Headlines from Dr. Rob Van Kirk’s SWE report filed yesterday for the Henry’s Fork drainage.
Headlines • Temperatures and snow melt were well above average yesterday, but current SWE is 10% above the average annual peak and 27% above average for the date. • Natural stream flow has increased 22% in the past two days and is 72% of average for the date. • Precipitation over the next week is now forecast to be heavy, with much cooler temperatures. • My forecast for April-September total natural stream flow is 105% of average for the watershed: 102% in upper Henry’s Fork, 103% in Fall River, and 106% in Teton River.
Headlines from Dr. Rob Van Kirk’s SWE Report Filed Yesterday for the Henry’s Fork Drainage
Precipitation overachieved yesterday, pushing SWE to yet another seasonal peak, at 119.2% of average.
Dry weather and a warming trend start today, which will greatly increase snow melt across the watershed.
Island Park Reservoir will start today’s freshet delivery at 91% full and just a few inches below the rubber spillway collar.
Outflow from Island Park will be increased to 2,000 cfs in four hourly increments this morning.
Outflow will be increased to 2,000 cfs in four hourly increments this morning and held at 2,000 cfs until at least Thursday and most likely until Friday morning. Post-freshet outflow is tentatively planned to be set at 650 cfs, but the exact figure will be determined by inflow rate. Current inflow is around 500 cfs, but I expect that to jump quickly over the next few days to 800 cfs and likely to exceed 1,000 cfs by the weekend. This will impact flow downstream to Ashton Reservoir and has purpose of helping to flush silt from the river bed.
Headlines From Dr. Rob Van Kirk’s SWE report for the Henry’s Fork drainage filed yesterday.
A little precipitation yesterday kept the water-year total at 115% of average.
Snowmelt was very modest yesterday, keeping SWE around 2% below its annual peak.
At an outflow of 271 cfs over the past week, Island Park Reservoir is 89% full and filling very slowly.
A freshet operation designed to transport fine sediment out of the Island Park to Riverside reach will occur next Tuesday-Friday. See details on the Henry’s Fork Foundation website.
We have posted a lot of information about the ecological benefits of the freshet, as well as information on other aspects of the freshet on the HFF web site and will continue to add to that information as the freshet approaches.
Although snow currently dominates the banks of the river and limits access, here is a soon-to-be-performed action that will impact flow and therefore fishing from Island Park Dam downstream at least to Ashton Reservoir. Normal flow out of Island Park Dam is around 200 cfs this time of the year.
The Drought Management Planning Committee (DMPC) approved a spring freshet for April 25-27. Freshets are most effective at sediment scour and thereby aquatic insect habitat improvement when flows increase rapidly, to high outflows, and last for 48-72 hours. Flows are expected to be bumped to 2,000 cfs on the 25th and remain there until morning of the 27th. This was not easy to accomplish and has been most of a year in the making. For more information on this action, visit the Henry’s Fork Foundation web site.
FAQ: What is a Spring Freshet?
Updated: 2 hours ago
Freshet:
A high rate of streamflow for a short amount of time. Typically a natural process in rivers caused by heavy rain or rapid snowmelt.
The Two Freshet Types on the Henry’s Fork
On the Henry’s Fork, a freshet can be:
Natural: from heavy rain or rapid snowmelt into a full or nearly full Island Park Reservoir, requiring a rapid increase in outflow from the dam
Managed: a planned freshet, negotiated with water managers and decision makers, to achieve (or even improve on) the benefits a natural freshet brings
The river below Ashton Dam is providing fishing with midge and BWO activity making for top water action during PMs. Flow is a bit below normal for the time of year. For browns and post spawning rainbows, try streamers around locations out of direct sunlight and during low light periods.
Headlines from Dr. Rob Van Kirk’s Henry’s Fork Drainage Snow-Water Equivalent Report filed Yesterday, April 10th
Natural stream flow has increased 10% since Thursday due to snow melt but is still only 59% of average.
At an outflow of 266 cfs, Island Park Reservoir is 88% full and filling very slowly.
At 9 degrees F above average, yesterday was the warmest day since last October 21.Based on updated data, SWE appears to have peaked for the year last Thursday at 117% of the average peak.Natural .
Here are Dr. Rob Van Kirk’s Snow-Water Equivalent (SWE) report headlines for the Henry’s Fork drainage today, March 28th.
Headlines
Mean temperature yesterday was 13 degrees F below average, extending what has been a very cold winter into spring. Precipitation was light yesterday, but after updating data, the water-year total is 112% of average, a little higher than I reported yesterday. Snow water equivalent (SWE) stayed at 119% of average for the date and 109% of the average seasonal peak.
There is no end to cold, wet weather in sight. The latest 7-day quantitative forecast is now calling for widespread precipitation exceeding 0.75 inch over even the driest parts of the watershed, with up to 3 inches of water equivalent at the highest elevations. Below-average temperatures are expected to continue well into the month of April, with high confidence in that outlook.
At an outflow of 250 cfs, Island Park Reservoir is 86% full, compared with 82% full on average. The three reservoirs in the Henry’s Fork watershed are 88% full, compared with 86% full on average.
Here is an important detail from Dr. Rob Van Kirk’s snow-water equivalent (SWE) report compiled yesterday for the Henry’s Fork watershed.
I will emphasize that average snow pack this winter—if we actually even achieve that—will not bring average stream flow. Stream base flows this winter are still in the range of 70-80% of average, and the springtime snow melt will be superimposed on that very low base—not on top of average stream flow. Average SWE this will winter will bring us natural stream flow in the range of 80-90% of average, which is much better than last year but still well below average.
Headlines from Dr. Rob Van Kirk’s Snow-Water Equivalent Report Compiled Yesterday
Yesterday’s watershed-averaged minimum and mean temperatures ended up just a fraction of a degree warmer than those on December 22, but most stations in the watershed set new low-temperature records for the date.
Water-year total precipitation is 104% of average, and SWE is at 106% of average.
If SWE stays at 106% of average the rest of the winter, expected April-September streamflow will be 90-95% of average.
At an outflow of 266 cfs, Island Park Reservoir gained 104 ac-ft yesterday and is 77% full, compared with 75% full on average.