Henry’s Fork, June 21st, 2022
Yesterday was a tough one along the upper river. Thunderstorms with rain and hail, air temperatures staying in the mid-forties in Degrees. F,, and hypothermic conditions for anyone getting wet. Today, the first partial day of summer, is different with good weather, air temperatures warming to seasonal levels, and aquatic insect hatches ( PMDs, some drakes, a few golden stones, March browns, and flavs, plus reliable PM caddis) becoming more active. Flow out of Henry’s Lake has been raised to around 200 cfs, which will flush some fish into the river below to benefit fishing in the Flat Ranch section. For a while wet flies ( streamers, bugger types, Henry’s Lake patterns) will be best bets here. Yesterday’s foul weather also hit the lower river but better weather now prevails to benefit hatches. Dry- dropper methods seem to work well anywhere on the river. Angler numbers are most numerous on the river from Ashton to Chester.and some social situations, mostly from boating anglers, are happening. Please be considerate.
Dr. Rob Van Kirk’s latest report on snow-water conditions in the Henry’s Fork drainage follows.
- Another cold, wet day moved water-year precipitation up a percentage point to 92% of average and added a little new snow at high elevations.
- Rain and cooler weather has kept both diversion and natural flow constant over the past few days.
- Island Park Reservoir stayed constant yesterday at 99.1% full, and outflow will be decreased to 600 cfs today.
- Drier, warmer weather is forecast for the rest of the week, albeit not quite as dry or warm as predicted a few days ago.
Details:
Mean temperature yesterday was 12 degrees F below average, and moderate precipitation was widespread across the watershed. Precipitation favored the Teton subwatershed, where Grand Targhee picked up 0.7 inch. Amounts in the Fall River and upper Henry’s Fork subwatersheds were fairly uniform at 0.1-0.3 inch. Valley areas received as much as 0.2 inch. Watershed average precipitation since the beginning of the water year moved back up to 92% of average. White Elephant and Lewis Lake Divide lead in that department, each at 98% of average. Because of cold temperatures, yesterday’s precipitation fell as snow at the highest elevations, keeping snow water equivalent (SWE) at 11% of the season’s peak. Over 13 inches of SWE remains yet to melt at Grand Targhee.
Much warmer and drier conditions are expected for the remainder of the week, but forecast temperatures are now a little lower than expected a few days ago. The current forecast shows temperatures staying near average for this time of year. In addition, monsoonal moisture is expected to creep up into our area from the south, bringing a chance of showers back into the forecast by Friday. The monsoonal (seasonal southerly moisture feed from the Gulf of Mexico) pattern always favors the southern end of the watershed, which has received less precipitation than the northern end over the past month. Water-year precipitation in the Teton subwatershed is 88% of average, compared with 95% of average in each of Fall River and upper Henry’s Fork. The monsoonal pattern also brings higher temperatures, especially at night, and forecasts at the 1-3 week time scale hint at above-average temperatures and near-average precipitation.
As far as drought recovery goes, all indicators point to strong short-term improvement and incremental long-term improvement. The one-year accumulated moisture availability index has increased by nearly 5 inches over the past month and is now less than 2 inches below average after spending most of the winter and early spring down around 6 inches below average. That index bottomed out at 11 inches below average last July. Three-year average precipitation has improved slightly over the past month, moving from 12.5% below average to 10% below average.
Although this spring has certainly been wetter and colder than average, it is nowhere near the coldest and wettest, even in recent decades. Mean temperature since April 1 is 3 degrees F below average, but that ranks only in the coldest 15% of years since 1989. The springs of 2010, 1998, and 2011 were all colder than this year’s. The spring of 2011 averaged 1.4 degrees colder than what we have experienced this year. Similarly, precipitation since April 1 this year is 22% above average, but that ranks only around the 70th percentile of years since 1989. The same three years mentioned above—2010, 2011, and 1998—were all wetter than this spring and some by a large margin. April 1 – June 20 precipitation in 2011 totaled 14.9 inches, compared with 10.6 inches this year.
Natural streamflow has remained fairly constant for the past three days, as rain has offset decreased snowmelt due to colder temperatures. Natural flow yesterday was 95% of average: 87% in upper Henry’s Fork, 96% in Fall River, and 101% in Teton River. Diversion has also stayed fairly constant and was 91% of average. Natural flow is 3300 cfs greater than demand, which is a little above average for the date, something that was extremely unlikely based on conditions in early April. Recall that last year at this time, Island Park Reservoir had been drafting for 10 days and was already less than 94% full. I expect diversion to increase back up to average by the end of the week, while natural flow will drop relatively slowly. Warmer temperatures will increase melt of remaining high-elevation snow, which should produce one more peak in Teton River streamflow. Fall River and upper Henry’s Fork will drop relatively slowly. I expect the positive effects of delayed snowmelt and recent rain on streamflow to persist into next week, delaying need for reservoir draft until then.
At an outflow of 800 cfs, Island Park Reservoir was constant yesterday at 99.1% full. Now that heavy precipitation appears to be over for the foreseeable future, outflow will be reduced today to 600 cfs to slowly bring the reservoir back to full pool until draft is needed to meet within-watershed irrigation demand. As mentioned above, that is not likely until at least next week, so expect relatively low and stable flow downstream of Island Park Dam during that time period. For reference, the long-term average outflow from Island Park Reservoir right now is 1,000 cfs. Lower outflow right now is due to an unusual combination of low natural inflow due to long-term drought, and no need for reservoir draft to meet irrigation demand due to a good short-term water supply.
The upper Snake River reservoir system continues to gain physical water and is 63% full, compared with its previous peak of 59% full back in April. The reservoir system has gained 296,000 ac-ft over the past few weeks after dropping slowly for much of April and May. Net gain since the mid-April peak is around 165,000 ac-ft. Outflow from Palisades Reservoir is around 9210 cfs, which is 62% of average for the date.
Rob Van Kirk, Ph.D.
Senior Scientist
P.O. Box 550
Ashton, ID 83420
208-881-3407 CELL
208-652-3568 FAX