Streamflow Still Above Average on Fall and Teton Rivers

Photo of Rainbow Trout.
  • 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 Watershed Natural Flow at Long-Term Average

Current natural streamflow is 1,044 cfs in Teton River above the Crosscut Canal, 1,109 cfs in Fall River, and 1,267 cfs in the upper Henry's Fork. Natural flow is above average in both Fall River and Teton River, reflecting above-average precipitation for the water year. However, natural flow is only 83% of average in the upper Henry's Fork subwatershed, reflecting slightly lower water-year precipitation there, as well as the cumulative effect of four years of drought on the deep Yellowstone Plateau aquifers that feed Big Springs, Buffalo River, Warm River, and other streams. The graph below shows that total watershed supply is right on average for the date, at around 3,400 cfs.

Graph of total watershed natural flow supply.

Variability in total watershed diversion has been driven by precipitation this spring and summer and is currently right at average for this time of year, around 3,300 cfs. Diversion was quite a bit lower by this time last year due somewhat to earlier planting but due mostly to lack of physical streamflow. All of the natural flow in both Teton River and Fall River was being diverted at this time last year, and the remaining demand was being met by high delivery of storage water from Island Park Reservoir. The graph below shows total watershed diversion, compared with long-term average and with last year.

Graph of total watershed diversion.

Storage Delivery Began July 14

Delivery of storage water from Island Park Reservoir began on July 14, compared with June 13 last year. The objective for storage delivery this year is to keep average daily flow in the Henry's Fork at St. Anthony at or above 1,000 cfs. As seen in the graph below, flow at St. Anthony fell below the 1,000 cfs target last Thursday through Sunday, as diversion increased during hot weather, while streamflow in Fall River was dropping rapidly. Steady increase in delivery from Island Park Reservoir last Friday through this Wednesday, as well as delivery of 50 cfs from Grassy Lake starting on Tuesday of this week, has increased flow at St. Anthony over the past few days. Since last Thursday, when flow at St. Anthony first dropped below 1,000 cfs, flow there has averaged 1,004 cfs. In 2016, flow at St. Anthony first dropped below 1,000 cfs on June 13.

Graph of streamflow at St. Anthony.

The graph below shows that Island Park Reservoir remained full from June 1 through July 15 and has dropped only a few percent since then. Last year at this time, the reservoir was already less than 50% full.

Graph of Island Park Reservoir volume.

As can be seen in the following graph, Island Park outflow followed the pattern of natural inflow during the time that the reservoir remained full but has since been increased to meet irrigation demand. Current outflow 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. The April-1 model has done well in predicting magnitude of outflow, but timing of storage delivery was delayed by almost three weeks because of higher-than expected natural flow in Fall River and Teton River through late June and early July.

Graph of outflow from Island Park Reservoir.

Forecast for Week of July 24

Looking ahead to next week, weather forecasts are starting to become more confident in measurable precipitation watershed-wide on Monday night and Tuesday. Some models are predicting very high moisture content in southerly flow at the same time as a low-pressure trough and cold front arrives. Depending on timing and location, precipitation could be locally heavy. If this occurs, streamflow in both Fall River and Teton River will increase starting on Tuesday, and irrigation demand will decrease. Even in absence of widespread rain the valley areas, I anticipate noticeable decreases in diversion to begin late next week as grain finishes. Regardless of precipitation, humidity will increase and temperature will decrease next week, reducing evapotranspiration. Although it is too early to tell 3-4 days out from the forecast precipitation event, it is possible that storage delivery has already reached its maximum for this irrigation season, and small cuts to delivery from Island Park Reservoir may be possible late next week. At worst, an additional 100-150 cfs will be needed next week to get over peak irrigation demand for the season.