Environmental Engineering Reference
In-Depth Information
10.2.2 Wound 2: Watershed, Stream, and Riparian Damage
Causes: Watersheds, stream channels, and riparian forest (bosques) have been severely
damaged by (1) trapping-out of beavers, (2) livestock grazing, (3) water diversions, (4)
groundwater pumping, (5) fuelwood cutting, (6) agricultural clearing, and (7) watershed
damage from a variety of human activities.
In the arid Sky Islands region, water is generally the limiting resource. Some 80% of
vertebrate species in the region are dependent on riparian areas for at least part of their
life cycle; over half of these cannot survive without access to riparian areas.
23
In Arizona
and New Mexico, more than a hundred federally and state-listed species are associated
with cottonwood (
Populus
)-willow (
Salix
) bosques.
23
Over half of the Threatened and
Endangered species in the U.S. portion of the Sky Islands region became so because of
riparian losses.
24
Arizona and New Mexico have lost 90% of presettlement riparian eco-
systems.
25
The Nature Conservancy lists the Fremont cottonwood (
Populus fremontii
)-
Goodding willow (
Salix gooddingii
) riparian community as highly imperiled.
In 1870, the total number of cattle in the Arizona Territory was only 5000. By 1891, the
population of cattle in the territory had grown to an estimated 1.5 million. In 1870, the
cattle population in 17 western states was estimated to be 4-5 million head; by 1890, that
had grown to 26.5 million. During this period, great numbers of sheep also grazed the
Sky Islands region, and herds of goats were common in some Sky Island ranges.
26
In this
grossly overstocked range, thunderstorms carried away the topsoil in sheets, and gully
washers turned placid streams into dry arroyos with 40 ft sheer banks. Arizona rancher
H.C. Hooker described the San Pedro.
San Pedro river valley in 1870 as “having an abundance of timber with large beds of saca-
ton and grama grasses. The river bed was shallow and grassy with its banks with luxuriant
growth of vegetation.” He gave a different description 30 years later, saying that “the river
had cut 10 to 40 feet below its banks (Figure 10.3) with its trees and underbrush gone, with
the mesas grazed by thousands of horses and cattle.”
27
Botanist Tourney
28
wrote, “There are
valleys [in the Sky Islands region] over which one can ride for several miles without find-
ing mature grasses sufficient for herbarium specimens without searching under bushes or
in similar places.” Before 1891, for example, the Santa Rita Mountains south of Tucson had
25,000 cattle and horses and 5,000 sheep grazing in them.
26
Drought struck Arizona and
New Mexico in 1891-1893, killing 50%-75% of the total cattle population. “Witnesses stated
that a person could stand at one carcass and throw rocks to others nearby.”*
Since the cattle crash 100 years ago, herds have built back up in the Sky Islands region.
Some desert grasslands were transformed into creosote bush (
Larrea tridentata
) desert by the
overgrazing/drought/soil erosion “triple-whammy”; thoughtful observers like rancher Jim
Winder believe that some of these areas can never be restored. In naturally occurring, peri-
odic droughts, livestock grazing is even more destructive than otherwise, as cattle will eat
everything they can before dying—after which vegetative recovery is nearly impossible. In
much of the Sky Islands region, in spite of the improvement from near desertified conditions
at the turn of the century, millions of acres of grazing lands remain in only poor or fair con-
dition. Riparian areas are considered by many authorities to be in their worst condition ever.
Aldo Leopold
3
wrote, “I sometimes wonder whether semi-arid mountains can be grazed at
all without ultimate deterioration.” His question remains unanswered.
During early settlement, bosques were heavily cut for fuelwood, fence posts, and mine
timbers.
26
This cutting of mesquite (
Prosopis velutina
), cottonwood, willow, and other tree
*
Ferguson and Ferguson, “Sacred Cows.”
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