From the roadside of Illinois Route 116 in southwestern Woodford County, just before it plunges 250 feet down the river bluffs, one can look outward onto the southwestward-oriented Illinois River valley. Like foothills from some nearby mountain range, the rounded, forested bluffs, which are ancient glacial moraines, rise abruptly from the gently meandering river valley and lower Peoria Lake. Fancy might suggest that this is a long forgotten, hidden river valley, especially if the traveler has just emerged from the rolling farmlands that cover most of central Illinois. But the river valley, of course, has a long history of human occupancy; and the results have been both positive and negative. One positive effect is called Cooper Park Wetlands, a 55-acre natural area located at the base of the river bluffs, along the margins of lower Peoria Lake.
Just before entering Tazewell County from the north, Route 116 descends the steep bluffs toward the edge of the river bottomlands; the ride is reminiscent of landing in a steady, slowly descending aircraft. The scenic view challenges the driver to remain alert. At the bottom of the bluffs, the highway soon follows along a narrow strip of land (about one quarter mile wide) between the river bluffs (rising to as much as 700 feet above sea level) and lower Peoria Lake (actually a vast widening of the Illinois River itself, which imperceptibly flows through the lake). Along this road, at the edge of East Peoria, a variety of small businesses and light industries, in strip-development fashion, have located along the highway’s frontage roads. But beyond the businesses west of Route 116, a stand of trees is barely visible. To the unknowing traveler, this stand of trees, part of Cooper Park Wetlands, may not cause much excitement, seemingly pushed by development to the edge of the lake. That response, however, would be mistaken.
While Cooper Park Wetlands is less than one quarter mile wide, it covers nearly one mile of lake front. A trail system runs through the entire length of the natural area as it passes through several distinctly different plant communities. The bird life is correspondingly diverse, especially during the breeding season.
The best place to begin a hike is probably at the north trail head just south of the Spindler Marina, which, like Cooper Park Wetlands, is owned and managed by the Fon du Lac Park District. A large sign proclaims that the hiker is about to enter a "Registered Reserve." Cooper Park Wetlands is, in fact, protected by state law from inappropriate development or direct destruction. A boardwalk begins after the entrance sign and leads through a wet floodplain forest dominated by silver maple, cottonwood, and green ash. From April through August, this is where the songbird chorus also begins. Warbling vireos and house wrens seemingly sound off from every direction. In spring and early summer, red-winged blackbirds and American robins are the most numerous and obvious species throughout the park. From mid-summer through September, anywhere along the trail, the laser-beam red of cardinal flower never fails to generate attention and comments.
Before too long the woods begin to open up, the ground gets wetter, and the trees more shrubby. Here the trail enters a shrub swamp dominated by black willow trees, buttonbush, and young silver maples. In areas of open marsh, inter-meshed within a mosaic of shrubs and young floodplain forest, wetland species such as arrowhead, arrow arum, and smartweed combine to create a much thicker ground cover than in the densely shaded floodplain forest, where, in places, ground cover can be rather sparse. In summer, in areas of open marsh, the large pinkish-white flowers of hibiscus and the less gaudy swamp milkweed are also very obvious. The shrub swamp is the best and most reliable place to see and hear yellow warblers, but only during the height of the breeding season; by July, they will be hard to find. In the marsh openings, one may see the occasional foraging great blue heron or the shy green heron, looking very much like a crow flying off, with a call that sounds a bit like a cat regurgitating a hair ball.
The boardwalk soon passes over a series of beaver ponds, sometimes wholly covered over with a mat of floating duckweed, a small flowering aquatic plant only a few tenths of an inch in width, that many mistakenly call green algae. Where one of the beaver dams is only a few feet away, it is easy to see the water slowly trickling through the dam. Further to the east is another beaver dam, and toward the lake is another, all in perfect repair and holding back just enough water to meet design specifications. Should a breach occur, it will be quickly detected by the beavers, who will soon effect sufficient repairs. This is a good location to search for tree stumps bearing teeth marks from the perspicacious, gnawing rodents. The ponds sometimes hold resting waterfowl, most likely mallards and wood ducks, but the beavers will probably remain elusive.
Beaver dam at Cooper Park Wetlands.
Along the boardwalk is the best place to view the decurrent false aster, a federally threatened plant that has its main population distribution limited to the Illinois River valley. The decurrent false aster, which blooms in August and September, is known as a "fugitive species," which tends to rapidly colonize soil left bare from disturbances. It soon loses out in competition with other plant species, and fails to survive and reproduce after only a few years. The species is, therefore, dependent upon the formation of other newly disturbed areas for colonization.
Decurrent false aster and arrowhead at Cooper Park Wetlands.
Yearly spring floods along the Illinois River are the main mechanism that clears areas of competing vegetation, allowing colonization by the decurrent false aster. The plant has adapted over thousands of years to the river’s natural hydrology, which is characterized by a gradual rise in river levels beginning in winter, a mid-spring peak flood, followed by a gradual decline to stable, low water levels throughout the summer.
The river’s annual spring flood, in fact, is the critical mechanism that is essential to maintaining all of the river’s natural communities of plants and animals. For many years, however, the Illinois River’s watershed has been highly modified by levee-building, ditching, drain tiles, urbanization, intensive row-cropping, and stream channelization. The result has been that for many decades water has been delivered to the main stem river at a much more rapid rate than before intensive agriculture. This allows for more frequent floods, even during the summer growing season, when the plants are "expecting" stable, low water levels. Water level changes also occur as a response to operation of the lock and dam system. And the river’s water is now seemingly perpetually turbid due to its high suspended sediment load from erosion across the watershed. Plants that are adapted to surviving under water for brief periods may now, thus, be enshrouded in darkness until the water recedes. It is little wonder, then, that a plant such as the decurrent false aster is threatened with extinction. The large Peoria lakes (lower and upper), in any case, tend to buffer water level changes during the growing season; and perhaps that is why the decurrent false aster persists at Cooper Park Wetlands.
[For a continuation of this story, see Nature on the Margins at Cooper Park Wetlands - Part 2 of 2.]
[Note: This story originally appeared in slightly different form in the fall 2002 issue of Illinois Audubon magazine, number 282.]
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