After enjoying four years of average or slightly above average water supply, plus near record trout populations, we knew a dry year would be around the corner. Remembering the four-year record drought from 2013- 2016, we hoped our next dry year wouldn’t be quite so bad. This is where the good news, bad news comes in.
The bad news is this year is looking a lot like 2016. As of April 1st, precipitation and snow-water equivalent (SWE) for the water year (Oct 1, 2020 – present) were both only 83 percent of average. Now, if you’re thinking in terms of school grades, that doesn’t seem so bad. But when we consider that 100 percent of average is just average, and anything below 100 percent is below average, the true picture of our water situation starts to come into view. The 83 percent of average SWE (snowpack) puts us in the 25th percentile; only seven of the last 33 years had lower snowpack than this year. We see this reflected in reservoir outflow this spring, as flows out of Island Park Reservoir had to be reduced to a near record low of 200 cfs because the reservoir wasn’t filling fast enough to reach full pool before draft would be needed to meet irrigation demand. Also, on several days this spring, streamflow at gages throughout the watershed was in the bottom 10th percentile of all years on record, even in the Henry’s Fork at Rexburg, which has a 111-year record. Natural streamflow has been in the bottom 10th percentile for extended periods this spring in Henrys Lake Outlet, Henry’s Fork at Island Park, and the upper Teton River.
After April, water-supply conditions continued to deteriorate. By early May, much of the Snake River headwaters area, including parts of the Centennial Range west of Island Park, was classified in severe drought (D2). By the middle of the summer, it’s expected that nearly all of the western half of the conterminous U.S. will be in some level of designated drought. In the Henry’s Fork watershed, dry conditions set in last July, so within a few months we will have experienced a full year of below average precipitation. Since July 1, 2020, the watershed has received only 73 percent of average precipitation.
Regarding summer flows, our low snowpack compared to recent years means that delivery from Island Park Reservoir will be needed earlier and at higher flows than we’ve experienced recently. Draft of Island Park Reservoir is expected to begin in early-mid June, compared to July 4 last year, and outflow is likely to increase from about 600 cfs to 1,000 cfs by early July, and stay around 1,500 cfs (or higher) until mid-August. There is a 5 percent chance of outflow near 2,000 cfs depending on summer precipitation, or lack thereof. Unfortunately, we can also expect turbidity downstream of the reservoir to be higher than average most of the summer due to these higher flows. In the lower river, mid-summer, low-flow conditions are expected to begin as early as mid-June and last until September.
Also, to put things into historical context, although the past four years (2017-2020) were much wetter than the previous four (2013-2016), natural streamflow in the Henry’s Fork watershed over the past four years was 111, 104, 99, and 93 percent of the 1978-2020 average, respectively. The four-year total streamflow was only 102 percent of average. Plus, natural streamflow has only been above the 1978-2020 average in seven of the past 21 years. And only one of those seven—2011—was truly a big water year, at 126 percent of average. For comparison, water supply ranged from 112 to 131 percent of average in the five consecutive years from 1982-86 and from 117 to 154 percent of average in 1995-99.
We are in a drought and it’s significant. That’s the bad news. But the good news is, we are still anticipating better fishing conditions than in 2016 thanks to high trout populations, a much cooler spring, and four near average water years coming into 2021. Remember, 2016 followed three dry years. The previous four years of good water supply and very good winter flows means that fish populations are expected to be high watershed-wide. This is true for populations ranging from Cutthroat Trout and Hybrids in Henrys Lake to Kokanee runs in the upper Henry’s Fork to Rainbow Trout downstream of Island Park Reservoir to Brown Trout downstream of St. Anthony. Also, cool temperatures in late winter will keep hatch timing watershed-wide near average if not even a little later between Stone Bridge and St. Anthony, and provide a good chance of favorable fishing conditions on Henrys Lake and mainstem Henry’s Fork into the summer. Normal hatch timing and strong trout populations are certainly a bright spot this year.
We still can’t control the weather, we still don’t have water rights, and we still can’t buy water in the sense that those with water rights can. But, 2016 was a turning point. We took stock of our programs and developed new, even more effective programs. We are significantly more prepared for the next drought year, which is now upon us, and whatever the future might bring.
HFF will continue to work with partners in pioneering efforts to conserve water and benefit trout populations, water quality, and the health of the fishery. From the Farms and Fish Program, precision water management partnerships, infrastructure improvements, predictive modeling and communications, to research and monitoring that has improved our understanding of water quality, and how Island Park Reservoir functions and impacts fisheries. We now have the capacity to save thousands of acre-feet of water in Island Park Reservoir and make noticeable, meaningful improvements to the fishery and fishing experience in years where drought and natural supply have us heading towards much worse conditions.
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