Aquatic Vegetation

Changes also occur to the aquatic vegetation within the Illinois River. Richardson documents theses losses in Peoria Lake in 1921:

Luxuriant growths of coarse aquatic plants (Potamogeton, Ceratophyllum, Scirpus, Vallisineria, etc.) that covered square miles of Peoria Lake at mid-summer and autumn levels between 1910 and 1914, and their rich fauna of small invertebrates along with them have disappeared now altogether in the upper and middle lake except for an occasional scraggly clump at the very edge (Mills 1966).

In other stretches of the river, an abundance of aquatic vegetation remains through the 1950s when it virtually disappears. The reasons are not fully understood. Siltation is believed to be a factor in that it increases turbidity that reduces light penetration and therefore photosynthesis, and creates a substrate that makes it difficult for plant roots to anchor. Sago pondweed (Potamogeton pectinatus) is more tolerant of reduced light and yet between 1938 and 1940, this species does not thrive in Lake Chautauqua where waters are deeper than 48 inches, and disappears completely in waters deeper than 56 inches (Mills 1966).

This disappearance of aquatic vegetation occurs throughout the river. In Peoria Lake, the construction of the Peoria Dam in 1938 stabilizes low water levels such that coontail, bushy pondweed, and sago, longleaf pondweeds, and wild celery increase dramatically, peaking in the 1940s. In the autumn of 1949, aquatic plant beds cover several thousand acres of the 10,000 acres of Peoria Lake. After this, however, the beds decline until they fully disappear. The reasons for this catastrophic decline can only partially be explained by increased turbidity (Mills 1966).

In Rice Lake, the aquatic vegetation is lush until the water level is increased artificially in the mid-1940s. By 1950, 360 acres of river bulrush (Scirpus fluviatilis) dwindles to less than 100, and by 1956 only 20 acres remain. Coontail and white waterlily (Castalia tuberose) are also eliminated from the lake by 1960. These losses are primarily attributed to the increase in water levels, and to the subsequent increase in wave action that disrupts the bottom sediments.