A three-day winter storm in the middle of the week set record cold temperatures for the date and greatly slowed snowmelt, saving that snow to melt later in the spring and summer. Elevations above 6,500 feet gained up to inch of new snow-water-equivalent this week, including the White Elephant site on the side of Mt. Sawtelle. As a result, current snowpack at the subwatershed scale is currently between 122% and 147% of median for the date. Although streamflow at most locations is below average this morning, warm temperatures forecast for the next 10 days will resume melt, leading to season-peak streamflows into Henry's Lake and in Fall River and Teton River around June 1. Henry's Lake and Island Park Reservoir will both fill around that time.
A well-above-average May snowpack has begun melting over the past week, resulting in above-average streamflow throught the watershed. Minimal amounts of water are being stored in the watershed's reservoirs, and irrigation diversion is only around 20% of total water supply. As a result, streamflows are very near their natural values--essentially the same flow the rivers would have in absence of reservoirs and diversions. These high, natural flows--occurring within the range of long-term average timing--are currently benefitting physical and ecological processes in stream channels and riparian areas throughout the watershed, mobilizing and removing fine sediment from the stream bottom, creating new habitat, and ensuring reproduction of cottonwood trees. After four years of drought, the sight of snowmelt filling rivers and floodplains is a welcome change.
With a very wet April now in the books, the 2017 water year is certain to be above average in terms of water supply in both the Henry's Fork watershed and in the upper Snake River basin as a whole. Highlights are:
April precipitation in the Henry's Fork watershed was 162% of the 1982-2016 mean and 261% of last year's value.
May 1 snow-water-equivalent in the HF watershed was 146% of the 1981-2010 median and 228% of last year's value.
April 30 natural streamflow in the Snake River at Milner was 171% of the 1988-2014 mean and 130% of last year's value.
Total diversion on April 30 in the upper Snake River basin was 82% of the 1988-2014 and only 72% of last year's value.
Four weeks ago, water-supply outlook for the upper Henry's Fork watershed was average at best because of an extended period of warm, dry weather in March that had already melted much of the low- and mid-elevation snowpack. However, April has turned out to be one of the wettest on record, greatly improving water supply and increasing the probability that relatively little Island Park Reservoir storage water will be needed to meet irrigation demand this summer. With three days left to go in the month, Island Park has received 4.5 inches of precipitation in April, compared with the 1981-2016 average of 2.3 inches and a maximum over that period of 4.2 inches in 1993. However, temperatures have remained above long-term averages throughout the month.
To provide streamflow information for all river stakeholders, we have constructed a computer simulation model of the Henry’s Fork watershed stream, reservoir, and irrigation system. Using early-April water-supply conditions and long-term temperature trends as inputs, we expect streamflow conditions to be generally better than average and much better than last year across the watershed. Focusing specifically on Island Park Reservoir and the river immediately downstream, we predict:
Streamflow during the second half of June at Island Park Dam will be roughly equal to the river’s natural flow. With 90% probability, this natural streamflow will range between 400 and 750 cfs.
Irrigation delivery from Island Park Reservoir will begin around July 1 and peak in mid-July. With 95% probability, releases from the reservoir during July will be lower than 1,400 cfs, and with over 95% probability will be much lower than 2016 releases between the middle of June and the first week of August.
With 95% probability, Island Park Reservoir contents at the end of the September will remain above 58,000 ac-ft (43% full), very close to the long-term average.
Because of lower outflows and higher reservoir contents, turbidity (how “dirty” the water appears) in the river immediately downstream of Island Park Dam is expected to remain lower than the high values observed in 2016 during the mid-July cyanobacteria bloom and late-summer reservoir drawdown.
On April 1, the traditional date of peak-snowpack measurements around the western U.S., snow-water equivalent (SWE) in the upper Henry's Fork watershed was at 106% of the 1981-2010 median. However, this value was only 98% of the long-term mean, based on records that go back into the 1930s at some sites. Among 32 river basins around Idaho, Montana, and Wyoming that are popular fishing and floating destinations, April 1 SWE was at or above the 1981-2010 median in all but 6 basins, which included the Gallatin, Clark Fork, Smith, Jefferson, and North Platte. The upper Henry's Fork ranked 22nd out of the 32 basins. Fall River came in 11th, and Teton River 15th. The basins with highest April 1 SWE this year were the Wind River, Big Wood, and Little Wood.
On March 12, snow-water-equivalent (SWE) in the Upper Henry's Fork subwatershed was 122% of the 30-year median. Unfortunately, the last two weeks brought record high temperatures, and we have already lost a large amount of snow. Today, SWE in the Upper Henry's Fork is only 104% of median. SWE is 127% of median in the Fall River subwatershed (down from its peak of 137%) and 125% of median in the Teton River subwatersed (down from its peak of 138%). Although some of the early snowmelt has been stored in Henry's Lake, Grassy Lake, and Island Park Reservoir--and in local and regional aquifers--the vast majority of recent record-high streamflows has flowed out of the upper Snake River basin. This water will not be available later in the summer when we need it most. Check back next week for detailed predictions of streamflow for the upcoming spring and summer.
Timing of snowmelt runoff affects river ecology, trout behavior, fishing experience, and mid-summer streamflow in natural and regulated river reaches.
Runoff timing is most strongly influenced by April-June temperatures (warmer = earlier runoff) and April 1 snow-water-equivalent (SWE; more snow = later runoff).
Since 1930, runoff timing in the upper Henry’s Fork has displayed a parabolic (upside-down “U”) trend over time, with the latest runoff occurring in the 1970s.
Date of 30-day peak streamflow in the upper Henry’s Fork in 2015 and 2016 ranked 2nd and 9th earliest, respectively, in the last 87 years. Earliest runoff on record occurred in 1934.
Over the past 30 years, mean April-June temperature at the watershed’s snow survey sites has warmed at 1.29°F per decade. All other factors being equal, this moves the 30-day peak flow window 5.4 days earlier each decade. Global climate models and data predict continued warming over the next few decades, so we can expect continued earlier runoff.
Runoff timing is poorly predicted by conditions in the month of March, so it is too early to make precise predictions of runoff timing in 2017. However, rapid loss of low- and mid-elevation snow since March 10 is greatly reducing the amount of snow on the ground. Thus, we are sure that runoff will be earlier than I would have predicted two weeks ago. Streamflow conditions will most likely be better than last year but not nearly as good as they could have been had the robust early-March snowpack stuck around until the beginning of April.