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.
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.
Earlier in November, the Henry’s Fork Watershed Council hosted a satellite location of Safeguarding Idaho’s Economy in a Changing Climate, a two-day a summit held November 16th and 17th. Invited speakers represented a broad sample of private businesses, public agencies, tribes, and NGOs in Idaho from Simplot and HP to the EPA and Idaho Dept. of Lands to Trout Unlimited. See the complete list of speakers here.
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.
This fall HFF initiated a new water-quality monitoring study designed to answer the following question: How far downstream of Island Park Dam (IP Dam) do high turbidity levels persist? I introduced this study in my blog post made on September 9, 2016. Please find details of the background and motivation for this study, as well as a map of the sampling sites, in the previous blog post. The current blog post summarizes what we found during our 2016 data collection season.
On Wednesday, September 7th HFF initiated an exciting addition to our water-sampling program designed to answer the following question: How far downstream of Island Park dam do high turbidity levels persist? These results will be important for predicting which reaches of the river will be most affected, and to what extent, during periods of high sediment delivery from Island Park reservoir.
Brief background: There are three genera and many species of cyanobacteria; only a handful of these produce a toxin harmful to mammals. Blue-green algae (or BGA) is the common name for cyanobacteria, which is a single-celled bacteria that photosynthesizes and is neither an algae nor a plant. All plants, algae, and cyanobacteria use chlorophyll A, a pigment used to capture light for photosynthesis. In addition to chlorophyll A, freshwater cyanobacteria also utilize phycocyanin for photosynthesis, which is a bluish pigment that gives them their color.
During our regular water-quality monitoring last week, we observed that recent turbidity levels are above average in all reaches of the river, except below Island Park Dam, and the highest observed turbidity last week is higher than any observed turbidity below Island Park Dam since we started regular water-quality monitoring there in the summer of 2013. The highest turbidity levels we observed last week were in the Warm River-to-Ashton Reservoir reach (18 NTU) and at Pinehaven (9 NTU).