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
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