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<br />I <br />I <br /> <br />10 <br /> <br />CLARKSON ET AL. <br /> <br />events created steep gradient rapids characterized by extreme turbulence and current veIocity <br />(LeopoId 1969, Oraf 1979, Kieffer 1985). Large pooIs formed immediately upstream of <br />these channel constrictions. PooI bottoms were filled with fine inorganic materials <br />transported by the sediment-rich pre-dam river. Flow separation and reattachment patterns <br />beIowdebris fan constrictions formed Iarge eddy compIexes of recirculating flow, and Iow- <br />velocity return channels or backwater habitats (Rubin et al. 1990, Schmidt 1990). <br /> <br />, <br /> <br />The ascending limb of the spring peak flow in the mainstern scoured peripheral and <br />midchannel habitats dominated by fine-grained substrates, such as eddies, eddy return <br />channcls, and pooI bottoms above and beIow rapids (Howard and Dolan 1981). Sediments <br />were deposited on the descending limb from a complex interaction between hydrology and <br />sediment load and particle size (Howard and Dolan 1981). Spring high flows were of <br />sufficient velocity to scour and redeposit larger substrates such as gravels, pebbIes and <br />cobbles, and rework tributary debris fans (Leopold 1969, Oraf 1979, Howard and Dolan <br />1981, Kieffer 1985). <br /> <br />Late summer and early autumn tributary floods transported large amounts of fine sediments <br />to the mainstem, where they were mostly aggraded on channeI bottoms (Howard and Dolan <br />1981); Highest suspended sediment concentrations in the pre-dam Colorado River occurred <br />during tributary flood events, occasionally exceeding 28,000 parts per million (Dolan et al. <br />1974). .. .;. <br /> <br />-.J..-.. <br /> <br />. ... . - ~-- - <br />Seasonal changes in water temperature lagged behind the patterns of hydrology and sediment. <br />Mean monthly temperatures of the pre-dam CoIorado River varied from 30C during January <br />low flows to near 2SoC in July and August on the descending limb of the peak flow <br />hydrograph(Figure 3). Teinpetatures typita1ly begantorisein~Febi\Jary_and.beg~ <br />d~~g in September. .-- . <br /> <br />c....._a.;,~.~:.. '. <br /> <br />"d,,' <br /> <br />Only a limited water temperature record is avaiIable for the LCR below BIue Spring, the <br />Stream's lower perennial source 21 Ian above the mouth. Temperatures during the period <br />1990-1993 rarely approached minima or maxima exhibited in the pre-dam ma!nstem (Figure <br />3)~~ .Constanttemperature of BIue Spring (20.60C) and other springs in the lower LCR <br />(LOughlin 1983) ameliorate the effects of extreme air temperatures in the lower perennial <br />reach. The pattern of seasonal temperature change in the LCR and Faria River are similar <br />to the pre-dam Colorado River (Figure 3). <br />21::~~l};J....~.' : ,- .:....,.' -.'-' "':".: ,':: _. ~I-H.~~~;. ". <br />1.;Ietilbolism in the pre-dam river was driven by heterotrophic pnicesses during periods of <br />higtr discharge and sediment load, where light limitation and 'scouring precluded most algal <br />'prodUctivity: Vast volumes of allochthonous organic matter transported from upstream <br />'teiiestrW . sources likely provided the maW{ source of biological nutrition during these <br />