Henry’s Fork, May 16th, 2022
Giant stone fly nymphs are concentrating along banks and shallows of the lower river, and a few adults may be flying to announce beginning of the main event. Already numerous boats are on the water getting started to enjoy the famed adult stone fly event, and fish responding to ongoing may fly and caddis activity.
Here are a few things of note. The repaving of U.S.Highway 20 between Ashton and the Fall River bridge should be complete in a few days making for no flaggers on one lane sections and the long awaited seven mile smooth ride. Fall River is in run-off mode impacting fishing below Chester Dam. Mosquitos are out in forested areas
Dr.Rob Van Kirk’s Henry’s Fork drainage snow-water report of yesterday is below.
Headlines:
- Yesterday was the warmest of the spring so far, and forecast showers did not materialize.
- Although cooler weather is forecast for the end of the week, it appears that our 6-week run of above-average precipitation has come to an end.
- Natural flow is increasing rapidly but is only 47% of average for the date at 71% of average for the water year.
- Island Park Reservoir gained over 400 ac-ft yesterday and is expected to fill next week.
Details:
Daily mean and daily maximum temperatures yesterday were both the warmest day since October 5. Showers did not materialize as forecast yesterday, but the Centennial Range received some measurable precipitation. The water-year total dropped a percentage point to 85% of average. Snowmelt was above average due to warm temperatures, and snow water equivalent at the three low-elevation stations is essentially zeroed out for the year. Relative to average, a fair amount of snow remains at the mid- and high-elevation stations in the northeastern corner of the watershed, where precipitation over the past few weeks has been heaviest.
Although cooler temperatures are still expected later in the week, forecasts have trended much drier over the past several days, and it looks like our 6-week run of above-average precipitation has ended. Only light precipitation in the northern-most areas of the watershed is expected over the next week, and long-range forecasts do not indicate any strong precipitation trend one way or the other.
Since April 6, the watershed has received 5.0 inches of precipitation, compared with 4.5 inches on average. Although this does not represent a large exceedance of average, the combination of moderate precipitation and very cold temperatures improved moisture availability in the agricultural regions by 2 inches. However, even with that improvement, one-year accumulated moisture availability is 4 inches below average. Three-year average precipitation improved slightly over the past six weeks but is still over 10% below average.
As anticipated, warm temperatures have increased natural flow watershed-wide but most notably in Fall River. However, natural flow is still exceptionally low—only 47% of average for the date and 71% of the 1978-2021 average for the water year to date. This is the lowest in the 1978-present record by a long shot—the next lowest year is 2002, at 79% of average. In the record of natural flow at Ashton (upper Henry’s Fork subwatershed) that goes back to 1930, accumulated natural flow for the water year to date has dropped to 4th lowest, behind 1935, 1934, and 1931.
Also as anticipated, diversion continues to increase rapidly and is now 81% of average. Increase in stream flow in Teton River yesterday exceeded increase in diversion there, increasing flow in the Teton River from 25 cfs in each of the North and South forks on Sunday to about 45 cfs in each yesterday. I expect both stream flow and diversion to continue to increase through Friday, but natural flow increase will outweigh increase in diversion during that time. Fall River should reach its second and probably highest peak for the year late this week before cooler weather slows snowmelt again. The Teton River will probably reach a higher peak in two weeks or so. At the system-wide level, the reservoir system has been dropping for nearly a week now and is 58% full. The American Falls storage right has been in priority for only a few short periods over the past two weeks, meaning that paper accrual of storage system-wide has been minimal.
At an average outflow of 212 cfs, Island Park Reservoir gained 414 ac-ft yesterday and is 98% full. Stream inflow increased a little yesterday in response to snowmelt, but evaporation loss reduced net inflow by 26 cfs (6% of stream inflow). The reservoir will continue to gain around 350-400 ac-ft/day, which will put it at full pool early next week.