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<br />.
<br />
<br />..
<br />
<br />As a consequence of dissolution of gypsum and limestone, the TDS load at
<br />the delta before regulation was about 380mgl-1. Present values exceed
<br />825 mg I-I (U.S. Bureau of Reclamation data) due to: (a) dissolution via irri-
<br />gation and impoundment, (b) diversions of low salinity headwaters, (c) flow
<br />depletion via evapotranspiration on irrigated land and (d) concentration via
<br />reservoir evaporation (cf. Pillsbury 1981). Intrabasin diversions now under
<br />construction (e,g. the Central Arizona Project) could deplete flows in the lower
<br />river by 2.5km3, increasing TDS to 1I50mgl-1 or more (U,S. Bureau of
<br />Reclamation data). Effluents from mines under development in the Upper Basin
<br />shales also may be high in TDS (cf. Turk 1982).
<br />In contrast, TDS values in the river below Lake Mead have decreased since
<br />1972. This has been dismissed as a transient phenomenon related to flow or
<br />perhaps less dissolution within the basins of Lakes Mead and Powell, but a more
<br />general explanation may come from analysis of the impact of mainstem reser-
<br />voirs on downstream TDS (Paulson & Baker 1983). Stanford & Ward (1984)
<br />showed that deep, mainstream reservoirs on the Gunnison River reduce down-
<br />stream TDS mainly by precipitation of SO; , and that nitrate levels increase
<br />from mineralisation in bottom waters. The reservoirs also limit the seasonal
<br />amplitude of ion strength by impounding floods which once diluted the natural
<br />flver,
<br />The chemistry of the river below Lake Powell is likewise determined by
<br />limnological processes in the reservoir, as 97.7% of the flow reaching Lee Ferry
<br />is from the lake. TDS values have not been appreciably altered by the lake's
<br />formation, although ionic proportions have changed (Table 1). Net losses of
<br />Ca2+ and HCO] via precipitation are offset by evaporation and gypsum dissol-
<br />ution. As the reservoir approaches steady-state with respect to its inherited salt
<br />burden (i,e. gypsum dissolution should decrease with time, while CaC03 pre-
<br />cipitation continues unabated), TDS in the river downstream should decrease
<br />(Gloss et af. 1981). Lake Mead evidently is at or near steady state, explaining
<br />the decreased TDS values (Paulson & Baker 1983).
<br />
<br />Ecological interactions
<br />
<br />Reservoir circulation, trophic status, morphometry and the timing and depth of
<br />releases are the primary determinants of tailwater ecology. Stream ecosystem
<br />processes (e.g. nutrient spiralling, temperature-flow relationships) progressively
<br />ameliorate the tailwater effects with distance downstream (Ward & Stanford
<br />1983; Stanford & Ward 1984). Unregulated side-flows may restore natural
<br />conditions in regulated streams if the tributary discharge approaches that of the
<br />receiving stream (cf. Hauer & Stanford 1982). For example, the middle Green
<br />River, formerly a warm, turbid stream with endemic cyprinids, has changed
<br />
<br />364
<br />
<br />..
<br />
<br />Table], Major iOIl concentrations (mg I-I) at Lee Ferry before impoundment of Lake Powell and
<br />in water discharged from the hypolimnion (after Gloss et at. 1981)
<br />
<br />Colorado River at Lee Ferry
<br />1948-62
<br />
<br />Lake Powell discharge
<br />1972-75
<br />
<br />Ca2+
<br />Mg2+
<br />Na+
<br />K+
<br />HCO)
<br />SO;
<br />C!-
<br />
<br />81.8
<br />24.2
<br />68.4
<br />4,2
<br />189,0
<br />218.0
<br />46,9
<br />
<br />73,6
<br />24,9
<br />75,7
<br />4,\
<br />159.0
<br />241.0
<br />51.1
<br />
<br />Total
<br />
<br />633,0
<br />
<br />629,0
<br />
<br />with regulation by Flaming Gorge Dam to a cold, clear trout stream. However,
<br />side-flows from the Yampa and White rivers (Fig. I) re-establish conditions
<br />favourable to the native fish (Vanicek et al. 1970). In fact, the Green River below
<br />the Yampa River, and the Colorado River from Lake Powell upstream to
<br />Glenwood Springs, Colorado, are the only reaches of the system where the lotic
<br />environment resembles historical conditions. In much of the Colorado drainage,
<br />reservoirs are so closely spaced that the nature of upstream impoundments
<br />determines that of. downstream reservoirs, as well as the intervening rivers
<br />(Stanford & Ward 1986b).
<br />The ecology of the virgin Colorado is largely a matter of speculation, The
<br />headwaters in many places remain as pristine rhithron streams with diverse
<br />zoo benthic communities and trout. The environment became increasingly hos-
<br />tile downstream, due to aridity and erosion. The riverine algae probably were
<br />diverse (cf. Czarnecki & Blinn 1978; Carothers & Minckley 1981), but scarce due
<br />to scouring by turbid flood-waters (Woodbury et af. 1959). Nutrient concen-
<br />trations were sufficient for considerable plant growth during clearwater flows
<br />(cf. Fisher et al. 1982). The zoo benthos, adapted to shifting, sandy bottoms
<br />(Ward et al. 1986), with autochthonous and allochthonous detritus, supported
<br />a unique fishery (Stanford & Ward 1986a).
<br />The post-regulation Colorado River is characterised by increased salinities,
<br />reduced temperature fluctuations, armoured substrata with profuse algal
<br />growths (mainly Cladophora glomerata), zoobenthos of low diversity and vary-
<br />ing productivity, and a diverse fish fauna. The different river segments are
<br />influenced by upstream reservoirs, the length of river between reservoirs, and
<br />side-flow influences. Regulation and competition with introduced species has
<br />reduced native fish populations such that some of the unique, endemic species
<br />face extinction (Stanford & Ward 1986a).
<br />
<br />365
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