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.