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<br />Bedrock Aquifers
<br />
<br />Bedrock aquifers in the study unit can be broadly
<br />grouped into Tertiary (Green River aquifcr), Mesozoic
<br />(Mesaverde, Mancos Shale unit, Dakota, Morrison,
<br />and Entrada aquifers), Paleozoic (Lcadville aquifer),
<br />and Precambrian crystalline unit. Tertiary rocks
<br />include fluvial sediments, marine sediments. and
<br />clastic deposits of sandstone and shale along with coal
<br />beds, Mesozuic rocks consist uf siltstone, sandstone,
<br />shale, and limestone, Palcuzoic rucks consist of
<br />carbonatc (limestone and dulomite) and clastic
<br />sedimentary rucks, Precambrian rucks are composed
<br />of metamorphic and granitic rucks,
<br />
<br />Wells completed in cunsolidated deposits COI11'
<br />monly are less than 500 ft in depth, but some can reach
<br />as much as 2,000 ft, The water yield frum these wells
<br />varies from 0,5 to possibly greater than 500 gal/min
<br />(table 7), The ability for bedrock to transmit water
<br />depends un the rock lithology and structure, The effec-
<br />tive porosity is largely affected by the wide range in
<br />lithulogies, where tightly cemented sandstones can
<br />have a porosity of less than 10 percent, and in more
<br />poorly sorted sandstones consisting of medium- to
<br />coarse-grained sands, porosity can be greater than
<br />30 percent. As indicated in table 7, the transmissivity
<br />ofthree bedrock aquifers is about 10 to 20 ft'/d, but can
<br />be a~ high as 600 ft'/d in tbe Green River aquifer,
<br />
<br />Water Movement
<br />
<br />Ground-water movement in the Upper Colorado
<br />River Ba~in is similarto surface-water fluw directions,
<br />In the eastern part of the study unit, the flow system
<br />primarily involves rechargc in the mountainous areas
<br />and discharge in the lower valleys, Recharge in the
<br />western part of the basin is due tu precipitation in the
<br />form of snow or rain and discharge occurs in the
<br />valleys (Chaney and uthers, 19&7), Most ground water
<br />is discharged into streanlS thruugh seeps along the sidc
<br />or buttom ufthe stream channel or to the land surface
<br />by springs,
<br />
<br />The rate and quantity uf ground' water movement
<br />depend on the hydraulic conductivity ufthe geologic
<br />formation and the hydraulic gradient. In the basin,
<br />alluvial deposits, other unconsolidated sedimentary
<br />deposits, and limestones have high hydraulic conduc-
<br />tivity and transmit water fairly readily, The transmis-
<br />sivity value listed in table 7 indicates the ability of
<br />alluvial depOSits to transmit water. In consolidated
<br />sedimentary, igneous, and metanlOrphic rocks, water
<br />moves primarily through fractures,
<br />
<br />Stream-Aquifer Relations
<br />
<br />Some aquifers in the basin are hydraulically
<br />connected to the surface water as gruund water dis-
<br />charges into main river channels through springs where
<br />the aquifer is near the surface or by upward movcment
<br />of the ground water if the aquifer is located at depth,
<br />During low flows, ground water helps sustain stream-
<br />flow on practically every perennial stream throughuut
<br />the year (Chaney and others, 1987), Perennial storage
<br />in alluvial aquifers, perennial snowfields, and reser-
<br />voirs in the basin provide sustained base flows, Bed-
<br />rock aquifers also can contribute to streamflow during
<br />luw- flow pcriods on perennial streams, However, the
<br />amount of water contributed tu perennial streams by
<br />bedrock aquifers varies seasunally, Water levels in
<br />the bedrock aquifers can change because of physical
<br />factors. such as climatic conditions, irrigation, and
<br />pumping wells, and because of the relative transmis-
<br />sive and storage properties of these aquifers (McLean
<br />and Johnson, (988), Ephemeral streams oc<:ur due to
<br />a drop in the water table below streambeds, a result of
<br />insufficient storage water within the aquifers,
<br />
<br />Aquatic Biological Characteristics
<br />
<br />The Upper Colorado River Basin provides
<br />diverse habitat~ for biological communities, reflecting
<br />the variations in climate, vegetatiun, and geology in the
<br />basin (Ward and others, 1986), In table 8, algae, fish,
<br />and macruinvertebratcs are listed that characterize the
<br />Southern Rocky Mountains and the Colorado Plateau
<br />physiographic provinces, This listing includes the
<br />more common taxa in each physiographic province but
<br />does nut include all algae, fish, and macro invertebrates
<br />in the study unit. Biological communities vary with
<br />altitude and physical habitat.
<br />Different algal species are affected by varying
<br />riparian vegetatiun as well as by the availability of
<br />nutrients, The dominant algae in the high-altitude
<br />streams are blue-green algae, diatoms, dinoflagellates,
<br />golden-brown algae, and green algae, In more saline
<br />environments, euglenoid algae may be present. In the
<br />lower altitudes, golden-brown and grccn algae are
<br />predominant.
<br />The high-altitude streams in the Southern Rocky
<br />Mountains arc dominated by brook, brown, cutthroat,
<br />and rainbow trout and other cold,water species, such as
<br />creek chubs, flathead minnuws, sculpin~, speckled
<br />dace, and white suckers, Lower altitudes, as character-
<br />ized by the Colorado Plateau, can contain culd-water
<br />and warm-water species because of overlap in transi-
<br />tion zones, Trout are present at the higher altitudes of
<br />
<br />28 Environmental SetUng and Implications on Water Quality, Upper Colorado River Basin, Colorado and Utah
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