WWRC 91-18
Flushing Flow Requirements of Mountain Stream Channels
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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