This is the 5th consecutive year I have predicted upcoming spring and summer streamflow conditions based on April-1 data, including streamflow for the winter just ended, Island Park Reservoir volume, snowpack, and air temperature. Streamflow during the upcoming spring and summer, along with Island Park Reservoir carryover, will certainly be the lowest since 2016. Thus, I made numerous improvements to my predictive model and greatly increased both breadth and depth of model outputs in order to provide the most detailed analysis possible for all watershed stakeholders. Including all of this information in blog format would serve neither me nor you well, so I'm including only bulleted summaries in the blog and putting all of the details, including graphs and tables, in a downloadable report.
In January of 2015, I started a new run of "fish of the month," a tradition I started years ago with long-time friend Tom Grimes, who is a guide at Henry's Fork Anglers. The idea is to catch a wild trout or whitefish every month of the year in our local waters, the streams and lakes of Montana, Wyoming, and Idaho in the Yellowstone region. My previous record was 55 months, from July 2004 to January 2009. I broke that record in 2019 and successfully completed month 72 during the pandemic year of 2020. Without travel and other diversions, I fished more in 2020 than I have in many years, and it was a great year to be on our local waters.
We are currently undertaking a large project to make all reports and papers produced, co-authored or funded by HFF and its staff--dating back the 1980s--available online to the public. Until we are able to finish that project, numerous requests for our recent work have prompted me to post some of it here, organized by the four major subject areas of our science and technology work: hydrology and water management, water quality and aquatic ecology, fisheries biology, and social science and economics.
Temperatures were below average and highly variable during the spring but above average for most of August and September, reflecting air temperature. Daily means were below stressful levels for trout except for a few days in late July and early August. Maximum instantaneous temperatures recorded were 73.5 degrees at Pinehaven and 75.7 degrees at St. Anthony, both on August 2.
Dissolved oxygen followed typical seasonal patterns, and daily means were well above stressful levels for trout, except at Island Park Dam, where late-summer values were very close to but still higher than the power plant’s requirement.
Phytoplankton and cyanobacteria production also followed typical seasonal patterns and was highest during the spring when sunlight and nutrients were first available. As expected, production was highest between Island Park Dam and Pinehaven.
Turbidity also followed seasonal patterns and was generally highest during spring runoff and following a few heavy rain events. Turbidity between Island Park Dam and Pinehaven was generally higher than the 2014-2019 average and the highest since 2016, as reservoir outflow and draft were higher this year than they have been since 2016. High turbidity during September is the longest-duration, highest-magnitude event we have recorded and was probably the highest since the 1992 sediment event.
Climate: Record-breaking cold in October, a generally dry winter, a 9th-inning rally that brought snowpack up to average just in time for it to melt, and a very dry summer. Mean temperature for water year 2020 was equal to the 1989-2019 average, but high variability in springtime temperatures melted snow rapidly during warm periods around May 1 and June 1. Water-year precipitation ended up at 90% of average, but July-September precipitation was only 52% of average.
Natural streamflow: 92% of the 1978-2019 average: 88% in upper Henry’s, 94% in Fall River, and 96% in Teton River. Streamflow dropped rapidly in all three subwatersheds after snowmelt peaks in early May and early June and after late June rain. Since 2009, natural flow in the upper Henry’s Fork has exceeded the 1930-2020 average in only three years: 2011, 2012, and 2019.
Irrigation Management: Draft of Island Park Reservoir started on July 4 (11 days later than average) and ended on September 9 (6 days earlier than average). Delivery of water to the Teton River through the Crosscut Canal started on July 8 and ended on September 24. Over the period of draft, streamflow in the Henry’s Fork downstream of all diversions averaged 469 cfs, compared with this year’s target of 350 cfs. Using the target downstream of all diversions rather than the St. Anthony gage saved 1,226 ac-ft in Island Park Reservoir while increasing streamflow downstream of diversions by 5.4%.
Island Park Reservoir: Winter flow during water year 2020 was 532 cfs, compared with 560 cfs in 2019 and a 1978-2019 average of 356 cfs. The reservoir reached a minimum of 54% full but ended the water year 56% full, compared with 46% full on average. Outflow during the upcoming winter is expected to be around 400 cfs.
Predictive Model Performance: Based on April 1 conditions, predictions were within 10% of actual values for hydrologic and water-management parameters that depended on spring and early summer conditions. However, the model under-predicted need for Island Park Reservoir draft by 20-30%, depending on the particular metric. Given observed flow in the lower Henry’s Fork, the model over-predicted Island Park Reservoir carryover by 27.2% (97,135 ac-ft vs. 76,366 ac-ft).
Percent mayflies, stoneflies, and caddisflies in invertebrate samples at Osborne Bridge averaged 57% in 2019, compared with 41% during and after the recent drought and 17% in 1993.
A high springtime freshet and low irrigation-season outflow from Island Park Reservoir, as occurred in 2018-2020, accelerates transport of sediment out of the Harriman reach, providing an explanation for improvement in hatches since 2017.
In the long run, net sediment transport out of the river between Island Park Dam and Pinehaven is around 1,600 tons per year. At that rate, 45-90% of the sediment deposited during the 1992 event has been removed to date, providing an explanation for substantial improvement since 1993.
HFF will continue to work with the Henry’s Fork Drought Management Planning Committee to implement the high springtime/low summertime flow regime that appears to be beneficial for aquatic insects.
Weather during the April-June spring period is a major factor determining summer water supply.
Although April-June temperature was average and 1.6 degrees below expectation, high variability resulted in three very warm periods that melted this year's average snowpack 10 days earlier than average.
The second half of June was very cold and wet, compensating for the early snowmelt and delaying need for Island Park Reservoir draft by one week relative to average.
As of July 14, 10 days into the draft period, Island Park Reservoir is still 91% full, compared with 83% full on average, setting up a fourth consecutive year of above-average carryover and high winter flows.