WWRC 87-25
Recommendation and Evaluation of a Mitigative
Flushing Flow Regime Below a High Mountain Diversion
Abstract
The recommendation of flushing flows as part of an overall instream flow
regime is becoming more common below water development projects in the western United
States. Typically, objectives of these high magnitude, short duration releases are to
maintain channel capacity and fish habitat quality by preventing riparian encroachment
and flushing accumulated fine sediments from the streambed. While several methods have
been developed for estimating flushing requirements, little evaluation has taken place
regarding the effectiveness of implemented releases. Water development-related
construction activity in the headwaters of south-central Wyoming's Little Snake River
during mid-1984 resulted in substantial deposition of fine and coarse sediment in the
stream channel. Given the potential for severe embeddedness of Colorado cutthroat trout
habitat, our study was conducted to 1) quantify the amount and composition of deposition;
2) monitor quantitative and qualitative changes in the deposition in response to varying
hydrologic, hydraulic and sediment transport characteristics; 3) recommend a flushing
flow regime to mitigate the sediment spill; and 4) evaluate the effectiveness of the
recommended regime. Flushing flow releases were somewhat successful in meeting
management objectives. Preliminary discussion of a new method for flushing flow
analysis, termed "sediment transport input-output modelling", is also presented.
KEY TERMS: Flushing flow, sediment transport, instream flow, fish habitat.
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