WSP00073

• File Number: 8277.400.200
• Description: Lower Virgin River Project
• State: NV
• Basin: Colorado Mainstem
• Date: 12/1/1976
• Author: USDA Soil Conservati
• Title: USDA Study Plan for The Virgin River Unit
• Water Supply Pro - Doc Type: Report/Study

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southeastward about 60 miles emptying into Lake Mead near Overton, Nevada.
Meadow Valley Wash is formed by the confluence of Spring Valley Creek and
Patterson Wash in the northern part of the Basin. It flows southward about
95 miles to join Muddy River near Glendale, Nevada.

The White River drainage originates in the northwestern portion of
the Basin, and contains 2,769 square miles. This drainage joins Muddy
River through Arrow Canyon at Upper Moapa Valley and is composed of a
series of ephemeral stretches supplied by important perennially flowing
springs. There are 14 major springs scattered throughout the length of the
White River drainage. These springs discharge about 63,000 AF annually.
After the water is used for irrigation it percolates back into the ground
water system, consequently, very little overland flow has reached the Muddy
River in recent times. Additional subsurface flows, also assumed to be
from the White River System, surface at Muddy River Springs which is the
source of Muddy River. These springs flow about 36,000 AF annually with a
TDS of about 620 mg/l.

The Las Vegas Valley drainage, which discharges through Las Vegas
Wash, heads in the Spring Mountain Range, and flows easterly into Lake
Mead. The cities of Las Vegas, North Las Vegas, and Henderson, Nevada,
are located in the valley.

The Lake Mead-Lower Colorado unit includes the Nevada portion of the
drainage bordering Lake Mead and the Lake Mohave-Davis Dam areas south to
the southern tip of Nevada. Included are areas around the Fort Mohave
Power Plant which are planned for extensive future urbanization.

The Grand Canyon drainage includes many intermittent tributaries that
flow directly into the Colorado River between Kanab Creek and Lake Mead.

Climate

The climate of the upper Virgin River Basin has been classified as
generally semiarid to subhumid, while the lower drainage basin including
most of the Muddy and White River basins is arid.

The frost-free season is long in the low valleys, averaging 240 days
in Nevada and 200 days in Utah; in the mountains the growing season may be
only 80 days. Based on observations at Lake Mead since 1936, the evapora-
tion from free water surface is estimated at 6.0 feet annually. The maximum
rate occurs during the summer with 13 percent of the total evaporation
occurring during anyone month.

The range of mean temperature for the Basin is from about 400 to 600 F.
annually with a monthly variation from winter to summer of 450 to 850 F. in
the low valleys.

Annual precipitation varies from less than 6 inches along the Colorado
River below Lake Mead to a maximum of 30 inches at the higher elevations.
Summer rainfall usually comes as convection storms while winter precipita-
tion comes in the form of snow in the north and rain in the south.

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