Henry’s Fork, January 5th, 2021

January 5, 2021

Many fly-fishers are interested in how winter’s progress will impact fishing quality in the Henry’s Fork and its drainage during the upcoming season. Dr. Rob Van Kirk, Henry’s Fork Foundation Senior Scientist, compiles and offers a water supply report sequentially throughout the year for the river and drainage. We at Jimmy’s believe that placing on our web site Rob’s presentation and analysis of data impacting Henry’s Fork water supply provides to a wider range of interested fly fishers more information with which to judge quality in its upcoming season. We therefore will provide Rob’s updates here during the winter season and into the irrigation season. The first of these is below.

Henry’s Fork water supply, Jan 05 2021

Headlines:

Another warm, wet day increased water-year precipitation to 82% of average and SWE to 85% of average.
At an outflow of 329 cfs, Island Park Reservoir gained 443 ac-ft yesterday, one-third of which was from direct precipitation on the reservoir surface.
The reservoir is 82% full and filling at well over twice the rate needed to reach the April-1 target.

Details:

Mean temperature yesterday was 10 degrees F above average, and precipitation was recorded at all stations except Alta. As predicted, precipitation heavily favored the upper Henry’s Fork for a change, where amounts through midnight ranged from 0.3 inch at Island Park to 0.7 inch at White Elephant and Black Bear. Ashton was again the winner in the valleys with 0.16 inch. Water-year total precipitation increased to 82% of average, and snow water equivalent (SWE) increased to 85% of average. Precipitation is continuing this morning, so storm totals will end up being quite a bit higher than reported through midnight. Only light precipitation is forecast for the next seven days, and the long-term outlooks call for generally dry conditions next week.

At an outflow of 329 cfs, Island Park Reservoir gained 443 ac-ft yesterday, one-third of which was from direct precipitation on the reservoir surface. The reservoir is 82% full, compared with 70% full on average. Fill rate has averaged 229 ac-ft/day since December 1, and a fill rate of only 103 ac-ft/day is needed for the rest of the winter to meet the April-1 target. Reservoir content is 1,880 ac-ft ahead of my November 19 prediction, which assumed an outflow of 330 cfs. As mentioned previously, higher-than-expected fill rate is due to a combination of slightly higher inflow and slightly lower outflow, at least according to our streamflow measurements earlier in December and to the power plant operator. According to USGS data, updated in mid-December, outflow since December 1 has averaged 344 cfs, compared with 341 cfs on average.

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