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.
On February 7, snow-water-equivalent (SWE) in the Upper Henry's Fork subwatershed was 99% of the 30-year median. The intervening month has brought heavy precipitation to the entire watershed, raising the Upper Henry's Fork SWE to 120% of median. SWE is 131% of median in the Fall River subwatershed (up from 121% on February 7) and 135% of median in the Teton River subwatersed (up from 133% on February 7). All three subwatersheds have already reached their median peak SWE for the whole season.
Ten days ago, snow-water-equivalent (SWE) in the Upper Henry's Fork subwatershed was 89% of the 30-year median. As forecast 10 days ago, storms over the past few days have favored the northern part of the watershed for a change, raising the Upper Henry's Fork SWE to 99% of median. SWE is 121% of median in the Fall River subwatershed (up from 113% nine days ago) and 130% of median in the Teton River subwatersed (up from 123% nine days ago). Forecast calls for four more days of wet weather, followed by a week-long dry period.
Nine days ago, snow-water-equivalent (SWE) in the Upper Henry's Fork subwatershed was 91% of the long-term median. Following another large storm that dropped heavy snowfall across the southern part of the state but relatively little across the north, Upper Henry's Fork SWE has dropped to 89% of median. SWE is 113% of median in the Fall River subwatershed (down from 120% nine days ago) and 123% of median in the Teton River subwatersed (compared with 124% nine days ago). Models hint at a more northerly track for the next round of precipitation, which would help the upper Henry's Fork.