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Effects of Sediment on Water Quality¹

ABSTRACT

Flowing water in a stream carries sediment which is considered a pollutant because it interferes with many uses of water. Other pollutants in the water may be attached to or greatly affected by the sediment. The ultimate source of most sediment is soil loss from sheet erosion which can be computed by the universal soil loss equation. It may be as high as 100 metric tons/ha per year on a steep, loose, cropped soil, or on any soil left bare during construction activity. Nearly level land in Illinois may lose 10 tons/ha per year regularly.

Sediment yield, or that delivered to a downstream point, can be estimated from the many measurements available. Yields are 15 tons/ha per year in Mississippi, the highest in the United States; yields are 10 in western Illinois and western Iowa, and less than 2 for the forest-covered east and the arid west. Sediment problems are usually proportionate to sediment yields. In the Upper Mississippi River Basin 27% of the land has sediment yield double the level described as being tolerable. In this basin total sediment damage of $25 million/year is about comparable in amount to the upstream damage done by all floods.

Rivers carry a load of suspended sediment as a crucial part of their total energy balance. This natural sediment load is reflected in the median turbidity of 33 Jackson turbidity units found to be representative of all river waters in Illinois. If the suspended sediment load of a river is reduced, the stream will erode its bed or banks to pick up sediment to re-establish its energy balance.

Traditional soil conservation measures will reduce soil loss from the farmer's field; this will also reduce sediment yield to a downstream reservoir. Reductions are often 70% to 90%. The river turbidity will be reduced, but it will never be eliminated. Stream turbidity affects many common uses of water. It is necessary to reduce this turbidity artificially in order to use the water.

Key Words: erosion • pollution • rivers

¹ Paper presented Aug. 18, 1971, in New York City at the annual meeting of the ASA and SSSA as part of the Division S-2 symposium "Sediments and Water Quality."

² Engineer, Illinois State Water Survey, Urbana. 61801.

Received for publication September 2, 1971.