Natural streamflow is still above average in Fall River and Teton River but below average in the upper Henry's Fork.
Total watershed natural flow is 3,400 cfs, right on average for this time of year. Total watershed diversion is also average, at 3,300 cfs.
Delivery of storage water from Island Park Reservoir began last week; current flow out of Island Park is 1,060 cfs, compared with the long-term average of 1,300 cfs for the date and last year's value of 1,420 cfs.
Island Park Reservoir is currently 96% full, compared with 49% full at this time last year.
Total natural streamflow in the Henry's Fork watershed is 122% of average and 2,500 cfs greater than irrigation diversion, which remains slightly below average.
No water has been delivered out of Island Park Reservoir yet this season, compared with 40,566 ac-ft (30% of reservoir capacity) last year at this time.
Island Park Reservoir outflow will remain roughly equal to inflow until release is needed to meet irrigation demand, which now looks to be another week out.
Island Park Reservoir, Grassy Lake and Henrys Lake have all filled.
Natural streamflow in the watershed is around 9,500 cfs and at its final peak for the season.
Total watershed-wide irrigation diversion is around 3,000 cfs, right at average for this time of year.
Outflow from Island Park Reservoir is being set to keep the reservoir full. Only small changes are anticipated over the next week, and outflows should remain around 950 cfs plus or minus 150 cfs, depending on rain.
Delivery of water from Island Park Reservoir in excess of inflow is not expected to be needed until early July.
Precipitation in May was about 50% of average, but temperatures were near average, preserving most of the high-elevation snow all the way through the month.
Snow remaining in the watershed at elevations above 7,000 feet was almost twice the median amount on June 1.
Warm temperatures are leading to increased streamflows in the Henry's Lake basin as well as in Fall River and Teton River.
Fall River and Teton River will peak in early June but supply enoughy water to meet irrigation demand through the month of June
Island Park Reservoir is 100% full, and outflow will roughly follow natural inflow as it gradually drops from 850 cfs now to around 500 cfs by July 1.
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.
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