Abstract Maintenance of instream fisheries habitat downstream of water development projects has long been recognized as environmentally desirable. More recently, concern has focused on the need for and the determination of flushing or channel maintenance flow releases. These high magnitude, short duration streamflow releases may mimic the natural runoff hydrograph and maintain channel conveyance capacity and spawning and rearing habitats for fish. Increasing interest in transbasin water diversion in the Central Rocky Mountain Region has focused attention on the flushing flow requirements of steep, rough mountain channels. My objectives were to describe the historic response of such channel types to flow depletion, evaluate sediment dynamics within discrete habitat types, and develop criteria for flushing flow determination.
A comparison of hydraulic geometry and channel morphology above and below diversion structures on different channel types indicated that low gradient (< 1.5 percent) reaches responded to flow depletion by significantly reducing their depth, area and conveyance capacity. Regression equations were developed to estimate these responses. Steeper gradient reaches (> 1.5 percent) maintained channel dimensions over extended time periods and are not as critical from the standpoint of flushing flow.
Investigations of bed material and sediment transport characteristics at nine study reaches on four mountain streams indicated very low gradient pools (< 0.30 percent) are the critical habitat type for flushing flow studies. Within these habitats, analysis of bedload transport relations indicated that a threshold of flushing is reached at flows of about 12 times the average annual discharge, above which net pool scour occurs. Flushing flows should exceed this threshold for a duration dependent on the transport differential entering and exiting the pools during intermediate and low flow events. A multiple regression model incorporating stream power, channel shape, bed material characteristics, and basin credibility was developed to aid in predicting this bedload transport differential. To maintain spawning gravel recruitment through these steep, rough channels, a flow event in the range of the peak flood having a five-year recurrence interval appears to be required.
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