Henry’s Fork, May 10th, 2022
When not blown away or put down by wind, BWO March brown mayflies and early season caddis can offer good top water fishing afternoons on the lower river. sweeping big stone fly nymph patterns along the bottom is also effective.
Excepts from Dr. Rob Van Kirk’s Henry’s Fork drainage snow-water report of May 9th are below.
Headlines:
- Mean temperature yesterday was 14 degrees F below average, and precipitation was heavy enough to move the water-year total up another notch to 85% of average.
- New snow accumulated at all SnoTel stations and was by far heaviest in the northern and eastern portions of the Island Park area.
- Natural flow dropped yesterday in response to colder weather but will increase in a day or two when yesterday’s new snow melts.
- At an outflow of 276 cfs, Island Park Reservoir gained 449 ac-ft yesterday and is still on track to fill on May 20.
Details:
Mean temperature yesterday was 14 degrees F below average, setting a new low for the date in the 1989-2022 record. Precipitation ranged from 0 in valley areas to nearly one inch of water equivalent in areas of the upper Henry’s Fork watershed and was enough to move the water year total up one more percentage point to 85% of average. For the two-day storm, mean temperature was 11 degrees below average, and precipitation totaled 0.42 inch over the watershed. Valley areas received only a few hundredths, while White Elephant and Black Bear SnoTel each received over 1 inch of new snow water equivalent (SWE). Precipitation heavily favored the Island Park area, where driving yesterday was very difficult and people reported 20 inches of new snow in places. Over the whole watershed, net change in SWE for the two-period was a gain of 0.5 inch, compared with an average change for the date of 0.7 inch of melt.
Very light precipitation is possible Thursday and Friday, before what appears to be an extended period of dry weather sets in. Temperatures remain below average into the weekend but will creep back up to average or even above early next week. So, it appears that our five-week run of cool, wet weather will end soon.
Natural stream flow dropped yesterday in response to cold weather, but I expect it to rise again in a few days as yesterday’s new snow melts. With temperatures back up to average by the weekend, I expect stream flow to increase substantially early next week, especially on Fall River. Watershed-total natural flow is 73% of average for the water year so far, the lowest in the 1978-2022 record by quite a margin. Natural flow system-wide has increased enough relative to diversion that the American Falls Reservoir storage right is accruing water again after a week or so without any accrual. Physical storage in the upper Snake River system has been increasing very slightly over the past few days but is still no higher than its peak a few weeks ago at 59% full.
At an average outflow of 276 cfs, Island Park Reservoir gained 449 ac-ft of physical water yesterday t0 reach 97% full. At this rate, the reservoir will fill around May 20. However, some of yesterday’s gain came from direct precipitation on the reservoir surface, and with only very light precipitation forecast for the next week, fill rate will likely be a little lower than yesterday’s. On the other hand, stream inflow will get a small boost as fresh snow from the past few days melts.
Rob Van Kirk, Ph.D.
Senior Scientist
P.O. Box 550
Ashton, ID 83420
208-881-3407 CELL
208-652-3568 FAX