Soil
Erosion and Sedimentation (continued)
This sedimentation entering the river and backwater lakes has immediate impacts to aquatic vegetation (Figure 53). Under natural conditions, backwater lakes are shallow, typically 4-6 feet deep, with some being 12-16 feet deep. They are filled with pondweeds, coontail, and water lilies, with arrowhead, marsh smartweed, and river bulrush common along the shore.
Figure 53: Sediment Entering a Body of Water
The lower Illinois suffers from high turbidity levels because of the soft-mud bottoms and heavy silt loads brought into this section from tributary streams. Turbidity levels can be extremely high, as reported from the Alton pool in June 1964. The Illinois Sanitary Water Board determined that following a flash flood, the turbidity is 2,000 turbidity units. Table 41 illustrates other examples of turbidity rates throughout the Illinois River. As is evident, turbidity is much reduced in the upper reaches.
Table 41: Ranges of Turbidity (Jackson turbidimeter) during minimum flow periods in the navigation pools of the Illinois River, 1963-1966 (Starrett 1971)
| Navigation Pool | Range in Jackson Turbidity Units |
| Alton | 36-320 |
| La Grange | 60-220 |
| Peoria | 15-140 |
| Starved Rock | 15-52 |
| Marseilles | 15-47 |
| Dresden | 15-27 |
Aquatic vegetation disappears from the backwater lakes in the late 1950s and 1960s due to the effects of sedimentation and fluctuating water levels (Havera 2002). The increased sediments create soft lake bottoms that make it difficult for plants to anchor into the substrate. These sediments also lead to increased turbidity that blocks sunlight and therefore reduces photosynthesis. As an example, turbidity readings at low river stages are two to three times higher in 1963 and 1964 than the benchmark records taken in 1896 (Havera 2002).
Increased sedimentation takes its toll on all natural resources with a connection to the Illinois River, including the backwater lakes, mussels, and waterfowl.
