Headlines: Water temperatures through the Ranch have been on the warm side. Temperature has climbed above 71˚F for a couple of hours each day over the last week. However, daily maximum temperatures have not been deadly for rainbow trout, and only a few daily averages have been at the lowest levels that would stress fish. This is the same pattern we saw in both 2015 and 2016, when streamflows were about 800 cfs higher than current flows. Thus, releasing additional water from IP dam does not result in significantly cooler summertime water temperatures, but we do know that increasing flow out of Island Park Dam decreases water clarity and will reduce winter flows, which reduces survival of juvenile trout next winter and hence recruitment of 2-year old fish into the population in 2019.
Island Park Reservoir, Grassy Lake and Henrys Lake have all filled.
Natural streamflow in the watershed is around 9,500 cfs and at its final peak for the season.
Total watershed-wide irrigation diversion is around 3,000 cfs, right at average for this time of year.
Outflow from Island Park Reservoir is being set to keep the reservoir full. Only small changes are anticipated over the next week, and outflows should remain around 950 cfs plus or minus 150 cfs, depending on rain.
Delivery of water from Island Park Reservoir in excess of inflow is not expected to be needed until early July.
Every year the Idaho Department of Fish and Game (IDFG) conducts multi-pass electrofishing surveys on various river reaches in the Upper Snake River region. These surveys provide valuable information on abundance, age-class structure, fish size, and species composition within the fishery. Most recently, IDFG finished its annual survey in the Box Canyon reach of the Henry’s Fork. This post will highlight and explain some of the results of that survey.
Two weeks ago salmonflies started showing up on the lower Henry’s Fork. Once the word got out, fisherman flooded from near and far for the opportunity to catch big fish on big dries. More than once I heard the classic “you don’t need to bring a boat, you can walk down the river on all the other ones” line. While the river was busy, it wasn’t that bad. After seeing lines of cars and trailers at Ora, Vernon, Chester, etc.
A three-day winter storm in the middle of the week set record cold temperatures for the date and greatly slowed snowmelt, saving that snow to melt later in the spring and summer. Elevations above 6,500 feet gained up to inch of new snow-water-equivalent this week, including the White Elephant site on the side of Mt. Sawtelle. As a result, current snowpack at the subwatershed scale is currently between 122% and 147% of median for the date. Although streamflow at most locations is below average this morning, warm temperatures forecast for the next 10 days will resume melt, leading to season-peak streamflows into Henry's Lake and in Fall River and Teton River around June 1. Henry's Lake and Island Park Reservoir will both fill around that time.