Environmental Engineering Reference
In-Depth Information
Watercourses in the Southwest have three general categories of surface-water flow. Only
the largest rivers have perennial flow, and these include the Green, Colorado, San Juan,
Escalante, upper Virgin, Verde, upper Salt, and upper Gila Rivers (Figure 4.1). During some
extreme droughts, some perennial rivers may have interrupted flow in reaches that cross
deep alluvial basins. Intermittent streams may have periods of sustained flow, particularly
during wet periods, but are dry at other times. This type of watercourse generally flows over
an alluvial basin with high groundwater levels. Ephemeral streams only flow during storm
runoff. Most mid- to low-elevation drainage basins are ephemeral, although intersection
with local or regional groundwater systems may create short reaches of perennial flow.
Owing to the combination of human influences and climatic fluctuations, intermittent and
ephemeral streams have become drier historically, and certain reaches that once may have
been perennial, such as the lower Gila River, are now mostly dry except during storm
runoff, urban wastewater discharge, or irrigation returns.
In intermediate-sized drainages, and particularly those with relatively low-elevation
headwaters, have alternating perennial and ephemeral reaches. Excellent examples
include the San Pedro River, where perennial flow extends northward from south of the
U.S.-Mexico border to downstream of Charleston and ephemeral flow occurs northward,
except in the reaches near Cascabel, north of Redington, and near the confluence with the
Gila River. Similarly, the Mojave River has perennial reaches in and north of Victorville,
historically near Camp Cady, and within Afton Canyon. As one moves downstream,
influent reaches increase in discharge owing to groundwater additions, transitioning to
effluent reaches where flow infiltrates into the aquifer. Alternatively, gaining streams are
influent while losing streams are effluent. Influent and effluent sections are associated
with geologic structures, including faults and shallowly buried bedrock. Rivers that once
had this natural configuration, such as the Santa Cruz and lower Gila Rivers, now have
artificial perennial-ephemeral reaches owing to excessive groundwater use, irrigation
returns, and wastewater effluent discharge.
Most towns and cities in the desert Southwest do not use significant surface water for
domestic or municipal supplies. Exceptions include municipalities served by Central
Arizona Project (CAP) water from the Colorado River and Salt River Project (SRP) water
from the Salt and Verde Rivers. The Colorado River Compact, signed in 1922 and modified
several times afterward, allocates 2.8 million acre-feet of water to Arizona in addition to the
1 million acre-feet supplied by the Gila River (including the Salt and Verde Rivers). The CAP
reached the Phoenix metropolitan area in 1983 and was completed to its current length in
1993.* The SRP delivers about a million acre-feet of water to the Phoenix metropolitan area
via a network of canals patterned, at least in part, after Hohokam canals built a millennium
before settlement. Roosevelt Dam, the largest of a series of dams on the Salt and Verde
Rivers, is one of the first large dams built in the United States and was completed in 1911. 23
Water development has far greater effects on streamflow than does climatic variation.
The lower Colorado River is a reach that has extremely high flow regulation and diversion
below Hoover Dam. 21 Comparison of the annual flow volume at gauging stations
downstream from this dam shows the magnitude of water diversion (Figure 4.4). At Yuma
(Figure 4.4D), the record before 1935 represents the pre-dam water flow of the lower
Colorado River. As dams and diversion structures are added, more water is removed until
flow at Yuma is less than 10% of the pre-dam volume. Despite intensive flow regulation
and flood control, the large regional floods in the late 1970s and early 1980s caused a small
but notable increase in annual flow at Yuma.
* http://www.cap-az.com/static/index.cfm?contentID=20 (accessed February 9, 2009).
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