9553 - Laserfiche WebLink

Home Browse Search

158 . VALDEZ AND MUTH UTAH - ARIZONA Farmington WYOMINQ P( COLORADO o 50 100 1~D 200 250 .., L i SCALE OF KILOMETERS [1 50 100 150 ~...,..- -A 1 · SCALE OF MILES - - -- - .---- --- NaVdfO Reservoir NEWMEXtCO Figure I.-The upper Colorado River basin and present distribution of wild Colorado pikeminnow Ptychocheilus lucius (shaded). progenitors of modern-day forms further evolved as the Colorado River became a single basin. Many small forms found in the lower basin are unique and have been unable to disperse upstream into the upper basin. Today, the upper basin originates at elevations of 3,000-4,000 m in high mountain meadows, and the Colorado River flows through a series of mid-elevation sandstone canyons (Figure 2) and in- tervening deep canyons with isolated upthrusts of hard Precambrian schist and gneiss. The charac- teristic geomorphic features of the upper basin pro- vide diverse and unique habitats to which the na- tive fishes have adapted over several million years. A long period of geologic isolation, steep stream gradient, high levels of water turbidity and con- ductance, and extreme seasonal variation in water temperatures and flows have led to unique mor- phologic and physiologic adaptations. These spe- cialized adaptations, together with low levels of competition and predation, have rendered native fish species highly susceptible to ecological changes from human activities including (a) flow regula- tion and diversion, (b) physical habitat destruction, alteration, and fragmentation, (c) introduction of nonnative fishes, and (d) degraded water quality (Miller 1961; Carlson andMuth 1989). Current Status and Ecology Fourteen species or subspecies of native fishes in- habit the upper basin of which eight (57%) are