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<br />
<br />, .
<br />
<br />.........-..- Colorado Riller
<br />.r-/"'- Mojor tributaries
<br />...---~ Basin boundary
<br />........ Tronsbasin diversion
<br />.... Nafive Fishery
<br />-{> El(oric Fishery
<br />
<br />~
<br />
<br />,
<br />\
<br />-'
<br />,
<br />I
<br />I
<br />I
<br />(
<br />,
<br />,"'f~..\ ,'...
<br />\.i\ \ ~ "
<br />\~ I I
<br />I ~ '\, ..._ ,,/ F'~C::II
<br />-,,-' '....~...'
<br />
<br />200
<br />
<br />km
<br />
<br />;ure 1. The Colorado Basin, Intermittent reaches are shown as broken lines, Important native
<br />,lid triangle) and exotic (open triangle) fisheries also are marked,
<br />
<br />th the spring freshet. The floodwaters renewed the vast desert marshes, cued
<br />awning of riverine fish and encouraged the growth of riparian plants. But
<br />: fate of the river was sealed; it remained only for flows to be allocated among
<br />? basin states.
<br />After years of argument (cf. Hundley 1975), the Colorado was legally divided
<br />o Upper (Wyoming, Colorado and Utah) and Lower Basins (Arizona, Cali-
<br />nia & New Mexico) at Lee Ferry, above the Grand Canyon (Fig, I),
<br />lowances for each state were based on the estimated average annual flow at
<br />c Ferry, A federal scheme of reservoirs and diversion canals facilitated
<br />ivery of water shares to each state, leaving only a small amount for Mexico,
<br />e major works have now been built, except for canals now under construction
<br />divert water from below Lake Mead into central Arizona. Except in small
<br />
<br />~,
<br />
<br />tributaries, flows in the system now are totally regulated. Unfortunately, the
<br />virgin flow of the river at Lee Ferry may have been over-~sti~ated, and many
<br />believe that the river will not supply present water allocatiOns m the long term
<br />
<br />(Spofford 1980). . .
<br />Regulation has changed the ecological character of the nver. Habitat destruc-
<br />tion has pushed many endemic fish to near extinction, Irrigation, with evapor-
<br />ation and mineral dissolution in reservoirs, has increased salinities in the lower
<br />reaches to levels which adversely affect agricultural and municipal uses. Salinity
<br />control projects, including a desalination plant, have been developed to reduce
<br />salt loads and deliver treaty water requirements to Mexico. .
<br />In this chapter we review the physiographic evolution of the Colora~~ River,
<br />describe the ecology of the pristine river and its riparian plant commumhes, and
<br />contrast these with the conditions imposed by regulation.
<br />
<br />Physiography and fluvial morphology
<br />
<br />Evolution of the Colorado
<br />
<br />The topography of western North America results from crustal deformations
<br />which uplifted the great mountain ranges, and erosion of the Cordillera by thr,ee
<br />great river systems: the Columbia, Sacramento-San Jo~quin and Colo,rado (Fig.
<br />2). Mesozoic orogenies produced the Rocky Mountams and the vaned ranges
<br />of the western Cordillera, including the block-faulted Sierra Nevada to the
<br />south and the volcanic Cascades to the north. Precambrian granites were
<br />uplifted through Palaeozoic and Mesozoic sediments to elevations exceeding
<br />4000 m. Most land between the ranges is the eroded surface of these old
<br />sedimentary formations. During this process the interior Cordillera subsided,
<br />and erosion sculpted the plateaus and plains of the Great Basin. Another
<br />portion, the Colorado Plateau, was uplifted to its present elevation of
<br />1500--2500 m (tilted downward E-W), with little of the compression that fold~d
<br />and faulted the sediments of the Great Basin and the flanks of the Rockies
<br />(Clark & Stearn 1960),
<br />Drainage from the developing Cordillera was influenced by the rain-shadow
<br />effect of the western ranges on weather systems moving from the Pacific Ocean;
<br />this accounts for the deserts between 36-390N (Fig. 2). Historically, the Great
<br />Basin has received little precipitation compared to the mountains on either side,
<br />although latitudinal shifts in climate may have produced vegetational changes
<br />in the last 24000 years (Cole 1982). Drainage from the western side of the
<br />southern Rockies proceeded across the Colorado Plateau to the southern
<br />perimeter of the Great Basin, where little more water was generated. The
<br />Colorado River assumed its present position by the Pliocene, if not earlier (King
<br />1958, 1959).
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<br />355
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