2021 · February · Henry's Fork ·Feb 02, 2021

Henry's Fork

Midge hatches are beginning to appear on the lower river, but access can be a problem at many points. Are you interested in doing a trip to fish the upper Henry's Fork and drainage during the upcoming season? Would seeing how snow and water conditions appear to be stacking up to help in planning? The information Rob VanKirk compiles may help. Rob's analyses, and reports on water and snow conditions is as good as it gets. Below is Rob's latest update. Henry's Fork Water Supply, Feb 02 2021

After a dry day, water-year total precipitation and SWE are both at 80% of average.
Island Park Reservoir gained 99 ac-ft yesterday, compared with 65 ac-ft/day needed to reach the target volume of 120,000 ac-ft by April 1.
The reservoir is 86% full, compared with 74% full on average.

Details: Yesterday was warm and dry. Although the daily mean temperature was similar to that on Saturday, the afternoon high was the warmest since December 9. Water-year precipitation and snow water equivalent (SWE) are both at 80% of average. Precipitation is expected again tonight through Friday. The seven-day quantitative forecast calls for around 0.25 inch in the valleys, 0.5-0.75 inch in Island Park, and up to 2 inches in the Teton Range. Precipitation over the next week is expected to favor the southeastern corner of the watershed due to northwesterly flow. The new monthly outlook issued yesterday gives better-than-even odds for above-average precipitation during the month of February, but one- and three-month outlooks have not performed well so far this winter. As mentioned yesterday, precipitation has been below average in three of the four months in this water year so far, long-range outlooks to the contrary. Natural flow in the upper Henry's Fork and Teton River continue to bounce around in the range of 90-95% of average. Contrary to my expectations, warm weather last week has not resulted in new streamflow data from ice-affected stations in the lower watershed. Island Park Reservoir gained 99 ac-ft yesterday, compared with 65 ac-ft/day needed to reach the target volume of 120,000 ac-ft by April 1. The reservoir is 86% full, compared with 74% full on average. Reservoir volume is 116,143 ac-ft, 3,481 ac-ft ahead of my November-19 projection. If the seven-day precipitation forecast proves to be correct—and these short-range forecasts have performed well this winter—the reservoir will gain 310 ac-ft from direct precipitation and a total of around 1,000 ac-ft over the next week. Graphics:

Watershed SWE
Island Park Reservoir inflow/outflow: 15-minute data
Island Park Reservoir volume: 15-minute data
Island Park Reservoir volume: predicted vs. observed