Water-year 2018 ended up close to average, at 102% of average precipitation and 105% of average streamflow. Upper Henry's Fork subwatershed was below average in precipitation and streamflow, vs. above-average values in Fall and Teton rivers. However, Upper Henry's Fork water supply improved from 70% of average in 2016 to 91% in 2017 and 94% in 2018, indicating recovery of deep aquifers from 2013-2016 drought. May and June rain compensated for early snowmelt and resulted in below-average irrigation diversion. Despite very dry conditions and below-average streamflow during July, August and September, Island Park Reservoir ended the water year at 73% full, compared with 43% full on average, thanks to careful and precise water management. This blog is even longer and more detailed than my usual long, detailed blogs, but there was a lot to learn from water-year 2018. This blog is a slightly edited compilation of the four daily water reports I wrote during the first week of the new water year. The four reports appear in four different sections in this blog: Climate Natural flow and diversion Streamflow and water management in lower watershed Streamflow and reservoir management in upper watershed Tables and graphs appear at the end of
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Temperatures and rainfall were near average in August. Natural streamflow and diversion were both around 90% of average for the month. As of September 10, Island Park Reservoir is 73% full, compared with 46% full on average. August Weather Near Average August provided a little reprieve in the middle of what has otherwise been a warm, dry summer. Temperatures were warm during the beginning of the month and cool at the end, resulting in a monthly mean about 1 degree F below average. Graph of watershed mean temperature, relative to long-term average. A very deep autumn-like trough of low pressure brought substantial rain and near-freezing temperature to the whole watershed on August 27, adding to lighter showers that fell at scattered locations earlier in the month. The result was near-average precipitation at most stations for the month. Precipitation favored Teton Valley and the Fall River watershed; the upper Henry's Fork and most valley locations recieved below-average precipitation. Ashton was an exception, receiving nearly one inch for the month. Temperatures during the late-August storm were cold enough that snow fell above 7,000 feet and stayed on the ground for a few days at the highest elevations. Water-year precipitation ended the month
The month of July was a little warmer than average but very dry. Natural flow and diversion were both around 90% of average for most of the month. Moisture from May and June rain has kept total diversion for the season about 100,000 ac-ft below average. As of August 3, Island Park Reservoir is still 82% full, compared with an average of 70% full at this time of year. Above-average Temperatures Continue through July Temperature over the month of July was 2 degrees F above average in the Henry’s Fork watershed, continuing the pattern of above-average temperatures. Mean April-June temperature was also 2 degrees above average. Cool nights throughout July offset very warm afternoon highs to keep daily means close to average. As of August 2, Pocatello had set a new record of 30 consecutive days with high temperature at or above 90 degrees. According to the National Weather Service, the current heat wave in southeastern Idaho is record-setting in persistence and duration but not extremes. Graph above sows temperature relative to average. Note consistently above-average temperature since early July. Spring Rain Keeps Irrigation Demand Low, Despite Dry July After abundant rain in May and June, July was very dry.
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. Recall my blog from June 8 where we quantified the amount of sediment that was flushed out of each reach of the Henry’s Fork, from Island Park Dam to Ashton Dam, during the major runoff events we experienced during April and May. These spring rains brought the highest spring runoff flows 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. We’ve seen much higher summertime flows than these in the last 7 years, but what was significant about this spring’s
Prepared by Melissa Muradian and Jack McLaren What happened? 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 their 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. How does this affect the river’s water quality? Temperature: Water delivered through the dam gates is from the bottom of the reservoir and as long as thermal stratification remains in the reservoir, the gates will deliver cooler water than the power plant. However, the response we saw in temperature was about 1 ˚C (approx. 2 ˚F) at our IP East sonde. We saw no discernable difference in water temperature by the time that water made it to Pinehaven. Thus the cooler water temps that result from total flow coming out of the gates only last until some point upstream of Pinehaven, and most likely only last until part-way through Box Canyon. Dissolved Oxygen (DO): DO was hovering at
Watershed temperature over the month of June was 1 degree F above average. June precipitation was 135% of average, but average natural streamflow at Island Park for the month of June was only 92% of average. As of July 6, watershed-wide natural flow has dropped to 89% of average. Delivery of Island Park Reservoir storage water began on July 3. Watershed temperature over the April-June period was 2 degrees F above average, continuing the 40-year trend of increasing springtime temperature. April-June precipitation was 134% of average. Precipitation 135% of average in June After a warm May, very little snowpack remained across the Henry's Fork watershed, leaving rain as the only mechanism to keep streamflow high. Fortunately, that happened, as watershed-total precipitation was 135% of average during June. Monthly climate summary for June, 2018 is given in this table. Moreover, precipitation in the agricultural areas of the watershed was 183% of average in June. In fact, the valleys received more precipitation during June than the high-elevation areas of the Teton subwatershed. As a result, irrigation demand was lower than average over the month. Total diversion in the Henry's Fork watershed so far in irrigation year 2018, compared with average and 2017.
