difficult to restore to its previous condition. In the Illinois River, clear vegetated backwaters and lakes became excessively turbid, barren areas, and gamefish and duck populations declined drastically from 1958 to 1961 in what was formerly the most biologically productive reach of the river: the lower 321 km (200 miles) (Bellrose et al., 1979). These degraded conditions persist to this day and are the subject of restoration efforts ranging from the scale of the entire drainage basin to experimental plots a few meters square.


The changes in biological productivity were associated with, and are probably attributable to, increases in sediment loading and sediment resuspension in the Illinois River (Bellrose et al., 1979). Sediment loading increased because of land use changes in the drainage basin and floodplain. Cropland accounts for 70.4 percent of the land area of Illinois, and so changes in farming practices have a major impact on streams and rivers (Herman, 1987). In the Illinois River basin, row cropland increased about 67 percent between 1945 and 1986, at the expense of pasture, forage, and small grains, which better protect the soil from erosion (Bellrose et al., 1979). As farms became larger and more specialized in row crops, fences and fence rows were taken out (Illinois Environmental Protection Agency, 1979). The size of farm machinery also increased, making it more difficult to do contour farming, and many contours and old terraces were removed (Illinois Environmental Protection Agency, 1979). The common practice of plowing fields soon after harvest in the fall leaves land

The National Academies | 500 Fifth St. N.W. | Washington, D.C. 20001
Copyright © National Academy of Sciences. All rights reserved.
Terms of Use and Privacy Statement