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<br />. <br /> <br />-6- <br />summer (Figures 10 and 11). The sag ~as not f~lly measures this year but is <br /> <br />probably not of a :r.agni -:uuc to affect.:"he fishes. It may be the for~r'Jm:er <br /> <br />of ~he more defined winter situation mentioned on page 5. <br /> <br />Samples taken in the dens i ty current sho.,. movement of Colorado Ri ".fer <br /> <br />water into the abysial region in tne narrow canyons immediately above t~e <br /> <br />dam. How the large releases of water made from the dam since its c~csure <br /> <br />have affected the extent and rate of this move~ent is unknownj however, it <br /> <br />certainly acts to influence a region kno'JO in other reservoirs to become <br /> <br /> <br />unfit for activity by most higher biological forms. This region in Lake <br /> <br /> <br />Powell has always appeared to maintain an adequate environment for fishes. <br /> <br /> <br />The summer thermocline situation on Lake Po\.ell closely follo~s classical <br /> <br />descriptions (Figure 10). During the period of well defined stratification <br /> <br />from June to September the epilimnion shows little temperature change with <br /> <br />depth. The thermocline normally ex~ibits a rate of temperature change near <br /> <br />0.5 to 1.00 F. per foot} and ranges from about 15 to 30 feet in thickness. <br /> <br />The thermocline gradually increases in depth up reservoir. It is found at <br /> <br />a depth of about 25 feet near the dam and 55 feet in the upper reaches of <br /> <br /> <br />the reservoir where it lies under the inflowing Colorado River. A nearly <br /> <br />homothermic condition exists from the bottom of the thermocline to a depth <br /> <br /> <br />of abO'.lt 120 feet. From this point there is a steady decline in terr;~erQ.ture <br /> <br />to the lake bottom. The deeper position of the thermocline in the upper <br /> <br />reservoir constric ts this homothermic a.rea until it disnppearG a fCH r.rilcs <br /> <br />above Bullfrog Bay. <br /> <br />The reservoir shows a distinctly differer..t pnttcl'n of cooli::[; th~n has <br /> <br />been recorded for most natural bodies of water. uJring summer there is very <br /> <br />little different in ma.:<imum surface temperatures over t.he entire reservoir. <br /> <br />Cooling begins in October and proceeds much ffi~ce rapidly in the upper <br /> <br />reservoir, above the Escalante or Rincon areas. Temperature isobars :'11',13 <br />