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<br />for conversion to agriculture. Major areas of bottomland hardwood
<br />forests have been cleared, drained, or converted to agriculture. Agricul-
<br />tural uses were estimated to account for 54% of the 300,000 acres lost
<br />annually from the mid-1970s to the mid-1980s.
<br />
<br />. Riparian ecosystems are being degraded and destroyed throughout the
<br />United States. The lower 48 states originally comained 75-100 million
<br />acres of indigenous, woody riparian habitat, but today only 35 million
<br />remain in nearly natural condition. The rest have been inundated by
<br />reservoirs, channelized, dammed, riprapped, converted to agricultural
<br />use, overgrazed, paved, or altered by a combination of factors that have
<br />impeded their ability to stabilize and maintain the biological diversity
<br />of their own watersheds. Riparian habitats have been lost in every'
<br />region of the country.
<br />
<br />. Channelization and other flood control projects can destroy riparian
<br />habitat by clearing vegetation; eliminating sandbars, islands, and pro-
<br />ductive backwater areas; and accelerating bank erosion. Bet\....een 1940
<br />and 1971 the U.S. Army Corps of Engineers assisted in navigation and
<br />flood control projects to alter 11,000 miles of streams. The Soil Conser-
<br />vation Service has installed 10,700 miles of channel modifications.
<br />
<br />. Dams can alter riparian habitat in many ways, such as dro......ning it
<br />under reservoirs, desiccating it by downstream dewatering, or render-
<br />ing it non-regenerative by interrupting the natural flood cycle. The
<br />nation's 68,153 nonfederal dams have altered or destroyed tens or hun-
<br />dreds of thousands of miles of riparian habitat. Impoundments by the
<br />federal government have transformed major river systems, including
<br />the Columbia, Colorado, !\:Iissouri, and Tennessee, into a series of
<br />artificial lakes, severely decreasing the diversity of habitats available
<br />to wildlife but creating other habitats and environments.
<br />
<br />. By overgrazing, trampling vegetation, compacting the soil, and break-
<br />ing down streambanks, livestock have seriously damaged watersheds
<br />and riparian zones. These impacts have led to increased soil erosion,
<br />higher nutrient load in streams, bank erosion, and lo-.....ering of water
<br />tables. Inadequate livestock management has been responsible for the
<br />serious lack of riparian habitat regeneration on federal rangelands in
<br />the \Vest.
<br />
<br />. LO\...ering of the water table in arid and semi-arid regions causes a
<br />drastic and often permanent degradation of the floodplain. [n many
<br />areas, a high \vater table and accompanying pools and springs are the
<br />only sources of moisture for riparian vegetation and native animals.
<br />Introduction of non-native plants has also significantly contributed
<br />to alteration of floodplain habitat. Salt cedar, for example, which was
<br />imported to North America during the 19th century, has become the
<br />predominant riparian tree species on the lower Colorado, the lower Rio
<br />Grande, and Pecos rivers. Ie covers some 500 square miles in those basins
<br />alone, and makes the riparian areas less suitable to many native birds.
<br />
<br />
<br />
<br />Alteration is widely used to amtrol flooding b)' inaeasing
<br />1M carrYIng capiUity 0/ a stream channel. 7i~hniques in-
<br />dude straightening, dupening, widening, or paving the
<br />chn1llul; rmwving dLhris; raising or enlarging bridges and
<br />culverts; remon'ng dnms and other obstru.ctions; and in-
<br />sto.lling u.nderJ!,ound conduilJ. HowelXr, unless carefuil)'
<br />planned and executed, srd dw.nnel rmuuji<ation can sig-
<br />nifi<ant/;' offect riparian habitat
<br />Artificial dw.mul (buried conduit) Ilnder construction,
<br />LaPlace, Louisiana.
<br />
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<br />' ~"'.l.A ~ '.,... '.c.
<br />',.. '."I/~~'- ~"'"' .-"'-
<br />!t- '!.........-. _., ,..~..
<br />.- - ---.1fC' of . \,
<br />~-::-,.~ ..! l , . ~,."".,
<br />..-'.---. . . <--. . . i! iW..,. "~ I
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<br />
<br />Thi introduction f!.( cattle to the Ameruan nest has Iw.d a
<br />fUnda.7TU'1ltal ifftct on the no.tion's landscape-in particu-
<br />lar, on riparian lands in semi.arid em'ironm,mts. In mall}
<br />COSts thi result has been soil compaction, loss 0/ vegeta-
<br />tion, increased erosion, and thi consequent dttenoration 0/
<br />floodpUiins, river banks, and river waleT gun/it)'.
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