Geological Survey Water-Supply Paper 1475-Q Present and Future Water Supply for Mammoth Cave National Park, Kentucky

Mammoth Cave National Park is located on the Mammoth Cave plateau, which is underlain by a thick sequence of limestone formations capped by sandstone (fig. 73). The rocks dip gently northwestward toward the Western Coal Field. The Green River bisects the park in a general east-west direction and cuts deeply into the thick limestones. The limestones are soluble; and south of the Green River, in the vicinity of Mammoth Cave, extensive solution action has taken place to approximately the level of the river. Solutional features include the famed caverns of Mammoth Cave. In this area the flat-topped plateau surface is deeply and irregularly dissected by northwest-trending closed valleys. Each valley consists of many sinkholes that coalesce to form a master valley. There is no surface flow in any of the valleys except during periods of heavy precipitation when some water may overflow from one sinkhole to adjacent ones. Subsurface channels carry all the drainage during normal flow. Only two permanent streams, Echo and Styx Rivers, enter the Green River from the south in the park. Both are very short streams emerging from caverns as springs.

FIGURE 73.—Topography and geology control the occurrence and movement of ground water in the Mammoth Cave area. (click on image for an enlargement in a new window)

Dendritic surface streams that are mostly perennial characterize the extreme western part of the general area south of the Green River. Drainage is generally northward via Beaver Dam Creek and short streams flowing into the Green River.

The Dripping Springs escarpment forms the southern boundary of the Mammoth Cave plateau and faces south to the lower Pennyroyal plain. Immediately below the escarpment, shallow sinkholes are especially numerous in an east-west belt about 2 miles wide. Elsewhere, the Pennyroyal plain is gently rolling and is drained by perennial streams that flow northwestward to the area of sinkholes. All streams terminate in one or more sinkholes and in solution channels under the Mammoth Cave plateau to the Green River.

Owing to the northwesterly dip, the insoluble Big Clifty Sandstone Member of the Golconda Formation and younger formations north of the Green River are much closer to the level of the river, and an extensive solution drainage pattern has not been formed. Short interrupted streams having steep gradients drain the area southward to the Green River.

All water that moves through or is stored in the Mammoth Cave area comes from precipitation. All surface and subsurface drainage from the Mammoth Cave plateau and Pennyroyal plain in the vicinity of the park is eventually into the Green River.

Precipitation in the area of Mammoth Cave National Park north of the Green River and in the extreme western part of the park south of the Green River runs off partly through surface streams and partly by subsurface drainage through the rocks. The larger surface streams that drain this area are the Nolin River and Beaver Dam, Bylew, Dog, and Buffalo Creeks (fig. 71).

Almost all precipitation on Mammoth Cave plateau south of the Green River in the vicinity of Mammoth Cave that does not evaporate or transpire moves downward through the rocks as ground water. It reaches the Green River as outflow from large springs such as Echo, Styx, Pike, and Sand Cave Springs and probably from numerous smaller springs and seeps. Downward movement through the rocks underlying the plateau is almost entirely in openings along joints and bedding planes, whose number and size control the rate of movement. Openings are numerous in the Hardinsburg Sandstone and Big Clifty Sandstone Member, but are few in number or nonexistent in the thin shale beds in the Girkin Formation and at the base of the Big Clifty and Haney Limestone Members. They are numerous and greatly enlarged by solution in the several thick limestone formations. Thus the rate of movement is rapid through the limestone, slower through the sandstone, and very slow through the shale beds. These differences in rate of movement have created several bodies of ground water beneath the plateau. A body of ground water is perched above the shale at the base of the Haney Limestone Member and is contained in solution openings in the limestone (fig. 73). It is this water that maintains the flow of Three Springs, Bransford Spring, and the several other springs on Flint Ridge. A second body of ground water is perched in the Big Clifty above the shale bed at its base. Some of this water discharges horizontally to the edges of the plateau and forms the "dripping springs" from which the escarpment is named. Other small and discontinuous bodies of ground water are perched above thin shale beds in the Girkin Formation.

Most of the water that falls on the plateau eventually finds its way down to the zone of basal ground water in the cavernous St. Louis and Ste. Genevieve Limestones. All rock openings below the water table or upper surface of the zone of basal ground water are filled with water. Owing to the ease with which water moves through the large solution channels, the basal water table is nearly flat. It slopes gently northwestward and is continuous with Green River. The basal ground water is recharged by water that moves directly down from the surface through sinkholes or open shafts in the limestone, by water that discharges from the edges of the perched zones, and by water moving through solution channels from the Pennyroyal plain. When the stage of the Green River rises faster than the level of the adjacent ground water, the flow of River Styx and Echo River is reversed and recharges the basal ground water in the immediate vicinity of the river.

A small part of the precipitation in the area and part of the runoff from the steep rock slopes flanking Green River move through and are temporarily stored in a narrow body of alluvium along the river. The body of water in the alluvium is connected hydraulically to the Green River. During periods of low flow, the water table in the alluvium slopes toward the Green River and water moves through the alluvium into the river. During periods of high flow, the alluvium is completely saturated.