. The Changing Illinois environment : critical trends : summary report of the Critical Trends Assessment Project. Man; Pollution; Environmental protection; Ecology; Environmental impact analysis. Historically the Illinois River's backwater lakes have been among the most important migra- tion areas for several species of ducks, including mallards. Since 1948, however, significantly fewer migrating mallards have alight- ed in the Illinois valley each fall. The trend is thought to reflect the general decline in mallard numbers across North America, but there is evidence that local conditions also
. The Changing Illinois environment : critical trends : summary report of the Critical Trends Assessment Project. Man; Pollution; Environmental protection; Ecology; Environmental impact analysis. Historically the Illinois River's backwater lakes have been among the most important migra- tion areas for several species of ducks, including mallards. Since 1948, however, significantly fewer migrating mallards have alight- ed in the Illinois valley each fall. The trend is thought to reflect the general decline in mallard numbers across North America, but there is evidence that local conditions also have deteriorated. The Illinois River's complex system of wetlands has long attracted more mallards than that of the Mississippi River. (In the late 1940s, mallards along the Illinois outnumbered those sojourning on the Since 1948 significantly fewer migrating mallards have alighted in the Illinois valley each fall. canvasbackâhave suffered more drastic population crashes on the Illinois River. Lesser scaups were abundant in the Illinois valley before the 1950s, especially on Upper Peoria Lake. More than 585,000 ducks were counted on one stretch of the river in 1954; three years later the number was around 10,000. Similar trends were recorded in populations of canvasbacks. During the 1952 migration more than birds were counted along the Illinois River north of Peoria; in 1971, only 120 were seen. The cause of the decline in numbers of lesser scaup and canvasback ducks in the Illinois River vallev is a scarcirv of 2,000 Illinois River Mississippi River E x> Z ra J.: S ^. o 3' ^ larger river nine to one.) The post-1948 decline in mallard counts along the Illinois has been relatively steeper than that recorded on the Mississippi, where sedimentation has not caused such drastic reductions in the amount and variety of natural plant foods available to migrating flocks. In addition, tillage of increased acreage of harvested corn fields in central Illinois during fal
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