Water Resources in the Desert Southwest - Design with the Desert: Conservation and Sustainable Development - page 71

4
Water Res ources in the Desert Southwest
Robert H. Webb and Stanley A. Leake
CONTENTS
4.1 Introduction .......................................................................................................................... 73
4.2 Hydrologic Settings in the Desert Southwest .................................................................. 74
4.3 Hydroclimatology ................................................................................................................ 75
4.4 Geomorphology ................................................................................................................... 78
4.5 Surface Water ........................................................................................................................ 79
4.6 Groundwater ........................................................................................................................ 82
4.7 Groundwater-Surface Water Connection ........................................................................ 85
4.8 Water Quality ....................................................................................................................... 85
4.9 Riparian Vegetation ............................................................................................................. 86
4.10 Conclusions ........................................................................................................................... 87
References ....................................................................................................................................... 87
4.1 Introduction
As the old saying goes, there is nothing more precious than water in the desert. The
Ancestral Puebloans, Hohokam, and other pre-Columbian cultures knew this and built
their civilizations near guaranteed water supplies. When the Spaniards arrived in present-
day Arizona, they found that the Tohono O'odham and Piman cultures had settled in prime
riverine sites, turning perennial flow through lush riparian ecosystems into irrigation water
for productive agriculture. The Spaniards followed suit, building their missions along
perennial reaches of the Santa Cruz River, including at one place aptly named “Punta de
Agua” (Point of Water) south of Tucson. When the Mormons spread southward from Utah
in the 1870s, their destinations were riverside settings on the Little Colorado, Salt, and San
Pedro Rivers (Figure 4.1). 1
At the beginning of the twenty-first century, water is even more precious in the desert
Southwest. The single greatest source of water in the region, the Colorado River, is
overallocated and drought depleted, 2 leaving an elaborate agricultural economy and growing
metropolitan areas at the mercy of sustained drought. From the late 1970s through the mid-
1990s, severe flooding in the Gila River system had reservoirs brimming while unplanned
releases forced draconian flood protection and zoning downstream; now, some of those
reservoirs store more sediment than water as the first decade of the early twenty-first century
73