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Era- them '" System u - o N o i:i u Series '" ~ E < " C> Holo- cene I and i Pleisto-I ceDe Undif- feren- tiated '" ~ 5 ... Hiocene Oligo- cene or Eocene Table 1. --SUDlDary of the geo~ogic fonu.tions and group~ in t:.he Upper Colorado River Basin and their phgsical, hgdrologic, and chemic.l characteristics [mg/L, milligrams per liter] E99! Formation and group Alluvial, landslide, eolian. and talus deposits Volcanic rocks Browns Park Formation North Park Formation I I Duchesne River Formation Hydro- geo- logic unit 1 Physical characteristics Pdmarily unconsolid9ted all,l- vial deposits. Some land- slide, eolian, and talus deposits. Deposits are largely clay, sand, silt, and gravel derived from stream erosion of the bedrock forma- tions. Generally only a few tens of feet thick. 2 Dense, black; resistant basalt. in lava flows having a thick- ness as much as 900 feet. Also includes ash-flow tuffs, breccias, .and volcanic con- glomerate. 3 These fo~ations consist pri- marily of sandstone and semi- consolidated conglomerate. The foctlUltions may be as much as 2,000 feet thick but I normally overlie older forma- ~ions in relatively thin beds 200 to 500 feet thick. Hydrologic characteristics Probably mote wells tap the allu- vial deposits tban any othet rocks. '{i.:~lds as much as sev- eral bundred gallons per miDute are common. Host of the alluvial deposits have close hydraulic connection with adjacent streams. Tbe water level in the deposits is generally within a few feet of the elevation of the water in the stream. Recharge occurs primar- ily as seepage losses from streams or from ground-water discharge from the underlying bedrock aquifers. Discharge from the deposits is mostly to streams and from ground-water pumpage. Because .ost stream valleys in tbe basio are Darrow, water with- drawn from wells offsets the flow io the stream within 8 few days. No data available. Favorably. located wells which tap these formations might yield as much as 300 gallons per minute. Recharge is from direct infil- tration of precipitation and snowmelt. Discharge is mainly to strealllS. In some areas. dis- charge is to underlying bedrock aquifers such as in the Vampa River basin where these forma- ~ions overlie tbe upturned Mesa- verde Group, but increasing amounts of ground-....ater pumpage in some areas also may be an important source of discharge. Chemical characteristics The dissolved-solids concentra- tion of water from the allu- vial deposits varies over a wide range, depending on the source of the parent material, the salioity level of the stream, and tbe amount of irri- gation. Host of tbe alluvial deposits yield water of only 200 to 300 mg/L dissolved- solids concentration. The alluvium is generally perme- able and most of the highly soluble mine~al8 b~ve been leacbed. Where tbe river- cbannel deposits have been eroded from saline formations such as marine shales, the dissolved-solids concentra~ tioo of the ground water from deposits may be as great as 1,000 mgjL. No data available. The dissolved-solids concentra- tion of water from the rocts is less than 1,000 mg/L and typically less than 500 mt/L.