1.—The bottomland (backwater) lakes of the Illinois River valley embrace about 28,500 ha (70,000 acres) and attract hundreds of thousands of waterfowl during their fall and spring migrations. All of these backwater lakes except Peoria Lake are lateral to the river channel.
2.—The Illinois River occupies a valley much older than the river itself as a result of a series of unique geological events. This valley in essence was the Mississippi River valley before the Wisconsinan glaciation. Its bottomland lakes developed because the river's remarkably low rate of fall resulted in its aggrading rather than degrading.
3.—Unfortunately, the very principles of sedimentation that created the lakes also set the stage for their extinction. Under pristine conditions this extinction would have taken hundreds, perhaps thousands, of years, but man, through intensive use of the land, has greatly accelerated the process.
4.—Aquatic and terrestrial habitats of the Illinois Valley have suffered a series of cataclysmic events since 1900: first, a permanent rise in water level from water diverted from Lake Michigan; second, the draining of more I Aug. 1979 Bellrose et al.; Waterfowl and the Changing Illinois Valley 49 than half of the 161,878-ha (400,000- acre) floodplain through the construction of levees and pumping stations; third, an upsurge in untreated urban and industrial pollution during the 1920's; fourth, the creation of a 2.7-m (9-ft) channel and its attendant navigation dams in the 1930's; and fifth, an acceleration in sedimentation rates following World War II, apparently resulting from an increase in the amount of open row crops grown within the basin.
5.—Waterfowl food plant resources have been dramatically altered by the many changes wrought by man. Factors that have directly affected the species composition and abundance of the wetland plants are (1) fluctuating water levels, (2) water turbidity, (3) water depth, and (4) competition between plant species.
6.—Fluctuating river levels adversely affect the development of aquatic and marsh vegetation on those bottomland lakes connected with the river at all stages. In the early years of the study, the more the lakes were separated from the river, the more extensive were their aquatic and marsh plant beds.
7.—During the earlier years of the study, aquatic and marsh plants disappeared from those lakes connected with the river at all water stages (and thus subject to water-level fluctuations). During the later years of the study, aquatic plants disappeared and the area of marsh plants greatly declined in all lakes, even in those enjoying a degree of separation from the river and minimal water-level fluctuations. Increases in water turbidity and bottom softness, stemming from sedimentation, appear to be responsible.
8.—However, low levees and pumps have increasingly been used to dewater all or part of the lake basins. This procedure controls small summer fluctuations and exposes mud flats for the development of moist-soil plants between 15 July and 15 October. Moist soil plants—millets, smartweeds, nutgrasses, rice cutgrass, water hemp, and teal grass—produce an abundance of seed palatable to many species of ducks. Low summer water levels permit or expedite dewatering. Summer rises that overtop low levees usually destroy moist-soil plant beds.
9.—Sedimentation is rapidly filling in the bottomland lakes of the Illinois Valley, reducing their size, degrading water quality, and minimizing the diversity of bottom depths. The fine silts and clays deposited on the bottoms when river waters invade bottomland lakes are readily resuspended by wave action and the activity of rough fish. The consequent turbidity reduces the euphotic zone to such a shallow depth that aquatic plants can no longer survive. Marsh plants have difficulty maintaining footings as bottom soils become softer.
10.—.Sedimentation occurs at a higher rate in deep water than in shallow water. Thus, most lakes now possess a uniform bottom instead of the turn-of-the century variation in bottom depths. (Peoria Lake, through which the river channel passes, is an exception.) Lake basins are now platter shaped. Estimated life expectancies are 33 years for Lake Depue, 92 years for Lake Chautauqua, and 90 years for Meredosia Bay.
11.—The abundance of certain species of waterfowl in the Illinois Valley is related to the abundance of native food resources. Among the dabbling ducks, the size of fall populations of the pintail, green-winged teal, and wigeon correlated with the abundance of wetland plants. Mallards feed extensively on waste grain in harvested fields, but even so, when annual variations in the continental mallard population were taken into account, moist-soil plant abundance influenced the abundance of mallards. Diving duck populations were unrelated to wetland plant abundance. However, when a catastrophic loss of fingernail clams occurred, diving duck numbers crashed. Neither this food resource nor the population of diving ducks has recovered in the ensuing two decades.
12.—Fall river levels determine the depths in bottomland lakes and thus the availability of moist-soil plant foods. If the river is low and mud flats are exposed, moist-soil plant seeds will be unavailable to waterfowl. If, on the other hand, the river is high and mud flats are too deeply submerged, the result is the same. The higher the fall rise in water, the greater the reduction in numbers of green-winged teal, with the same influence to a lesser degree on pintails, wigeons, and mallards.
13.— As a result of the disappearance of aquatic plants and the prohibition of baiting, private duck clubs, the Illinois Department of Conservation, and the U.S. Fish and Wildlife Service have placed increasing emphasis on controlled dewatering of wetland habitats. Private duck clubs control 23,198 ha (57,320 acres) of land and water in the Illinois Valley and have 6,723 ha (16,612 acres) under varying degrees of low water level control. State and federal agencies control 15,644 ha (38,656 acres) and have 4,688 ha (11,585 acres) under similar water-level management.
Effect of the north Atlantic oscillation on water level fluctuations in lakes of northern Poland
