Henry’s Fork, November 2nd, 2021
Presenting streamer patterns in deeper waters throughout the river is becoming very effective.
Excerpts from Dr. Rob Van Kirk’s Monday analysis of water conditions in the Henry’s fork drainage are below
Headlines:
- The first month of water year 2022 was a little on the warm side but wet, with 140% of average precipitation.
- Natural flow has dropped steadily since last Wednesday’s rain-driven peak and is 80% of average.
- At a mean outflow of 116 cfs, Island Park Reservoir has gained 535 ac-ft/day over the past three days and is 57% full, compared with 54% full on average.
Details:
Mean temperature for the month of October was 1 degree F above average, due primarily to consistently warm daily minimum temperatures over the last 12 days. The warm overnight lows have accompanied generally high humidity (for around here, anyway) and persistent precipitation, at least in some locations. For the month as a whole, precipitation was 140% of average. By subwatershed, monthly totals relative to average were 149% in Fall River headwaters, 144% in upper Henry’s, 136% in the Teton headwaters, and 120% in the valleys. The only station with below-average precipitation during the month was Ashton, coming in at 93% of average. Warm temperatures prevented much snow accumulation, despite above-average precipitation. The only two stations with appreciable snow water equivalent on the ground right now are the two highest in elevation: Grand Targhee at 81% of average and Black Bear at 67% of average.
Over the first month of the new water year, the three-year average watershed precipitation increased by almost 1 inch but remains 1 inch below average. One-year accumulated moisture availability in the agricultural areas increased 2.5 inches but remains 4.5 inches below average. New outlooks for the month of November issued yesterday give our area better-than even odds of continued warm temperatures, above-average precipitation, and slow improvement in drought conditions. In the short term, light to moderate precipitation is expected tonight/tomorrow morning and again Thursday night/Friday morning. Forecast confidence is low after that.
Watershed-total natural flow has dropped steadily since Wednesday’s rain-drive peak and was 80% of average yesterday. Accumulated natural flow for the water year so far is 77% of average, compared with 76% of average for the entirety of water year 2021 and only 69% of average for April-September. Diversion stayed fairly constant near the long-term average over the weekend but will decrease today as canals shut down after the end of administrative irrigation season 2021. Based on data available to date, diversion was 92% of average for irrigation year 2021. That figure may change a little once Water District 1 adds data from pumps that are not measured in real time and adjusts data for rating-curve shifts. The process of approving all diversion and water-rights accounting data for the previous irrigation year is usually completed in early March, but at the watershed scale, the approved data are usually not much different than data available at this point.
We measured outflow from Island Park Reservoir at 122 cfs on Friday, right on the trajectory determined by measurements over the previous two weeks. Mean outflow since Friday was 116 cfs, at which the reservoir gained 535 ac-ft/ day. The reservoir is 57% full, compared with 54% full on average. Total storage in three reservoirs in the Henry’s Fork watershed is 68% of capacity, around 630 ac-ft above average for the date.
Rob Van Kirk, Ph.D.
Senior Scientist
P.O. Box 550
Ashton, ID 83420
208-652-3567 OFFICE
208-881-3407 CELL
208-652-3568 FAX