Water Diversion
and Impoundment
--From Nabhan, Gary Paul and Andrew R. Holdsworth.1998.
State of the Sonoran Desert Biome: Uniqueness, Biodiversity,
Threats and the Adequacy of Protection in the Sonoran Bioregion.
Sponsored by The Wildlands Project. p. 36
Since 1908, hydrological engineers in the Sonoran Desert
have impounded and diverted water flows from virtually all
of the region's major rivers by constructing 41 major dams,
with 17 in Arizona, 21 in Sonora, 1 in Baja California, and
2 in Baja California Sur (Figure 4).
The dams in Arizona have impacted the entire Sonoran Desert
portions of the Salt, Gila, Bill Williams and Colorado Rivers.
These impacts include stream conversion to lakes above the
dams and the creation of dry stretches below them; the loss
of natural flooding processes, and related changes in riparian
dependent biotic communities. Below the Salt River's Granite
Reef Dam, for example, all water is diverted and there is
no riparian vegetation until the outpouring of treated sewage
effluent from Phoenix wastewater treatment plants. Where there
is riparian vegetation below dams, it is now most frequently
dominated by exotic salt cedar.
CHECK: JUST SIDEBAR QUOTE? The damming of the Colorado River
and the Rios Yaqui and Mayo in Sonora has dramatically reduced
the annual floods that carried the nutrient-rich sediments
to once highly-productive, biodiversity-rich delta and marine
ecosystems. Given that the total diversion capacity of the
Colorado River for municipal and agriculture projects is more
than the average flow of the river at Lee's Ferry, it is now
a rare event when the Colorado's flow reaches the Gulf of
California. When it does reach the Gulf, it is in the arid
West, water diversion - primarily for agriculture - has been
going on for thousands of years, albeit on a small scale until
the late nineteenth century. Native Americans sometimes diverted
water to grow crops, but they generally grew crops where the
water was, rather than moving water to the crops ... In the
twentieth century, this urge to capture and store water reached
its culmination with the building of the gigantic Hoover and
Glen Canyon Dams on the Colorado River. These large dams and
diversions cause much biological impoverishment due to habitat
fragmentation, alteration in water flow patterns, increases
in exotic species, and other changes."
- Allen Coopenider et al.,
Defending the Desert (1995)
In general, the very presence of the border has set up some
"natural experiments" where it becomes easy to compare
different fertilizer and pesticide residue. In combination
with fisheries overharvesting, the reduction of water flow,
sediment loads, and water quality from the Colorado River,
Rio Yaqui and Rio Mayo has resulted in the endangerment of
the totoaba (Totoaba macdonaldi), the "vaquita"
porpoise (Phocoena sinus) and sea turtles, and produced
substantial reductions in the Gulf shrimp harvest (Búrquez
and Martínez-Yrízar 1997).
Among U.S. Federal Register notices of listing plants and
animals as endangered species, water impoundment and diversion
are among the most frequently cited threats mentioned (Rick
Knight, pers. comm.). Inundating vegetation in reservoirs
behind dams, and changes in river flow are collectively among
the top seven cited pressures on threatened plants in the
U.S./Mexico borderlands (Nabhan et al. 1991).
Because so many native species in this arid region critically
depend upon the limited surface water which is episodically
available, water impoundment and diversion away from riparian
habitats has radically changed the distribution and abundance
of many organisms. As a case in point, Johnson, Bennett and
Haight (unpubl.) have documented the regional demise of 36
of the 82 breeding bird species which formerly used riparian
woodlands. These "bosques" - dominated by closed
mesquite canopies - have decreased in abundance since 1900
in the Sonoran Desert lowlands to the point that 12 of their
historic breeding bird species are now more frequently found
in desertscrub than in water-starved floodplain habitats.
Even small water impoundments and developments can unintentionally
serve to disrupt wildlife populations, and change relationships
between large ungulates, their predators, parasites and diseases.
Broyles (1995) has recently reviewed the ecological impacts
of desert wildlife water developments such as artificially-impounded
sheep tanks and bubblers, and calls to question whether these
"habitat improvements" cause more harm than good.
On a much larger scale, the heavily altered wetlands of the
Colorado River delta provide the greatest testimony for the
tragic effects of water diversion and impoundment. However,
there remains the possibility of restoring the continent's
grandest river delta ecosystem. Extensive agricultural production
and related river diversion and channelization have converted
cottonwood-willow gallery forest to low stature, salt tolerant
vegetation and once vegetated areas into salt and mud flats.
Salt cedar now dominates much of the delta, denying such birds
as the yellow-billed cuckoo and southwestern willow flycatcher
the nesting and feeding sites they prefer. The Sonoran Desert's
single endemic grass species, Palmer's Saltgrass (Distichlis
paimen), occurs only in the northern Gulf of California,
but has not been observed regenerating under current more
saline conditions there.
Nevertheless, the Colorado River delta is still the largest
wetland in the Sonoran Desert. Since 1973, its areal extent
has fluctuated between 5,800 and 63,000 hectares. Relatively
wet periods (such as 1980-1993) have breathed life into the
delta wetlands, allowing floodwaters to once again reach the
delta in near-record volumes. Saline agricultural drainage
supports Cienega de Santa Clara's habitat for 22 wetland plant
species, an important migratory waterfowl stopover site, and
the world's largest populations of Yuma clapper rails and
desert pupfish (Glenn et al. 1996). Additionally, the delta
provides an important nursery grounds for marine crustaceans
and fish.
Glenn et al. (1996) claim that "delta wetlands are not
directly in danger due to lack of water," for it is the
management of water that is pivotal. The Cienega de Santa
Clara would be dramatically altered if the Yuma Desalting
Plant started operating and utilized the agricultural drain
water that flows to it. Even though it is part of a biosphere
reserve, the delta's water requirements are not taken into
account in water management and land use planning. However,
sustaining the current delta wetlands could be accomplished
without undermining irrigation needs nor municipal demands
(Glenn et al. 1996). There are now efforts underway to define
management criteria that will further protect wetland values
in this delta and region.
- Allen Coopenider et al., Defending the Desert (1995)
(Búrquez and Martínez-Yrízar 1997).
Glenn et al. (1996)
Broyles (1995)
Johnson, Bennett and Haight (unpubl.)
(Rick Knight, pers. comm.).
(Nabhan et al. 1991).
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