The contrasts presented by the Illinois River at high water and at low water respectively are graphically set forth by Kofoid in his report on the plankton work of the Natural History Survey, published in Volume VI of the State Laboratory Bulletin. (1915)
During High Water
"A trip by boat," he says, "across the submerged bottom-lands from the Quiver shore [on the east bank, 2 miles above Havana] to the western bluff in the latter part of May would be far more enlightening than any description that might be given. As we leave the sandy shore of Quiver we traverse the clear, cold, and spring-fed water along the eastern bank with its rapidly growing carpet of Ceratophyllum [hornwort], and in a few rods note the increasing turbidity, rising temperature, and richer plankton of the water which has moved down from the more or less open and slightly submerged bottom of the north.
"As we cross the muddy bank of Quiver ridge and enter the main channel of the river we find rougher water, caused by the wind which usually sweeps up or down the stream with considerable force between the bordering forests. The water also appears much more turbid by reason of silt and plankton, and no trace of vegetation is to be seen save occasional masses of floating Ceratophyllum or isolated plants of Lemna, Wolffia, or Spirodela [duckweeds]. Huge masses of cattle-yard refuse, veritable floating gardens, may also at times be seen moving down the channel or stranded in some eddy along shore.
"As we plunge into the willow thicket on the western shore we have to pick our way through the accumulated drift lodged in the shoals or caught by the trunks of the trees or the submerged underbrush. The surface of the water is one mat of logs, brush, sticks, bark, and fragments of floating vegetation with its interstices filled with Lemnaceae [duckweed] dotted with the black statoblasts of Plumatella. From this dark labyrinth we emerge to the muddy but quiet waters of Seeb's Lake with its treacherous bottom of soft black ooze.
"We next enter a wider stretch of more open territory with scattered willows and maples and a rank growth of semiaquatic vegetation, principally Polygonums [smart-weed]. The water is clearer and of a brownish tinge (from the diatoms), while mats of algae adhere to the leaves and stems of the emerging plants. A flock of startled waterfowl leave their feeding grounds as we pass into the wide expanse of Flag Lake. We push our way through patches of lily-pads and beds of lotus, past the submerged domes of muskrat houses built of last year's rushes, and thread our way, through devious channels, among the fresh green flags and rushes just emerging from the water.
"Open patches of water here and there mark the areas occupied by the "moss" or Ceratophyllum, as yet at some depth below the surface. The Lemnaceae are everywhere lodged in mats and windrows, and, amidst their green, one occasionally catches sight of a bright cluster of Azolla. The water is clear and brownish save where our movements stir the treacherous and mobile bottom.
"We now enter a second time the partially wooded country, and cross the submerged ridge to the sandy eastern shore of Thompson's Lake. This ridge is covered by submerged vegetation which has as yet attained but little growth. The "breaks" of the startled fish show that we have invaded favorite feeding grounds. The waters are evidently moving towards the river, and they bear the rich plankton of Thompson's Lake, while their turbidity is doubtless increased by the movements of the fish. Schools of young fry can be seen feeding upon the plankton in the warm and quiet waters.
"Thompson's Lake, the largest expanse of water in the neighborhood, is wont to be rough in windy weather, but if the day be still we can see the rich aquatic vegetation which fringes its margin and lies in scattered masses toward its southern end. Its waters seem somewhat turbid, but more from plankton than from silt, though the deep soft mud which forms much of its bottom is easily stirred. The slender transparent limnetic young of the gizzard-shad may be seen swimming near the surface. There is a perceptible drift to the south in the open lake, though this current is deflected by the elevated banks of Spoon River towards the Illinois River, crossing the lower bottom-lands above this region.
"If we push on through the fringing willows at the south we find a series of open places locally known as "ponds." The warm still waters are turbid in places from the movements of fish, and at times we see the compact schools of young dogfish (Amia calva) and, if we are late enough in the season, the myriads of young black, tadpole-like catfish (Ameiurus), likewise in schools, while young carp (Cyprinus carpio) are everywhere. The new vegetation is already springing from the decaying and matted stems of the preceding summer.
Mouth of the Spoon River
"Turning back towards the river we pass through the heavy timber where the still brown water, cool and clear, overlies the decaying leaves and vegetation of last season's growth, now coated with the flood deposits of the winter. Emerging again upon the river channel, we may find a turbid yellow flood pouring out from Spoon River, bringing down its load of drift and earth, and marking its course down the stream as far as the eye can see.
During Low Water
"Contrast with the extent and variety of conditions at flood the limitations placed upon the stream at low water. Instead of an unbroken expanse of four or more miles we find now a stream only 500 feet in width, while the adjacent territory is dry land save where the sloughs, marshes, and lakes remain as reservoirs. Quiver Lake is now much reduced in width, and it may be choked with vegetation except in a narrow channel where the clear water shows little or no current.
"A half mile below we find the river water rushing in a narrow "cut-off" across the ridge of black alluvium into the lower end of the lake. The wooded banks which separate the river from Quiver and Seeb's lakes are now crowded with a rank growth of weeds and vines. The latter "lake" is reduced to a shallow stagnant arm of the river, whose warm turbid waters are foul with dead mollusks, and whose reeking mud-flats beneath the August sun shine green and red with a scum of Euglena.
"As we pick our way through the tangle of rank vegetation we come upon Flag Lake, now a sea of early summer, and its margins are even now dry, with gaping cracks. Beyond the marsh we pass to the shore of Thompson's Lake to find its southern end choked with vegetation, though the greater part to the north is open water. The woodland and open ground to the south are now pastures and fields of waving corn. The only outlet to this large body of water, now somewhat reduced in area but warm, turbid, and rich in plankton, is a tortuous slough six miles to the north. The discharge, however, is in any case but slight, the lake being, indeed, not infrequently the recipient of river water.
"Spoon River still pours a sluggish but constant stream into the river, but save for a waterbloom of livid green (Euglena) its waters yield but little plankton. Thus, of all the wide area contributing to the plankton of the channel at high water there are now remain only Thompson's and Quiver lakes and Spoon River, each much diminished in volume, but all diversified in character.
"Returning now to the river itself we find a gently sloping bank of black mud, baked and cracked by the sun's heat, extending towards the softer deposit at the water's margin. A low growth of grasses, sedges, and weeds springs up as the water recedes. The river margin does not often have much aquatic vegetation. In low-water years, such as 1894 and 1895, a considerable fringe is formed along the shore, but this is quickly cleaned out on the seining grounds, which occupy a large part of the shore, as soon as the fishing season opens in July.
"In years of normal high-water the vegetation rarely gets much of a foothold along the shores, even at low-water stages. Save for the few sandy banks where springs abound, such as those below Havana along the eastern bluff, there is little, at least in the La Grange pool, to vary this monotony of mud banks and fringing willows. The backwaters have been reduced to the lakes, sloughs, bayous, and marshes which abound everywhere in the bottom-lands. Many of these, as, for example, Phelps and Flag Lakes, have ceased in their reduced condition to contribute to the river.
"Others, like Thompson's Lake, maintain a connection with the river by means of a long and tortuous bayou or slough through which the current flows in or out as the relative levels of the two fluctuate. This lake receives but little water from a few springs and creeks along the bluffs, and like many others in the bottom-lands serves only as a reservoir from which the water is slowly drawn off as the river falls, but when once the lower stages are reached its contributions cease. Still others, like Quiver and Matanzas, maintain direct and open connection with the river, and since they receive tributary streams they continue to feed the river, but in reduced volume."