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<br /> <br />10S" <br /> <br />SAND <br />WASH <br /> <br />, Steamboat <br />Springs <br /> <br />3S~ <br /> <br />108' <br />- Structural basin boundary 1.-:...._:..,'.1 Valley fill aquifers <br /> <br />104" <br /> <br />38. <br /> <br />Multiple sedimentary <br />bedrock aquifers <br /> <br />1()4" <br />.. Dakota-Cheyenne <br />Group aquifer <br /> <br />.. High Plains Aquifer <br /> <br />Principal sedimentary aquifers and structural basins of Colorado <br /> <br />Colorado is the foundation of the <br />state's water resources. You might <br />say, "Geology guides ground <br />water." The geologic story deci- <br />phered from the rocks of Colorado <br />recounts multiple structural events <br />raising mountain ranges, later <br />eroded and partially buried in <br />their own debris, shallow seas with <br />their associated beaches sweeping <br />across the land, and deserts undu- <br />lating with dune fields. In more <br />recent geologic time, the rocks tell <br />of large active volcanic fields that <br />seared a land dominated by deltas <br />and swamps with lava and vol- <br />canic ash. Over much of Colorado, <br />the landscape resulting from this <br />geologic history has been modified <br /> <br />Colorado Geological Survey ROCKTALK Vol. 5, No.4 <br /> <br />- --. <br /> <br />by the work of glacial ice that <br />scraped off mountain peaks and <br />scoured valleys, leaving thick lay- <br />ers of accumulated sediments <br />across the land as glaciers retrea ted <br />and melted. <br />This complex geologic history <br />has divided the state into frac- <br />tured, crystalline rock mountain <br />ranges; deep basins and fault- <br />bounded valleys as well as areas of <br />relatively undisturbed flat lying <br />sedimentary deposits. Colorado's <br />principal aquifers are categorized <br />into: 1) unconsolidated Quater- <br />nary-age alluvial aquifers associat- <br />ed with our major river systems, <br />2) poorly consolidated or uncon- <br />solidated sediments, 3) consolidat- <br /> <br />ed sedimentary rock aquifers, and <br />4) volcanic and crystalline rock <br />aquifers. <br />Alluvial aquifers are uncon- <br />fined and contain ground water <br />stored in stream-deposited uncon- <br />solidated sediment along river <br />valleys. Ground water in alluvial <br />aquifers usually interacts with <br />surface water of the stream system, <br />and ground-water levels may <br />exhibit seasonal variation in <br />response to surface-water flow. <br />Perched or confined ground water <br />can also occur in alluvial aquifers <br />if clay layers are present in the <br />stream sediments. Alluvial <br />deposits associated with ten of <br />Colorado's major river watersheds, <br /> <br />5 <br />