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. . I , '---- - , GROUND-WATER SYSTEM Hydrogeologic Setting Allu_ium underlying the South Platte River valley and the tributary rvalleys consists of Pleistocene and Holocene terrace deposits and Holocene !flood-plain deposits. The terrace deposits form the major part of the allu- :vium and are continuous upstream along Lost, Kiowa, Bijou, Antelope, BaJger, !and Beaver Creeks, and contain the major alluvial aquifers in these areas !(Bjorklund and Brown, 1957, p. 30). The alluvium consists of interbedded and !lenticular deposits of clay, silt, sand, and gravel. McGovern (1964, p. 19) ,reported, "The Beaver Creek drainage is a complex system of meandering 'streams that have cut and refilled their channels periodically throughout ;most of Pleistocene time. Some of the streams presently occupy their original 'channels, but others do not." McGovern also reported that the alluvial de- ,posits of Beaver Creek valley were generally of local origin. These comments ;for the Beaver Creek drainage basin also generally apply to the Badger Creek ;drainage basin. I i The maximum thickness of :60 ft at the Morgan County line iCreek, the maximum thickness of alluvium along Beaver to nearly 100 ft near the alluvium is about Creek ranges from about Brush. Along Badger 100 f t. I : Dune sand lover lies pa rts ;the areas of ;the und< rlying , ! occurs both east and west of the two alluvial valleys and of the alluvium, especially west of Bea'ler Creek. "In general dune-sand deposits are good infiltration areas for recharge to ~1I1uvial material" (Bjorklund and Brown, 1957, p. 33). The Pierre Shale of Late Cretaceous age underlies the alluvium and dune sand. "The Pierre Shale consists of bluish-black marine shale and silt and :interbedded tan to yellowish-brown sand and sandy shale in the upper part, or 'transition zone. Many beds of bentonite and large bluish-grey limestone ,concretions are present throughout the formation" (Bjorklund and Brown, 1957, p. 19). This formation provides a relatively impermeable base beneath and ~djacent to the alluvium and sand dunes. ' Natural recharge to ground water beneath the alluvial valleys occurs 'from precipitation on the valley floors and on the adjacent sand dunes. Except on the sand dunes, "only a small part of the precipitation reaches the 'ground-water reservoir; most of the ,'ater is lost by evapotranspiration before it can percolate dmm\'lard to the water table" (McGovern, 1964, p. 22). ~~he potential evapotranspiration rate exceeds the average precipitation ~ate; thus only during very wet periods is the opportunity favorable for ap- preciable amounts of water to escape downward to the water table" (McGovern, :1964, p. 22). Also, because the surface area of the alluvial valleys (about 40 mi2) is considerably smaller than the surface area of the contributing dune sand (estimated to be 200 mi2), most of the natural recharge to the al- !luvium along Badger and Beaver Creeks comes from the dune sand adjacent to rhe alluvial valleys. 32