. The Changing Illinois environment : critical trends : summary report of the Critical Trends Assessment Project. Man; Pollution; Environmental protection; Ecology; Environmental impact analysis. 22 • The Changing Illinois Environment Human use has significantly altered natural stream function. • More than 25% of the total length of sizeable streams in four main river basins has been straightened. Page 20 • Almost every sizeable Illinois stream is dammed in at least one spot. Page 21 Figure 3-8 Number of Dams Constructed in Illinois in Each Decade and Mean Normal Storage 1900-1990 30 ,
. The Changing Illinois environment : critical trends : summary report of the Critical Trends Assessment Project. Man; Pollution; Environmental protection; Ecology; Environmental impact analysis. 22 • The Changing Illinois Environment Human use has significantly altered natural stream function. • More than 25% of the total length of sizeable streams in four main river basins has been straightened. Page 20 • Almost every sizeable Illinois stream is dammed in at least one spot. Page 21 Figure 3-8 Number of Dams Constructed in Illinois in Each Decade and Mean Normal Storage 1900-1990 30 , 20 E ra Q 10 Acre - Feel Dams. , 00 10s 20s 30s 40s 50s 60s 70s 80s Source: Ecological Resources, Illinois Natural Histor>' Surrey, 1994 natural flooding and dr>- ing cycle in the flood- plains that formerly main- tained a highly productive and diverse biota. Fish and wildlife Of the larger animal species present in Illinois streams at the turn of the century, about one in five of the fish, one in three of amphibians and reptiles, more than half the fresh- water mussels, and one in five of the crayfish have been extirpated or are threatened by extinction. Information is not available to gauge the survival status of insects and aquatic plants. Forty-four of the 100 species of amphibians and reptiles known in Illinois have a stream-dependent larval stage lasting from several months to a few years, and nearly ail of thtm also deposit their eggs in water. These complex life cycles are especially depen- dent on high-quality, varied stream habitat. For example, the hellbender salamander requires fast-running clear water and gravelly streambeds of the sort that have been buried by silt throughout much of Illinois; the animal has been seen only once in Illinois since the 1950s. Sixteen exotic fish species are reproducing in Illinois streams and rivers. .Most of them were introduced by accident. Some native species, such as the red shiner, that are toleran
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