• Over the month of February, water-year precipitation improved from 93% of average to 98% of average, and snow-water equivalent improved from 96% of average to 101% of average.
• Mean outflow from Island Park Dam over the December-February time period most critical for survival of juvenile trout was 509 cfs, 147% of average and the highest since the winter of 2011-2012.
• Streamflow throughout the watershed remained above average at most locations.

Good precipitation + cold temperatures = improved snowpack

February 2018 started out very warm, cooled to near average in the middle of the month, and closed with the coldest 11-day period of the winter. Averaged over the whole month, temperature was 2 degrees F below the 30-year average.

Graph of watershed-averaged temperature.

Precipitation during February was above average in the upper Henry’s Fork, well above average in the Fall River subwatershed, near average in the Teton subwatershed, and well below average in the valleys. As a result, water-year precipitation improved from 93% of average to 98% of average over the month. This has increased to 101% of average over the first three days of March.

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

Snow-water-equivalent (SWE) accumulation followed the same pattern. Over the month of February, SWE accumulation improved from 96% of average to 101% of average. Over the first three days of March, this has increased to 105% of average.

Graph of watershed-total SWE.

Equally important as the improved SWE are cold temperatures to keep that snow from melting early. The tables below summarize climate and SWE information for the month of February.

Monthly climate summary, by subwatershed.

Monthly SWE summary, by subwatershed.

Streamflow remains above average

Despite the coldest weather of the winter at the end of February, natural streamflow was at or above average all month, led by Fall River, at 120% of average. Streamflow in the upper Henry’s Fork and in Teton River were both very close to average over the month.

Graph of natural streamflow in the Henry’s Fork Watershed. Some streamflow data are still missing from the mid-winter time period.

Winter outflow from Island Park Reservoir well above average

Outflow from Island Park Reservoir averaged 509 cfs over the three-month period from December 1 to February 28. This is 147% of average and the highest since the winter of 2011-2012.

Graph of outflow from Island Park Reservoir.

The December-February period is the most critical for survival of juvenile trout in Box Canyon. This year’s good winter flow is expected to produce very strong recruitment of 2-year old fish in 2019. Unfortunately, predicted recruitment this year is near where it has been over the past three years, due to low flows last winter.

Graph of recruitment of 2-year old Rainbow Trout into the Box Canyon population as a function of total winter flow through Box Canyon. Actual values for 2015, 2016, and 2017 are labeled. Predicted values for 2018 and 2019 are shown in red. The vertical red bars indicate the statistical margin of error in the predictions.

Despite high outflows, Island Park Reservoir has remained nearly constant over the winter, thanks to high inflows and direct precipitation on the reservoir surface. The most encouraging figure is that natural reach gain between Henry’s Lake and Island Park was 107% of the long-term average over the three-month period, continuing to show good outflow from Big Springs and other springs flowing out of the Yellowstone Plateau aquifer.

Inflow to and outflow from Island Park Reservoir over the three-month winter period.

Looking ahead

Cold, wet weather is forecast to continue through March. If this forecast holds, SWE will be above average on April 1, with the snowpack in good shape to melt slowly later in the spring. Based on current conditions, April 1 – September 30 streamflow is predicted to be a little above average over the whole watershed, led again by Fall River at around 110% of average. Streamflow in the upper Henry’s Fork and Teton River is expected to be average, but a wet March will improve that outlook.