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Publication Abstract

The Causes and Effects of Sedimentation in Lake Decatur Brown, Carl B., J.B. Stall, and E.E. DeTurk, 1947  Illinois State Water Survey, Champaign, IL,  ISWS B-37    Full Text Available

For more than a hundred years surface water has been impounded in reservoirs and purified for public and industrial uses in some sections of the United States. For that long, at least, sediment has been recognized as a major problem in utilizing surface water supplies. Chemists early found that minute sediment particles were the principal cause of turbidity—the cloudy or muddy quality that must be removed before water is acceptable on the dinner table, in the bath, or in the factory boiler. Moreover, engineers began to recognize some decades ago that sediment carried by flowing streams settles in impounding reservoirs built on these streams and, in some cases, rapidly reduces the capacity of the reservoirs to store water. As recently as 15 years ago, however, there was little quantitative data on the rates at which reservoirs were being filled with sediment. Moreover, there was little appreciation of the intimate relationship between the sources of sediment on the lands of the drainage basin and the increasing losses of reservoir capacity or the increased costs of water purification.

The need for more information on the effects of sediment on surface water supplies began to be recognized in Illinois as early as 1930. In 1931 and 1932 the State Water Survey Division undertook preliminary investigations of sedimentation in Lake Decatur in cooperation with the Decatur Water Supply Company, former owner of this municipal reservoir. Impetus was given to investigation of sedimentation problems with the spread of the movement to control soil erosion, which led to the creation of the Soil Conservation Service as a permanent agency of the U. S. Department of Agriculture in 1935. This agency undertook a systematic study of rates of reservoir sedimentation in all parts of the country in an effort to evaluate the effects of accelerated soil erosion on public water supply and other services rendered by storage reservoirs. Related studies were made of land use and erosion on the drainage areas. One of the early projects under this program was a survey of Lake Decatur and its watershed made in April-June, 1936, in cooperation with the State Water Survey Division, the State Agricultural Experiment Station, and the city of Decatur (See Figure 1).

A second survey of Lake Decatur was made by these agencies in May and June, 1946. Additional investigations designed to cover all of the important relationships between reservoir capacity, water needs, effects of sediment on treatment problems, water supply from the Sangamon River, and effects of land use and agricultural practices in this drainage basin have been made during 1946. The results of these studies are reported in this publication. The findings have led to a prediction of the remaining useful life of Lake Decatur. They show the nature of the problem confronting the city. They give a basis for long-range planning for the protection and maintenance of the city's water supply. Furthermore, these investigations have shown for the first time in this section of the country the intimate relationship between public water supply and problems of the land. They give a measure of the effects of land use and treatment on the quality of water and the maintenance of storage reservoirs. The results are useful not only in solving the problem of the city of Decatur; they also indicate the general nature of the problems confronting other cities and towns in Illinois that now depend on surface water supplies and many more that will be developing surface supplies in the future as ground-water supplies become more and more fully developed or overdeveloped.



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