Photo of Brown Trout.
  • As of December 10, water-year precipitation is 87% of average, and snow-water-equivalent is only 75% of average.
  • Precipitation deficit since July 1 is over 5 inches.
  • Natural streamflow has been around 95% of average since early October.
  • Outflow from Island Park Dam has averaged 502 cfs since December 1, 143% of average.

Water year 2019 has started out the same way 2018 ended, dry and with slightly below-average streamflow. Through December 9, accumulated water-year precipitation is 87% of average, and snow-water-equivalent (SWE) at the watershed’s nine SnoTel stations is only 75% of average. So far this winter, precipitation has favored the southern half of the watershed, including the Teton headwater areas and the valleys. Upper Henry’s Fork lags the other subwatersheds by quite a bit. In addition to the current water-year precipitaiton deficit of around one inch, the four-inch deficit accumulated during July, August and September puts us over five inches down over the past five months. The winter will have to be much wetter than average to make up the deficit.

Graph of precipitation as percent of average

Graph of accumulated water-year precipitation as a percent of average.

Graph of snow-water-equivalent

Graph of snow-water-equivalent (SWE) across all SnoTel sites in the watershed.

Below-average SWE is not the result of warm temperatures but rather due to low precipitation. Mean temperature so far this water year is 2 degrees F below average. Since Halloween, the week around Thanksgiving was the only extended period of above-average temperatures, and even then, temperatures were above average by only a few degrees. The first week of December brought the coldest temperatures seen since last February.

Graph of mean temperature in the HF watershed

Graph of mean watershed temperature since October 1.

  Graph of temperature departure from average

Graph of mean temperature departure from average.

Despite dry weather over the past five months, streamflow remains near average, in response to precipitation that was well above average between September of 2016 and June of 2018. Watershed-total natural streamflow has hovered around 95% of average so far this water year. Since October 1, total gain of natural streamflow between Henry’s Lake and Island Park, a good indicator of groundwater flow from the Yellowstone Plateau aquifers, is 97% of average. Based on conditions last summer, I predicted gain between Henry’s Lake and Island Park to be 97.8% of average this winter. So far, so good.

Graph of streamflow gain between Henry's Lake and Island Park

Graph of natural reach gain between Henry’s Lake and Island Park.

Good inflow, along with very high reservoir carryover from the 2018 irrigation season, allowed managers to fill Island Park Reservoir to its normal April level in October and November by dropping outflow to around 275 cfs until the last day of November. 

Graph of Island Park Reservoir volume

Graph of Island Park Reservoir volume.

This means that no water needs to be stored between now and mid-April, and outflow from Island Park Reservoir will simply match inflow. Over the first 9 days of the winter period most critical to survival of juvenile trout downstream of Island Park Dam, outflow has averaged 502 cfs, a hair below last year’s 513 cfs but 43% higher than the 1978-2018 average winter flow of 351 cfs. Without the precision management that kept Island Park Reservoir above 72% full all summer, winter flow would have been about 180 cfs lower than it is now. Fremont-Madison Irrigation District, U.S. Bureau of Reclamation, and all of the other members of the Henry’s Fork Drought Management Planning Committee deserve a lot of credit for managing the reservoir and the entire irrigation system so carefully during the 2018 irrigation season.

Graph of outflow from Island Park Reservoir

Graph of outflow from Island Park Reservoir.