Water Resources in the Desert Southwest - Design with the Desert: Conservation and Sustainable Development - page 85

in the late twentieth century, although major declines occurred in areas with major ground-
water development. 21 In many reaches of perennial and ephemeral flow, riparian vegetation
increased in the twentieth century, although much of that increase was in nonnative tama-
risk or other invasive species. Riparian vegetation, while promoted by above-average winter
precipitation and flooding in the late twentieth century, is easily removed by groundwater
overdraft, construction associated with channelization and installation of bank protection
for flood mitigation, diversion of most or all surface-water flow, construction of reservoirs
and the resultant downstream alteration of seasonal flow, or a combination of these factors.
Following its removal, and at least initially, riparian vegetation reestablishment is pro-
moted by wastewater effluent and bank protection. Xerophytic riparian vegetation—trees
and shrubs that thrive with extra water but do not need it—colonized bank-protected
channels through the Phoenix metropolitan area, and riparian vegetation has thrived in
the Santa Cruz and San Pedro Rivers. Bank protection thwarts lateral channel change and
focuses flow and groundwater discharge on a fixed channel location, creating a semblance
of stability of water availability. As a result, dense stands of vegetation increases flow
roughness and decreases channel conveyance, thereby increasing flood hazard. Because
many people would rather see green riparian vegetation than bare channels, a tension has
developed between environmental concerns and flood-hazard mitigation; soil-cemented
reaches as originally designed can only effectively pass floods if vegetation is reduced or
eliminated. One of the central design issues for urban areas is to create channels that can
serve the dual purpose of environmental esthetics and flood-hazard mitigation.
4.10 Conclusions
River channels, surface water, and groundwater are integral to the southwest desert
landscape, on the one hand representing a natural wildness and on the other a resource
to be tamed and used. High variability of precipitation and surface runoff is one strong
characteristic of arid regions; some hydrologic records display nonstationarity with less
predictable water-supply characteristics. As a result, channels through urban areas are
modified to account for this variability and uncertainty, leaving the seeming paradox of
wide channels that are dry most of the time. The largest rivers in the region are regulated,
either solely for flood control, for both flood control and water diversion or storage, or
just for water use downstream. Groundwater systems, if unaffected by withdrawal, are
less variable, but development has caused substantial lowering of water levels in the large
alluvial basins of the region. Water quality in the region is highly variable but may be
decreasing owing to declining surface-water supplies, high evaporation rates in water-
delivery systems, and depletion of groundwater aquifers.
References
1. McClintock, J.H., Mormon Settlement in Arizona (1985 reprint) (Tucson, AZ: University of
Arizona Press, 1921).
2. Webb, R.H., R. Hereford, and G.J. McCabe, Climatic fluctuations, drought, and flow in the
Colorado River, in S.P. Gloss, J.E. Lovich, and T.S. Melis, eds., The State of the Colorado River
Ecosystem in Grand Canyon (Flagstaff, AZ: U.S. Geological Survey Circular 1282, 2005), pp. 59-69.