Non-point Source Pollution (continued)

In 1986, the ISGS conducts an assessment of sedimentation rates and contamination levels from 18 sampling stations located in the backwater lakes. A total of 112 samples are tested. The surface area, shoreline length, maximum depth, and condition (Table 36) are assessed. In general, the lakes are in poor condition due to unstable water levels, high turbidity, and high sediment deposition (Cahill 1986).

Table 36: Physical parameters for backwater lakes sampled along the Illinois River (Cahill 1986)

Source: 1978 Assessment and classification of Illinois Lakes, IEPA

Sedimentation rates as measured by Frank Bellrose in 1983 are found in Table 37, and the rates measured in 1984 M.T. Lee are in Table 38. The discrepancies between the two data sets are due in part to fluctuating water levels (Cahill 1986). Sedimentation rates range from 1.0 cm/year in Swan Lake to 3.2 cm/year in Depue Lake.

Table 37: Sedimentation Rates and Depth at Different Periods (Cahill 1986)

Source: Bellrose et al, 1983

Sediment accumulation is extreme in several lakes, such as Peoria Lake and Muscooten Bay. The sedimentation rate in Upper Peoria Lake is nearly two times greater during 1965-1976 than the long-term average of 1.67 cm/yr from 1903-1976. This increased rate over an 11-year period results in 34 cm of sediment being deposited - nearly one-quarter of the total amount of sediment deposited.

Table 38: Sedimentation rates in Illinois River backwater lakes (Cahill 1986)

Source: data for Table 38 taken from Lee, 1984

The combination of large surface areas and shallow depths alters these lakes from ones with high diversity of aquatic plants to “saucer-shaped basins filled with soft sediments that support little or no life” (Cahill 1986). These lakes are further degraded from wind- and barge-generated waves and recreational boating, which cause resuspension of bottom sediments. This resuspension degrades the lakes by:

• Increasing turbidity;
• Reducing photosynthesis; and
• Reintroducing contaminants that may have otherwise settled to the bottom

Depue Lake is located in the Upper Illinois River, downstream of Spring Valley. It is connected to the Illinois River by a narrow channel. This lake was popular for boating before World War II when the depth was reported to be 18-20 feet. Sediment has since been carried in during flood stages of the river, resulting in a decrease in storage capacity of 72.6% between 1903 and 1975. The average water depth is now 1½ feet (0.5 m) (Cahill 1986).

Sediments are tested for concentrations of metals. Zinc (Zn), lead (Pb), and cadmium (Cd) concentrations are higher in upstream lakes as compared to downstream lakes. Zinc concentrations range from 5,000 ppm at Depue Lake to 132 ppm at Silver Lake