Environmental Engineering Reference
In-Depth Information
Wash developed into the Park (see chapter 13) is a linear stretch of seven miles,
located upstream of Lake Mead and eight miles from the downtown Las Vegas Strip,
at the lowest point in the valley.
The Las Vegas valley has a very interesting topography, hydrology, and geology
(France 2011). The tallest peak is twelve thousand feet in elevation and all the water
or snow that falls in the greater valley must run through the five major tributaries
and then through the Wash before reaching Lake Mead. One interesting geologic
feature that is contiguous to the Wash is the Rainbow Gardens, which include all of
the major rock formation types that exist in the nearby Grand Canyon.
The area is, like the marshlands of southern Iraq and the Azraq wetlands in
Jordan (see chapter 7), truly an oasis in the desert. Given the context of being located
in a part of the Mojave Desert that gets only four inches of annual rainfall, the Wash
is one of the few hydroriparian areas in the entire southwestern United States with a
rich floral biodiversity. Indeed, over a hundred species of plants have been identified,
seven of which are currently candidates by the U.S. Fish and Wildlife Service for
listing as threatened or endangered species. Wildlife biodiversity in the Wash is also
atypically high, with over two dozen threatened or endangered species, including the
peregrine falcon, desert tortoise, several types of bats, and some species of birds that
interestingly are also present in the Iraqi marshlands (Scharnhorst 2004).
The Las Vegas Wash is truly a unique place, but like any urban area there are atten-
dant disbenefits associated with the presence of human development (Scharnhorst
2004). With construction of the Hoover Dam in the 1930s, the valley suddenly dis-
covered that it had an abundant supply of electricity which would eventually lead to
inexpensive air conditioning, which in turn would lead to the extreme urbanization
of the valley floor. Indeed, the area today has the highest growth rate in the entire
United States. Further, one of the first undertakings by the workers on the Hoover
Dam and some of the early residents was to begin cutting down the mesquite trees
for firewood. With the disappearance of the woody plants in the Wash, soils were no
longer able to be held in place.
Following the increase in population, ephemeral streams became perennial as
a result of wastewater being discharged to the lowest point in the valley. In conse-
quence, stream channels began to become highly incised under this increased flow
regime (Scharnhorst 2004). This resulted in the former broad floodplain becom-
ing, by 1975, a raging river channel following the occasional high-intensity thunder-
storms. And to give a perspective about how much water goes through this area at
such times, the flow rate of the Colorado River that attracts rapid-running adventur-
ers is generally about twenty-five thousand cubic feet per second, not too different
from the amount of water that will actually go down the Las Vegas Wash during a
hundred-year storm. The damage resulting from this increased discharge was enor-
mous, with some reaches experiencing channel cutting in the order of thirty feet
between 1978 and 1984 (France 2011).
The National Park Service used to have a road that went around Lake Mead that
was built at grade with box culverts underneath. A particularly severe storm in 1975
washed the culverts out and the Park Service decided to improve the road by raising
it up forty feet with a very long span bridge, thereby allowing the Las Vegas Wash
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