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ing spring runoff and after heavy rains, and <br />substrates are generally rocky with occasional ex- <br />panses of sand. Benthic invertebrates are generally <br />abundant only where substrates are rocky, and <br />primary production is higher than in other zones. <br />Salmonid fishes are less common than in the upper <br />zone, and cyprinids and catostomids are predomi- <br />nant. The lower (large-river) zone has warm, turbid <br />water and can be subdivided into two distinct sub- <br />units -canyon areas of steep gradient and meander- <br />ing sections with low gradient in flat terrain. <br />Substrates in high-gradient canyons are of sand, <br />gravel, and rubble; in low-gradient canyons or on <br />flats, sand substrates predominate. Primary produc- <br />tion is virtually absent in this zone, and production <br />of benthic invertebrates depends on the availability <br />of gravel-rubble substrate. Allochthonous materials <br />are the basic energy source for this zone's aquatic <br />communities. The fish components of large-river <br />communities are cyprinids and catostomids, and the <br />relative abundance of various species differs con- <br />siderably in the two subdivisions of the zone. <br />Bishop and Porcella (1980) identified water- <br />quality problems in the Upper Colorado River <br />System by determining where federal or state <br />water-quality standards were exceeded. Problems <br />were considered primarily local and included acid <br />mine drainage and heavy metals pollution, energy <br />impact, reservoir eutrophication and sedimentation, <br />biochemical oxygen and dissolved oxygen interac- <br />tions below treatment facilities, and potential health <br />problems associated with municipal sewage dis- <br />charge. The most serious water-quality problem, in <br />a general sense, is increasing salinity (total dis- <br />solved solids). Salinity increases downstream <br />because of concentration of salt in subsurface <br />waters by range and forest tracts and <br />evapotranspiration by phreatophytes and marshy <br />areas along the river. Man's activities have also con- <br />tributed salts, and salts have been concentrated by <br />irrigation of crops, reservoir evaporation, water <br />diversions, and municipal and industrial water uses. <br />In 1974, at the behest of the Environmental Protec- <br />tion Agency, the seven states of the Colorado River <br />Basin agreed to maintain salinity in the Lower Basin <br />at or below levels measured in 1972. The U.S. also <br />agreed in 1974 to deliver Colorado River water to <br />Mexico at Morales Dam in an amount that does not <br />exceed the average salinity at Imperial Dam (north <br />of Yuma, Arizona) by more than 115 (t 30) mglliter. <br />Total sediment load has decreased substantially <br />since construction of Colorado River Storage Pro- <br />ject dams on the Upper Colorado River; Lake Powell <br />and Navajo, Fontenelle, Flaming Gorge, Blue Mesa, <br />and Morrow Point reservoirs trap about 75-80% of <br />the sediment that normally flowed into Lake Mead <br />on the lower Colorado (Joseph et aL 1977). These and <br />other reservoirs have also significantly altered <br />stream temperatures and discharge in the Upper <br />Colorado River Basin. Joseph et al. (1977) discussed <br />temperature, pH, and dissolved oxygen and sum- <br />marized voluminous U.S. Geological Survey records <br />on carbonates, calcium, chloride, conductivity, <br />magnesium, phosphate, potassium, silica, sodium, <br />sulfate, and turbidity in the Upper Colorado River <br />Basin. <br />STUDIES OF FISHES OF THE UPPER COLORADO RIVER SYSTEM <br />The present Colorado River drainage has existed <br />since the Pliocene and has had no broad connections <br />with surrounding river basins for millions of years <br />(Behnke 1980). This long period of isolation has led <br />to a high degree of endemism in the fish fauna of the <br />river (Behnke 1980; Behnke and Benson 1980). Miller <br />(1959) stated that the Colorado River drainage <br />system was second of seven centers of endemism <br />studied in degree of endemism of fish species. It <br />ranked highest (87%) in endemism of primary <br />(strictly freshwater) fishes. Miller listed 35 species, <br />22 genera, and 11 families of native fishes for the <br />Colorado River System. Hubbard (1980) reported 30 <br />species, 18 genera, and 6 families of native <br />freshwater fishes in the Colorado River Basin, with <br />73% of the species and 39% of the genera being <br />endemic. <br />Some disagreement regarding numbers of native <br />and introduced fishes in the Upper Colorado River <br />System is reflected in recent reports. Wydoski <br />(1980) referred to an unpublished 1976 Colorado <br />Wildlife Council list of 50 species and 4 subspecies of <br />fish in the Upper Colorado River. Twenty species <br />and 4 subspecies were said to be native to one or <br />more states in the Upper Basin. Raleigh (1980) cited <br />a 1975 unpublished report of the Utah Water <br />Research Laboratory listing 13 native and 31 in- <br />troduced fish species in the Upper Colorado River <br />drainage system. Joseph et aL (1977) and Behnke <br />and Benson (1980) listed 13 species of fishes native to <br />the Upper Basin. The report by Tyus et al, in this <br />symposium contains the most recent and authori- <br />tative data on this subject. <br />Fishes of the Green Sub-basin <br />Fontenelle and Flaming Gorge reservoirs have <br />had a profound effect on flow and water quality in <br />the Green River; lower summer water temperatures <br />have resulted, and spawning of native fishes in <br />these areas has virtually ceased. The changed <br />habitat immediately downstream favors introduced <br />salmonids which compete with native species <br />(Joseph et aL 1977). Generally, introduced fishes ap- <br />pear to be thriving in the Upper Colorado River <br />Basin. <br />Banks (1974) discussed the fishery resource of the <br />Green River in the Fontenelle tailwater (between <br />the Fontenelle and Flaming Gorge impoundments), <br />where a very productive trout fishery competes <br />with industry, agriculture, and municipal interests