Backwater
Lakes
Backwater lakes (Figure 54) are one of the greatest natural resources, supporting diverse fishery, waterfowl, mussel, and plant populations, and a thriving economy based on hunting, logging, recreation, navigation, and agriculture (Schneider 2000). Before settlement, the Illinois River floodplain contained so many lakes that it “resembled a boundless marsh” (IDNR 1998). Unfortunately, today only 11 of these backwater lakes remain.
The productivity of the Illinois River is inextricably linked to the backwater lakes for two reasons. First, the shallow waters provide breeding and feeding grounds for fishes. Second, the lakes supply food for fishes that thrive in the main channel of the river. Plankton, the base of the aquatic food chain, is produced exclusively in the backwater lakes. In the main channel, the flowing water will transport the plankton downstream, leaving the river devoid of its food supply. Floods, however, wash plankton from the backwater lakes into the river, thus replenishing the food supply. As levees are built and the floodplain is drained, the fishery collapses (Schneider 2000).
Paradoxically, both the origin and demise of backwater lakes are in the deposition
of silt. As a river floods, sediment is deposited in the floodplain, raising
its level. At the margin of the channel, sediment, deposited in greater volumes,
creates a natural levee 4 or 5 feet higher than the surrounding floodplain.
Rivers also move horizontally within the floodplain; their outer bank erodes
and sediment is deposited on their inner bank. Another consequence of the horizontal
movement of the river is the creation of depressions or swales that fill with
water during floods, forming floodplain lakes, sloughs, and marshes. During
flood conditions, these swales are connected to the river; as the floods recede,
they become isolated from the river.
Thus, siltation results in the formation
of natural levees, the movement of the river creates swales, and both form
backwater lakes (Schneider 2000).
The same processes that create backwater lakes also doom them. Silt continues to be deposited under flood conditions, which results in backwater lakes gradually filling in - shallow lakes become mud flats, and mud flats are gradually colonized by trees such as willows and cottonwoods, and eventually become a bottomland forest. Under natural conditions, backwater lakes are being created and filled in simultaneously (IDNR 1998).
Unfortunately, with the construction of agricultural levees, humans have accelerated the process of sedimentation and have virtually stopped the creation of new lakes and sloughs. Currently, these lakes are filling in faster than would occur under natural conditions because of human-induced erosion. Since settlement, backwater lakes have lost 70% of their capacity to sedimentation. The materials entering the river are mostly silts and clays that are more difficult to remove by dredging than is the sand that is the problem in the main channel of the Illinois River (IDNR 1998).
The depth of Upper Peoria Lake has decreased from 7.6 feet in 1903 to 5.3 in 1998. Peoria Lake suffers even more dramatic changes, decreasing from 8.0 feet to 2.6 feet during the same period. If these sedimentation rates do not change, these lakes will lose an additional 50% of their volume over the next 200 years (IDNR 1998).
