Construction of locks and dams along the Illinois River,
especially in the reach extending from Grafton to the Lake DePue
area, has created a large number of backwater areas within the
main floodplains of the river. These backwater areas are called
backwater lakes, and they are subject to natural variations in
water depths and sediment deposition. Moreover, over the last
100 years or so all the river basins in Illinois have been
subjected to intense human activities and alterations. As a
result of these natural and human interventions, the Illinois
River valley has experienced a tremendous amount of sediment
deposition in recent years. Many of the 53 or so backwater
lakes along this river have lost 30% to 100% of their capacity
to sediment deposition. Peoria Lake, a bottomland lake, has
lost 68% of its 1903 capacity, and upper Peoria Lake will
eventually attain the appearance of an incised river with broad
and shallow wetlands on both sides. On the average, about 20.6
million tons of sediment is deposited annually over the entire
valley, with a deposition rate of 0.81 to 2.1 inches per year.
Recently implemented nonpoint source pollution control measures
are showing their impacts on the receiving bodies of water
through substantially lower concentrations of trace elements
within the recently deposited sediment. Numerous alternative
solutions can be considered for managing these backwater lakes,
including constructing closing structures, periodic and
selective dredging, using dredged materials to create artificial
islands, diverting high flows, installing management practices
on the watershed, and converting some of the backwater lakes to
marsh and wetland habitats. A concerted effort, in which
various physical, biological, and environmental factors are
considered, is needed for the development of an acceptable
management scenario for these backwater lakes.
