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Bedrock Aquifers
<br />Bedrock aquifers in the study unit can be broadly
<br />grouped into Tertiary (Green River aquifer), Mesozoic
<br />(Mesaverde, Mancos Shale unit, Dakota, Morrison,
<br />and Entrada aquifers), Paleozoic (Leadville 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. Mesozoic rocks consist of siltstone, sandstone,
<br />shale, and limestone. Paleozoic rocks consist of
<br />carbonate (limestone and dolomite) and clastic
<br />sedimentary rocks. Precambrian rocks are composed
<br />of metamorphic and granitic rocks.
<br />Wells completed in consolidated deposits com-
<br />monly are less than 500 8 in depth, but some can reach
<br />as much as 2,000 ft. The water yield from 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 on the rock lithology and structure. The effec-
<br />tiveporosity is largely affected by the wide range in
<br />lithologies, 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 />of three bedrock aquifers is about 10 to 20 ftz/d, but can
<br />be as high as 600 ftz/d in the Green River aquifer.
<br />Water Movement
<br />Ground-water movement in the Upper Colorado
<br />River Basin is similar to surface-water flow directions.
<br />In the eastern part of the study unit, the flow system
<br />primarily involves recharge in the mountainous areas
<br />and discharge in the lower valleys. Recharge in the
<br />western part of the basin is due to precipitation in the
<br />form of snow or rain and discharge occurs in the
<br />valleys (Chaney and others, 1987). Most ground water
<br />is discharged into streams through seeps along the side
<br />or bottom of the stream channel or to the land surface
<br />by springs.
<br />The rate and quantity ofground-watermovement
<br />depend on the hydraulic conductivity of the 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 metamorphic rocks, water
<br />moves primarily through fractures.
<br />Stream-Aquifer Relations
<br />Some aquifers in the basin are hydraulically
<br />connected to the surface water as ground water dis-
<br />chargesinto main river channels through springs where
<br />the aquifer is near the surface or by upward movement
<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 throughout
<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 />low- flow periods on perennial streams. However, the
<br />amount of water contributed to perennial streams by
<br />bedrock aquifers varies seasonally. 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, 1988). Ephemeral streams occur due to
<br />a drop in the water table below streambeds, a result of
<br />insufpicient storage water within the aquifers.
<br />Aquatic Biological Characteristics
<br />The Upper Colorado River Basin provides
<br />diverse habitats for biological communities, reflecting
<br />the variations in climate, vegetation, and geology in the
<br />basin (Ward and others, 1986). In table 8, algae, fish,
<br />and macroinvertebrates 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 not include all algae, fish, and macroinvertebrates
<br />in the study unit. Biological communities vary with
<br />altitude and physical habitat.
<br />Different algal species are affected by varying
<br />riparian vegetation 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 green algae are
<br />predominant.
<br />The high-altitude streams in the Southern Rocky
<br />Mountains are dominated by brook, brown, cutthroat,
<br />and rainbow trout and other cold-water species, such as
<br />creek chubs, flathead minnows, sculpins, speckled
<br />dace, and white suckers. Lower altitudes, as character-
<br />ized by the Colorado Plateau, can contain cold-water
<br />and warm-water species because of overlap in transi-
<br />tion zones. Trout are present at the higher altitudes of
<br />26 Environmental Setting and Implications on Water t~uality, Upper Colorado River Basin, Colorado and Utah
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