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<br />GRAND CANYON NATIVE FISH MANAGEMEm' <br /> <br />9 <br /> <br />,:I <br />",:' <br /> <br />et aI. 1978, Howard and Dolan 1981, Rubin et aI. 1990). For comparative purposes, we <br />reIy upon relevant information from areas upstream in the CoIorado River basin that remain <br />reIativeIy unimpacted by flow manipulations (e.g., Cataract Canyon), and other more general <br />references. <br /> <br />;.? <br /> <br />J <br /> <br />,I <br />f,l <br />~ <br /> <br />The annual hydrograph of the pre-dam CoIorado River in Grand Canyon typically was <br />unimodal, and the singIe peak in flow was produced from snowmelt waters carried from high <br />mountains in the upper basin. Based on U.S. GeoIogica1 Survey (USGS) gaging station <br />records near Phantom Ranch in the 20 years before closure of the GIen Canyon Dam (1942- <br />1962), mean monthly flows varied between approximateIy 5,000 and 10,000 cubic feet per <br />second (efs) from September through February (Figure I). Spring flows rose to-a mean of <br />approximateIy 50,000 cfs in June. Peak flows then receded relatively abruptly and <br />approached base Ievels by August. ConsiderabIe variation in the timing, magnitude, and <br />duration of these discharges occurred among years, but the overall flow pattern was <br />predictabIe. <br /> <br />HydroIogica1 patterns of the major sediment-producing tributaries in Grand Canyon generally <br />exhibited two peak periods of discharge; one associated with spring snowmelt and the other <br />with runoff from late summer convective ("monsoon") storms. As exemplified by USGS <br />flow records at the Cameron gauge on the Little Colorado River (LCR), the,timing, <br />magnitude, and duration of tributary discharge inputs were more variabIe than mainstem <br />discharges (Figure 1). Discharge at the mouth of the Faria River is similar to the LCR <br />patterii (Figure 1). The extreme temporal and spatial variability of late summer precipitation <br />events in watersheds of these streams are well documented (Sellers andHiIl1974).. .;. <br /> <br />" <br /> <br /><: <br /> <br />>- <br /> <br />~~ <br />., <br />-~;;~ <br />?~ <br /> <br />(::~ <br /> <br />.. <br /> <br />~t <br /> <br />~( <br /> <br />"'<-. <br /> <br />"-" <br /> <br />~~~ <br />~~~: <br />." <br />;.':-:1 <br />.,<", <br />~~ <br />'?t;: <br />;~ <br />., <br />;J~ <br />;,';. <br /> <br />. Mean suspended sediment discharge through Grand Canyon was 66.1inillion tons per annum .. <br />at Lee's Ferry, and approached 86 million tons at Phantom Ranch (Andrews 1991; Figure <br />2). The Faria and Little CoIorado rivers, entering the CoIorado between these points, <br />contributed annual means of 3.0 and 9.3 million tons of sediment, respectively., CoIorado <br />River; sediment transport patterns at Lee's Ferry more cIosely approximated the seasonal <br />pattern of hydroIogy than these tributaries (Figure 2), where sediment inpu~.. were <br />diSproportionately associated with late. summer precipitation events (Andrews 1991). The <br />tributaries also -contributed a much greater proportion of sediment load to the Colorado River <br />. than Water discharge levels would indicate (Andrews 1991). <br /> <br />.,-">-' <br /> <br />IDgh spring flows inundated canyon-bound tributary mouths and created large, ponded areas <br />'; with low current veIocities at inflows and in rare unconfined reaches of channel where river <br />'teiraces were overtopped (Howard and DoIan 1981). Terrace deposits were, repeatedly <br />: reworked by, these annual discharge events. Massive gravel-boulder debris fans that were <br />; periodically deposited at the confluen\zones following high magnitude tributary flood <br />