Electrical Power for Valdez and the Copper River Basin-1981

alteration of the temperature regime through mixing base flows inAllison Creek with project flows. An extension of the one yearrecording period may be necessary.As additional information is available for a thorough assessment ofimpacts due to potential changes in flow and temperature regimes,other alternatives for the discharge to Port Valdez may be acceptableor recommended. For example: (1) operation of the project only forbase load pmver production would eliminate the radical flow variationsassociated wi th a peaking facility, (2) alterations in the dischargeof flow· from the tailrace in response to power demand could be doneincrementally by a specified discharge in a given time period (ex.10 cfs/hour), (3) discharge of excess flows directly into PortValdez could be done via .a flume or manmade channel and dischargedsub tidally only from June through September.Recent information on spawning populations in Allison Creek is alsolacking. Beginning in 1980, escapement counts should be taken atleast once a month in July, August, and September of each year.These surveys should continue through the planning, construction,and operation phase of the project to allow assessment of projectimpacts upon salmon populations.A dual tailrace design as proposed for the lower powerhouse alternativeshould be included in plans for the upper powerhouse alternativeas well. The impacts associated with the potentfal changesto flow and temperature regimes described previously would occur ateither powerhouse alternative unless appropriate mitigation featuresare incorporated into project design. In fact, construction andoperation of the upper powerhouse with the dual tailrace feature isfavored slightly because stabilizing the flows in that stream reachbetween the lower and upper site would benefit fish resources in agreater portion of their habitat.To prevent scouring and downstream sedimentation, energy dissipatorsshould be installed in both the tailrace and outlet of the 6 inchdiversion pipe to Allison Creek. Design of the dissipators shouldinsure that the velocity of the discharge into Allison Creek willnot exceed the optimum velocity of the natural stream for fishmaintenance.The timing of construction will be of considerable importance inminimizing impacts to fish. The work should be done to avoid criticalbiological life stages. Disturbance of the water quality orstreambed morphology while eggs are incubating or fry are emergingcan result in direct mortality through suffocation by burial orphysical damage. Disturbance ,",hile adults are present can disruptor prevent spawning and l:Lmit production of future generations. Thetiming of any inwater construction activity or construction on th.ebanks of Allison Creek should he coordinated with the FWS, NationalMarine Fisheries Service (ID-IFS) ,and the ADF&G to avoid unnecessaryimpact on the salmon population. Also, because highest densities ofpopulations of spawning salmon occur in odd years, major constructionaffecting flows should be done on even years.