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. These surveys provide valuable information on abundance, age-class structure, fish size, and species composition within the fishery. IDFG recently published the results in their annual brochure and this blog will highlight and explain some of those results. Largest Rainbow Trout sampled during the 2018 population survey in th Box Canyon reach. Highlights Box Canyon Abundance estimate for fish greater than 6 inches in length: 2,796 Rainbow Trout per mile 2017 abundance estimate: 2,913 Rainbow Trout per mile Average length 10.1 inches 2017 average length: 11.5 inches Max length: 22 inches Abundance estimate of age-2 class fish was higher than predicted for a second year in a row (see below for more details) Chester to Fun Farm 1,225 trout per mile (727 Rainbow Trout and 498 Brown Trout) Average length Brown Trout: 15 inches Rainbow Trout: 14 inches Max length Brown Trout: 23 inches Rainbow Trout: 20.5 inches Vernon to Chester 1,612 trout per mile (1,071 Rainbow Trout and
Henry's Fork Days XXXIV - Membership Meeting At the annual membership meeting as part of our Henry's Fork Days celebration, HFF staff gave presentations on topics ranging from science and technology, outreach and education, and communications to stewardship and collaboration. Thank you to everyone who joined us for breakfast and presentations this year! We had a wonderful turnout and many great questions and points of discussion. The membership meeting was recorded and informational clips from each talk are forthcoming in a new HFF "educational series". In the meantime, we'd like to provide the slides from each presentation for those who were unable to attend or would like to take a second look. As always, please don't hesitate to give us a call or send us an email with any questions. Below is a complete list of presentations. Agenda and Presentation Slides Hydrology and water supply - Rob Van Kirk Market-based approaches to water conservation - Bryce Contor Status of Henry's Fork wild trout populations - Bryce Oldemeyer Water quality in the Henry's Fork and tributaries - Melissa Muradian and Jack McLaren Temporal and spatial trends in aquatic invertebrates, Youth Education programs, and Economic value of angling and river recreation -
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 spawning 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 above the hydroelectric facility. This data allows us to quantify run size, run timing, number of spawners, number of return spawners, and other valuable information needed to monitor and understand the Henry’s Fork Rainbow Trout population. Last week (June 11, 2018) we opened the Buffalo River fish trap and concluded our monitoring of the 2018 Rainbow Trout spawning migration. Below is a quick summary of the 2018 data. Buffalo River and Buffalo River fish ladder looking down stream from the hydroelectric facility. Notable Numbers 905 fish captured (February 16 through June 11) Rainbow Trout 112 spawning sized fish (greater than 12 inches) 15.8 inches - median size 23 inches - largest fish 576 juvenile fish
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. Unfortunately, the second period coincided with Memorial Day weekend—a reality that put a murky, cloudy, and disappointing damper on dry-fly fishing. So, why did this high flow and high turbidity event coincide with the Memorial Day weekend? Island Park (IP) Reservoir was already full when the watershed received over an inch of rain on May 23–24. The resulting increase in inflow was necessarily passed through IP Reservoir. For better or worse, IP Reservoir was not designed to discharge over the dam. Thus managers lack even the operational and mechanical option to pass the necessary flow over the dam. (Also this would flood the road over the top of the dam.) Bottom release does deliver more turbid water, but also much cooler water than from surface release. As many