• Temperatures and rainfall were near average in August.
• Natural streamflow and diversion were both around 90% of average for the month.
• As of September 10, Island Park Reservoir is 73% full, compared with 46% full on average.

August Weather Near Average

August provided a little reprieve in the middle of what has otherwise been a warm, dry summer. Temperatures were warm during the beginning of the month and cool at the end, resulting in a monthly mean about 1 degree F below average.

Graph of watershed mean temperature, relative to long-term average.

A very deep autumn-like trough of low pressure brought substantial rain and near-freezing temperature to the whole watershed on August 27, adding to lighter showers that fell at scattered locations earlier in the month. The result was near-average precipitation at most stations for the month. Precipitation favored Teton Valley and the Fall River watershed; the upper Henry’s Fork and most valley locations recieved below-average precipitation. Ashton was an exception, receiving nearly one inch for the month. Temperatures during the late-August storm were cold enough that snow fell above 7,000 feet and stayed on the ground for a few days at the highest elevations. Water-year precipitation ended the month at 107% of average.

Climate summary table for August 2018.

Streamflow a Little Below Average

Natural streamflow stayed at around 90% of average during August, although streams rose a little after the rain late in the month. Fall River led the subwatersheds at around 110% of average flow for the month, compared to around 85% of average in both the upper Henry’s Fork and Teton River. Rate of flow recession was about average during the month, as natural flow fell from around 2,800 cfs at the start of the month to 2,500 cfs at the end. 

Graph of natural streamflow in Henry’s Fork watershed through September 9.

Diversion was also around 90% of average for the month. A period of above-average diversion during the middle of the month was offset by lower diversion rates at the beginning and end of the month. After dropping a little in the wake of the late-August rain, diversion increased a little during the first part of September as weather turned warm and dry again. Diversion from Fall River was below average for the whole month, whereas that on the Teton River was generally a little above average. As a result of high diversion and low natural flow on the Teton River, delivery of 100-200 cfs from the Henry’s Fork to the Teton River via the Crosscut Canal was necessary to meet irrigation demand.

Graph of diversion in the Henry’s Fork watershed through September 9.

Island Park Reservoir Stays Well Above Average

Outflow from Island Park Reservoir was well below average during August, reaching a maximum of 930 cfs, compared with a maximum of 1,140 cfs at the peak of irrigation demand in July. Most of the storage water delivered from the reservoir during August was needed for delivery to the Teton River.

Outflow from Island Park Reservoir through September 9.

As a result of below-average outflow, the reservoir dropped only 11,574 ac-ft during the month of August, 8.6% of the reservoir’s capacity. As of September 10, reservoir content is 73% full, compared with an average of 46% full for the date. The reservoir contained 36,967 ac-ft more than average on September 10. All other things being equal, this is worth about 187 cfs of additional winter outflow from Island Park Dam.

Outflow from Island Park Reservoir through September 9.

Lower Henry’s Fork Flow Objective Met with High Precision

This summer, the flow objective for the lower Henry’s Fork was 1,000 cfs at the St. Anthony stream gage, as long as Island Park Reservoir is being drafted. Outflow from Island Park Dam is changed as needed to meet irrigation demand and meet this flow objective. Since Island Park delivery began on July 3, flow at the St. Anthony gage has averaged 1,087 cfs. Daily average flow has dropped below 1,000 cfs on only 9 days, and the average over those 9 days was 979 cfs. This high degree of precision in meeting the St. Anthony flow objective is due to careful management and communication among Fremont-Madison Irrigation District, HFF, U.S. Bureau of Reclamation, and Fall River Electric Cooperative. As of September 10, only a few days of Island Park storage delivery remain, so the St. Anthony flow statistics will remain very close to what they are today.

Streamflow in Henry’s Fork at St. Anthony through September 9.

Outlook for Autumn

Outflow from Island Park Reservoir will continue to be reduced in small increments until inflow and outflow are equal and reservoir volume stays constant. This will occur by September 15, at an outflow of around 410 cfs. The reservoir will then be at its lowest point for the year, around 73% full. Outflow will remain at this level until the middle of October. By then, the reservoir will probably be filling a little, as cooler weather decreases evaporation from the reservoir surface. Some precipitation would help as well. If needed, outflow may be reduced by 25-50 cfs during late October and November if needed to fill the reservoir a little more before winter. Regardless, winter outflow is likely to be between 400 and 450 cfs–not as high as last year’s 506 cfs but higher than the long-term average of 354 cfs.