Environmental Engineering Reference
In-Depth Information
Where Does Our Water Come From?
The water our lives depend on originates in the world's oceans, from
where it evaporates and is carried by air currents over land surfaces. The
chief proximate sources are large river systems such as the Mississippi
and Ohio in the East and Midwest, and the Colorado and Rio Grande
in the West; large lakes such as the fi ve Great Lakes along the Canadian
border; and underground aquifers such as the Ogallala in the Midcon-
tinent from north Texas to South Dakota. The water in each of these
sources is either decreasing or experiencing increased pollution from the
artifi cial chemicals we inject into it—or both. The Colorado and Rio
Grande no longer reach the sea year round because a growing share of
their waters are claimed for various uses.
The Colorado River
The Colorado River, with an annual fl ow of 5 trillion gallons of water,
is perhaps the best example of the unsustainable overuse of river water
in the United States. A common misconception of water use in the basin
and in the West in general is that rapidly growing urban areas are the
main users of the region's limited water. In fact, 85 to 90 percent of the
water is used in agriculture, mainly to grow food for cattle. 6 Only 10 to
15 percent of the water is used directly by the 25 million people served
by the river who live in Los Angeles, Phoenix, and other communities.
How much of the water is used to keep swimming pools fi lled and lawns
watered in this dry climate is unknown. But clearly the river's water is
oversubscribed, because the river's channel is dry at its entrance into the
Gulf of California (fi gure 1.1). Five trillion gallons of water per year is
not enough to satisfy both the needs and wants of 25 million people.
The shortage of water in the Colorado River was recognized many
decades ago, and there have been many lawsuits by those who felt
slighted by their legislated allocations. The problem was most severe in
years when annual rainfall was less than average, so to alleviate this
problem, the federal government built many dams and reservoirs along
the river to store water and smooth out yearly variations. But lawsuits
persisted.
Finally, after years of wrangling and facing the worst drought in a
century, and with the prediction that climate change will probably make
the Southwest drier in the future, federal offi cials in 2007 forged a new
pact with the states on how to allocate water if the river runs short. The
pact puts in place new measures to encourage conservation and manage
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