WSP06951

• File Number: 8273.300
• Description: Colorado River Basin Salinity Control - Federal Agencies - USGS
• Basin: Colorado Mainstem
• Water Division: 5
• Date: 1/1/1985
• Author: USGS
• Title: Ground-Water Contribution to the Salinity of the Upper Colorado River Basin
• Water Supply Pro - Doc Type: Report/Study

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mountainous watersheds produce the major perennial streams. Much runoff
oCcurs in the spring and early summer and is caused primarily by melting of
the mountain snowpack. The base-flow period is generally from late summer to
early spring of the following year. During this time, streamflow'is
relatively uniform and consists primarily of ground-water discharge.

Geology and Ground Water

Hydrogeologic conditions in the Upper Colorado River Basin are complex.
The geology of the area is the principal factor controlling the occurrence,
movement, and the chemical quality of ground water. Rocks underlying the
study area are mainly consolidated sedimentary deposits, Igneous and
metamorphic rocks comprise most of the mountainous regions. Unconsolidated
alluvial deposits border and underlie most of the major streams. Ground water
occurs in all of the geologic formations in the study area. However, because
of the diverse hydraulic properties and mineral composition of these
formations, the quantity and chemical quality of the ground water varies
considerably.

The source of almost all of the ground water in the study area is
precipitation that falls within the study area. The principal areas of
ground-water recharge are in the higher elevation areas. Normally, ground
water moves only a short distance from the area of recharge to points of
discharge. However, in some instances, ground water may move relatively long
distances from the area of recharge to points of discharge. For instance,
water in the Leadville Limestone of Mississippian age (table 1) moves many
miles from the area of recharge to points of discharge. Most ground water is
discharged to streams as diffuse nonpoint sources, discharged into the
atmosphere by phreatophytes, or discharged from springs.

The rate and quantity of ground-water movement primarily depend on the
hydraulic conductivity of the geologic formation and the hydraulic gradient.
In general, alluvial deposits, other unconsolidated sedimentary deposits, and
limestones have the largest hydraulic conductivities and are capable of
transmitting water readily. Shales have the least hydraulic conductivity and
are capable of transmitting water only slowly. Water movement in many
consolidated sedimentary rocks and igneous and metamorphic rocks is primarily
through fractures. The density and the degree of fracturing of the rocks
determine the amount of water that can be stored and the rate at which it can
be transmitted. Extensively fractured rocks are capable of transmitting water
considerable distances.

The availability of recharge to the formation also determines the amount
of water that can be transmitted. A permeable formation in an arid region
where potential recharge is slight would transmit small quantities of water.
Also, a relatively impermeable formation in an area of significant
precipitation might still transmit only small 'quantities of water.

Chemical quality of ground water is dependent on the mineral composition
and hydraulic properties of the aquifer, such as surface area of contact,
porosity, and rate of water movement. Because water moves slowly through most

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