Assessing instream flows and reservoir operations on an eastern Idaho River

A methodology for assessing reservoir management was applied to the histori cal conflict between winter fish and wildlife flows below Island Park Reser voir on Henrys Fork of the Snake River and the fulfillment of storage water rights. The methodology consists of (1) identifying impacts of flow regula tion, (2) quantifying relationships among variables affecting physical rese rvoir fill, and (3) assessing effects of these discharges on the fulfillmen t of water rights in the context of a larger system of interrelated reservo irs. Winter (storage season) flows are critical to management of fish and w ildlife populations below Island Park Dam, but flow regulation has resulted in decreased winter discharge. Allowable winter flows are a function of in flow, length of storage season, reservoir content at the start of storage s eason, and potential for downstream capture of excess storage season water discharged at Island Park. Modeling results indicate that winter flows in t he range of those recommended for fish and wildlife management are attainab le during average years but not during years when initial reservoir content is low. The methodology was successful in quantifying information useful t o decision makers in a variety of agencies and disciplines and could be app lied to solve water management problems on other regulated river systems.