WWRC 89-49
Response of Mountain Stream Channels and Associated Riparian
Areas to Flow Regulation
Abstract
The need for channel maintenance flow
releases below water development projects in the central
Rocky Mountain region is currently being debated. The
objective of our study was to measure the change in
hydraulic geometry of mountain stream channels in
response to water diversion. Channel width, depth, width
to depth ratio, cross-sectional area, and conveyance capacity
were measured above and below diversion structures on
20 stream reaches in Wyoming and Colorado to determine
the extent of channel adjustment since regulation. Ocular
estimates of riparian vegetation density were also made.
Diversion structures ranged in age from 12 to 106 years and
reduced streamflow up to 90%.
Our preliminary analysis using paired t-teats indicates
no significant (P> 0.05) differences in channel dimensions
above and below diversions on steep (slope > 4.0%) and
moderate (slope 1.5 to 4.0%) gradient channels. Low gradient
(slope < 1.5%) channels responded to streamflow
depletion by significantly reducing their depth, area, and
conveyance capacity. In a majority of cases, we observed
no differences in vegetation density above and below diversion
structures.
Our preliminary results indicate that moderate to high
gradient mountain stream channels located in the forest
snowpack zone may be maintained with reduced streamflow
regimes. Low gradient channels, where encroachment
and aggradation appear more likely to occur, should be the
primary focus of channel maintenance flow studies.
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