Water Resources in the Desert Southwest - Design with the Desert: Conservation and Sustainable Development - page 73

America 3 due, in no small part, to the complexity of the regional geologic framework
(see Chapter 2). Here, we restrict our discussion to the Mojave and Sonoran Deserts and
the Colorado Plateau, although our information generally applies to the other deserts as
well. The Mojave and Sonoran Deserts are part of the Basin and Range Province, and fault-
block mountains bound deep alluvial basins. The Colorado Plateau is a physiographic
province characterized by low tectonic activity and few active faults, and the landscape
either is bare bedrock or consists of shallow sediments over bedrock.
Most of the drainage systems on the Colorado Plateau and in the Sonoran Desert drain
to the Colorado River and ultimately to the Gulf of California, while most in the Mojave
and Great Basin Deserts drain to closed basins called playas (see Chapter 1). Precipitation
in closed basins does not runoff directly to the oceans; instead, water pools in intermittent
lakes and either recharges to groundwater or evaporates. The principal through-flowing
rivers include the Green, Colorado, and San Juan Rivers in the north; the Gila River system
in the south, the principal drainage of most of Arizona, consists of the Verde, Salt, and Gila
Rivers with the lesser San Pedro and Santa Cruz River draining southeastern Arizona
(Figure 4.1). In the Mojave Desert, the Owens and Mojave Rivers are the only major drain-
ages, and each terminates in a major playa system.
All of the principal drainage basins feeding into the Colorado River originate in high-
land areas that receive far more annual precipitation than the desert lowlands. For the
entire Colorado River drainage, the area of highest precipitation is in the Rocky Mountains
of Colorado, and the area of lowest precipitation is in the delta area south and west of
Yuma, Arizona (Figure 4.1). In the adjacent Great Basin, again the highest precipitation is
in the higher elevation mountains and the lowest precipitation is in low-elevation closed
basins such as Death Valley. Given this disparity in higher precipitation in headwaters
areas, combined with more agriculture and urbanization in the desert lowlands, it is not
surprising that extensive surface-water regulation networks exist to capture, store, and
transfer water from where it is generated to where it is used.
In terms of areal extent, groundwater systems generally do not correspond to surface-
water drainage basins.* Instead, groundwater basins correspond to subsurface geologic
structure and rock permeability. For example, major springs in Death Valley are an out-
flow from an extremely large groundwater basin that extends into central Nevada. 6,7 On
the Colorado Plateau, sandstone units form the principal aquifers, and these can be iso-
lated from other aquifers by intervening shale or limestone units. 8 Through most of the
Mojave and Sonoran Desert, however, groundwater basins are smaller and more closely
related to the Basin and Range structural framework with basin-fill sediments adjacent to
bedrock of the mountains (Figure 4.2).
4.3 Hydroclimatology
Climate, as previously discussed in this topic (see Chapter 3), strongly affects the hydrol-
ogy of the Southwest. Hydroclimatology is the study of interactions between climate and
hydrologic processes. An understanding of the origins and interannual and seasonal
fluctuations of surface water, as well as longer-term fluctuations in groundwater levels,
* For a comparison of surface-water drainage basins with groundwater basin areas in Arizona, see Seaber et al. 4
and Arizona Department of Water Resources. 5