Killey, Mryna M.

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C ILLINOIS STATE GEOLOGICAL SURVEY Urbana, Illinois 61801 fUlNOIS GEOLOGICAL SURVEY LtBRARY DO YOU LIVE ABOVE AN UNDERGROUND RIVER? Myrna M. Killey GEOGRAM 6 October 1975 Do you think of an underground river as a hidden stream rushing through a tunnel in solid rock? Such subterranean rivers do exist in some states-in Ala-bama and Missouri, for example. In Illinois, h0tvever, except in a few areas where water flows through cracks and channels it has created by dissolving the lime-stone bedrock, underground "rivers" are not really rivers at all. The Mahomet "river" that underlies part of east-central Illinois is a good example. So is the eastem part of this "river," which is called the Teays (rhymes with "days"). Such rivers are vital to many towns, for they are a reliable source of water. The Mahomet-Teays river system was discovered more than 25 years ago when numerous water wells were drilled in the eastern and midwestern United States. The story of this vast river system has been pieced together largely from informa-tion obtained from records made during the drilling of the wells. More than a million years ago, before the glaciers of the Great Ice Age crept down over the Midwest, a river as large as the present Mississippi flowed generally westward from its probable source in the mountains of West Virginia, crossed Ohio and Indiana, and traversed east-central Illinois from Hoopeston to Havana. At Havana it joined another ancient river system that occupied what is now the Illinois River Valley (see map). All along its course it cut a deep val-ley in the bedrock. When the successive glaciers invaded Illinois from Canada, the fringes of the ice melted during the warmer periods, and the water (meltwater) carried with it great quantities of sand and gravel that had been embedded in the ice. This material, called outwash, was deposited in thick layers in the Mahomet Valley. As the later glaciers advanced southward, both the valley and its outwash were bur-ied by ice. When the ice finally melted, tremendous amounts of unsorted rock de-bris (pebbly, sandy clay called till) that had been held in the ice blanketed the land surface, including the former river valley, to depths of 50 to more than 100 feet. (The outwash and till deposits are collectively called drift.) The great Ma-homet River Valley was obliterated from the landscape and the river no longer exis-ted. Instead, on the new land surface the river patterns we know today developed. The buried Mahomet Valley is invaluable to east-central Illinois because its porous sand and gravel deposits act as vast underground sponges, storing the rainwater that seeps downward from the land surface. Water flows easily through the sand and gravel into wells drilled in the porous materials. In contrast, gla-cial till is too fine-grained to allow the water it holds to flow easily and, therefore, cannot supply large amounts of water to wells. Towns such as Hoopeston, Champaign-Urbana, Mahomet, Monticello, and Clinton that are situated above the buried Mahomet Valley have large ground-water supplies available to them, but towns away from the valley have more difficulty obtaining their water. Perhaps the term "underground river" is still applied to the Mahomet Valley because it is easier to imagine great volumes of well water coming from a river than from beds of sand and gravel in a buried valley. ~ Buried bedrock '1' valleys i Bedrock va 11 eys which coincide with present valleys and lowlands 0 20 40 Miles I E---3 F-3 f' The Mahomet Valley has been traced r for about 150 miles across Illinois, it _ lies at an average depth of more than 200 feet below land surface, and its bottom is at an average elevation of 350 feet above sea level. In some places the ancient valley varies in width from 5 miles at the Indiana line to almost 10 miles near Clinton in De Witt County. Another major "underground river" is the Princeton Bedrock Valley in the north-central part of Illinois. Many smaller bedrock valleys in the state contain sand and gravel deposited by glacial meltwater. The Mississippi, Il-linois, Kaskaskia, and Wabash Rivers also contain beds of outwash deposited by glacial meltwaters, but their cours-es were not obliterated by the gla-ciers, and their valleys have remained open as drainageways. The water supplies in these depos-its in the ancient river valleys of Il-linois are one of many resources con- (~-tributing to the state's natural wealth. -Of the 3. 3 billion gallons of water a day used by Illinois, about 450 million gallons are pumped from sand and gravel deposits, mainly of glacial origin. The value of ground water from these depos-its is over $115 million per year. Do you live above an underground "river"? Look at the map and see. Lo-cate the source of the water you use in your town. If you should see a well being drilled, stop and ask if you can look at the earth materials brought up from the well. These are the kinds of material used to interpret the geologic history of Illinois. 

Do you live above an underground river?

https://www.ideals.illinois.edu/bitstream/2142/100298/2/geogram_6.pdf

Do you live above an underground river?

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