Impacts of Turbidity

High concentrations of particulate matter can modify light penetration, cause shallow lakes and bays to fill in faster, and smother benthic habitats - impacting both organisms and eggs. As particles of silt, clay, and other organic materials settle to the bottom, they can suffocate newly hatched larvae and fill in spaces between rocks which could have been used by aquatic organisms as habitat. Fine particulate material can also clog or damage sensitive gill structures, decrease their resistance to disease, prevent proper egg and larval development, and potentially interfere with particle feeding activities. If light penetration is reduced significantly, macrophyte growth may be decreased which would in turn impact the organisms dependent upon them for food and cover. Reduced photosynthesis can also result in a lower daytime release of oxygen into the water. Effects on phytoplankton growth are complex depending on too many factors to generalize. Very high levels of turbidity for a short period of time may not be significant and may even be less of a problem than a lower level that persists longer. The figure below shows how aquatic organisms are generally affected (Lake Access 2005).

Figure 36: Impacts to Fish from Turbidity Over Time (Lake Access 2005)

The Illinois River is relatively clear, although it always carries some silt load, primarily during periods of high water. As Euroamerican settlers begin agricultural practices, changes to the river begin. In 1896, Kofoid measures the clarity of the Illinois River by submerging a white plate of semi-porcelain. He states,

As might be expected in the river environment, when floods occur the turbidity is often extreme, and is exceedingly variable according to the locality and the river levels. The extreme range of our records extends from 1.3 cm [½ inch] in a Spoon River flood, to 260 cm [8 ½ feet], in Quiver lake, under ice (Mills 1966).

Jackson and Starrett write in 1959:

The sediments in Lake Chautauqua are mostly of a fine texture and form a loose, flocculent ‘false bottom’ (not similar to the type found in bog lakes) over the original lake bottom. A slight disturbance of the ‘false bottom’ causes particles to become resuspended and so increases the turbidity of the water (Sparks and Starrett 1975).

An increase in wind velocity from light to strong increases turbidity from 162 to 700 Jackson turbidimeter units (JTU) and that requires 7-12 calm days for the sediment to settle from Lake Chautauqua.

This increased turbidity affects fish production. An experiment using fish ponds divided into three turbidity classes, show the following fish production after two seasons:

• Clear ponds (less than 25 JTU) 161.5 lbs/acre (181.0 kg/acre)
• Intermediate ponds (25-100 JTU) 94.0 lbs/acre (105.4 kg/acre)
• Muddy ponds (>100 JTU) 29.3 lbs/acre (32.8 kg/acre)

Production in the muddy ponds decreases by 82% compared to the clear ponds.

As of 1965, siltation is having a profound effect on conditions in the Illinois River. Starrett and Fritz write:

Today Quiver Lake is devoid of aquatic plants. The formerly deep basin of the lake has been filled in with 4- 8-foot deposits of silt. Turbid water at depths of over 3 feet and a soft, flocculent bottom prevent the establishment of aquatic plants in the lake. Conditions in Quiver Lake are duplicated in many of the other floodplain lakes of the Illinois River; that is, in the past 35 years siltation has greatly changed the ecology of these lakes (Mills 1966).

The silt load in the Illinois River is greater in the lower reaches, and the conditions are generally more turbid because of the movement of tow boats up and down the main channel (Mills 1966). The readings in Table 20 are made in 1963 and 1964 under minimum flow conditions in the fall. During flood conditions, the turbidity readings can reach as high 2,000 units.

Table 20: Turbidity Readings in Navigation Pools, Fall 1963 and 1964 (Mills 1966)