Main message: The great spring sediment flush we observed earlier this year not only impacted fishing during Memorial Day weekend—by contributing to high flows and turbidity—but has continued to impact the fishing experience from Box Canyon through Riverside this season in two plainly obvious ways: depressed hatches and higher than average amounts of uprooted floating plant material. In addition, the rainy and cloudy weather that contributed to the spring flush may also have contributed to later hatch timing of some species earlier this summer.
Due to warm water temperatures and low dissolved oxygen in the water at the intake to the Island Park hydroelectric plant, the plant’s aerators have been unable to maintain dissolved oxygen in power plant outflow above the level required by Fall River Electric's operating license. Thus, the plant shut down yesterday morning (July 15), and all outflow from Island Park Reservoir was shifted to the bottom-withdrawal gates on the west side of the dam.
Every year the Idaho Department of Fish and Game (IDFG) conducts fish population surveys on various river reaches in the Upper Snake River region. In the spring of 2018, surveys were conducted on the Chester to Fun Farm, Vernon to Chester, and Box Canyon, reaches of the Henry’s Fork River.
Henry’s Fork Rainbow Trout migrating to spawn in the Buffalo River have to pass through the fish ladder at the Buffalo River hydroelectric facility to access upstream habitat. At the end of the fish ladder we, HFF, operate a fish trap from early February through the middle of June. Three times a week we check the Buffalo River fish trap and collect data on species, length, sex, and life histories via passive integrated transponders (PIT) tags if one is present, before passing the fish upstream of the hydroelectric facility.
The main message: Spring of 2018 brought the highest runoff event in 7 years to the upper Henry’s Fork watershed! Our network of water quality monitors showed that these flows were strong enough to provide a major springtime sediment flush--a natural rhythm of our local hydrology that provides significant benefit to trout and aquatic insect habitat. These favorably high natural flows came in two periods during April and May of this year.
May 22-24 precipitation totals were over 1 inch at most locations; water-year precipitation jumped from 105% of average to 109%.
Snowmelt continues at average rates, and SWE remains at 102% of average.
Watershed-total natural flow has increased to its highest level so far this year and higher than last year’s peak.
Inflow to Island Park Reservoir is around 1,600 cfs, and outflow is currently just a hair over inflow, allowing the reservoir to drop very slowly. Current reservoir content is a little higher than full pool.