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<br />11 <br /> <br />Dua235 <br /> <br />There were 33 transects on the 18 study reaches that were <br />classified as critical riffle. Seventy-six percent of the time, <br />average depth was considered first limiting or co-limiting accord- <br />ing to the specified criteria (Table 3). Fifteen percent of the <br />time, percent wetted perimeter was first limiting or co-limiting. <br />Nineteen percent of the time, average velocity was first limiting <br />or co-limiting. This indicates that average depth is the most <br />important criterion in minimum flow recommendations as the para- <br />meters are set at the present time. <br /> <br />The primary assumptions made in using the critical riffl~ <br />concept are that average depth, average velocity, and percent <br />wetted perimeter change most rapidl~ across critical riffles. <br />If these parameters are maintained at or above minimum acce~table <br />levels across the riffle areas, the~ will be maintained in other <br />,habitat types such as pools and runs as well so that adequate <br />habitat exists for maintenance of most life stages of fish and <br />~uatic invertebrates. Examination of the data (Table 5) tends <br />to bear out these assumptions. Observations made during electro- <br />shocking and cross section evaluations throughout the summer of <br />1978 also further substantiate these contentions. On Sangre de <br />Cristo Creek early fall flows were reduced to less than 0.1 cfs <br />and remained that way for approximately 60 days. Despite no <br />opportunity for movement up or downstream to better refuge areas, <br />this stream supported a standing crop' of 27 lbs ~er surface acre <br />biomass of Rio Grande cutthroat trout (SaZma cZarki virginaZis) <br />.during the extended period of near zero flow com~ared to 23 lb/ <br />acre in June 1978 at near optimum (17 cfs) flow conditions. <br /> <br />va..f <br /> <br />Low 'FLOC......, <br /> <br />l <br />I <br /> <br />Average water velocities of 1 - 1 1/2 ft/sec have been shown <br />~ various investigators to produce both op.timum number~_o{_~quatic <br />invertebrates as well as good spawning conditions fo:r:.most_species <br />of trout (Giger, 1973; Hooper, 1973; Hoppe and Finnell, 1970). <br />These limits fall within the criteria range set for average velocity <br />with the Colorado R-2 Cross Method. I found that 77% of the time <br />the R-2 Cross overestimates the actual average velocity across the <br />transect by almost 45%, i.e., a predicted average velocity of 1.45 <br />ft/sec would only be 1 ft/sec. Under the proper set of circumstances <br />,this could 'result in a minimum flow recommendation that might not, <br />adequately sustain aquatic invertebrate populations or incubating <br />trout eggs. Hoppe and Finnell (1970) found that incubating trout <br />~ggs suffocated when average water velocities dro~~ed l>~lowJ, ft/see. <br /> <br />Stalnaker and Arnette (1976), Hooper (1973), and the Wyoming <br />Water Resources Research Institute (1978) have all presented excel- <br />lent reviews of the literature concerning depth, velocity, and <br />substrate preferences for the species of trout occurring in the <br />stream reaches included in this study. The average depth and <br />average velocity criteria used in setting the minimum flow recom- <br />mendations for the R-2 Cross fall well within the accepted ranges <br />as summarized by the above investigators. <br />