Summary and Conclusions Channel width, depth, width to depth ratio, cross-sectional area and conveyance capacity were measured above and below diversion structures on 20 mountain stream reaches in Wyoming and Colorado. Diversion structures ranged in age from 12 to 106 years and reduced streamflow up to 90 percent. Statistical analysis indicated no significant differences in channel dimensions above and below diversions on steep (slope > 4.0 percent) and moderate (slope 1.5 to 4.0 percent) gradient channels. Low gradient (slope < 1.5 percent) channels responded to streamflow depletion by significantly reducing their depth, area and capacity.
Based upon these findings, additional comparisons were made on low gradient reaches of foothill and basin streams. Results were similar. Channel width, depth, area and capacity were significantly reduced below diversion structures. Using regression analysis, equations were developed expressing these channel properties as a function of discharge.
Our results indicate that moderate to high gradient mountain stream channels located in the forest snowpack zone may be maintained with reduced streamflow regimes. Channel maintenance flow studies should focus on low gradient stream reaches where encroachment and aggradation are more likely to occur. The regression equations presented can be used to estimate the physical response of this channel type to flow depletion.
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