Water Supply Still Strong Well Into July

Photo of Rainbow Trout.
  • 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.

Streamflow Above Average; Diversions Below Average

Despite the arrival of the hottest weather of the summer this past week, showers and high-elevation snowmelt continue to maintain above-average streamflow in Fall River and Teton River. The cumulative effects of above-average precipitation since the beginning of the water year last October are also helping to keep streamflow above average and irrigation demand below average. Current natural streamflow in the watershed is 5,690 cfs, compared with an average of 4,648 cfs, and with paltry 2,334 cfs at this time last year. Meanwhile, total irrigation diversion is around 3,000 cfs, a little below the long-term average. The graphs below illustrate total supply and total diversion.

Graph of natural streamflow supply.

Graph of total diversion

No Delivery Needed from Island Park Reservoir Yet

The graph below shows that streamflow in the Henry's Fork at St. Anthony has remained well above the 1,000-cfs target that generally signals need for delivery of water from Island Park Reservoir. Furthermore, flow at St. Anthony is well above my April-1 prediction and even above the top of the 90% prediction interval. This means that given snowpack and watershed conditions on April 1, there was less than a 10% chance of observing streamflows this good at this time of year.

Graph of streamflow in Henry's Fork at St. Anthony

Given current streamflow recession rates, it is likely that irrigation demand will be met well into next week without need for delivery of water from Island Park Reservoir. On average, peak diversion occurs on July 10, so this means that the watershed's natural water supply will most likely meet demand until after demand is already declining.

The graphs below show volume in and outflow from Island Park Reservoir. The reservoir has been full now for over five weeks, and outflow has equaled inflow for that whole time. Since Henry's Lake has also been full and passing inflow, outflow from Island Park has been essentially equal to the flow that would be in the river in absence of the two reservoirs. Note that last year at this time, over 40,000 ac-ft (30% of the reservoir's capacity) had already been delivered, and outflow was at 1,590 cfs.

Graph of Island Park Reservoir volume

Looking Ahead

Weather forecasts call for above-average temperatures but average precipitation for the next two weeks. This should keep irrigation demand close to the long-term average. Streamflow will continue to recede, but current recession rates are very slow, and Teton River flow has actually increased over the past few days. Outflow from Island Park Reservoir will be set roughly equal to inflow until delivery is needed from the reservoir to meet irrigation demand and keep flow at St. Anthony above 1,000 cfs. As each week goes by, the date on which delivery will be needed keeps getting pushed farther and farther out, greatly increasing the amount of water that will remain in the reservoir at the end of the irrigation season. HFF's science unequivocally shows that keeping as much water in Island Park Reservoir as possible all summer results in:

  1. better water clarity,
  2. lower suspended sediment transport out of the reservoir,
  3. lower water temperatures below Island Park Dam, and
  4. higher flow next winter, leading to higher trout survival and a higher trout population in future years.

Right now, all data indicate that we are on track to have the best summer-time water quality and highest winter flows since 2011-2012.