December precipitaion well below average

The month of December turned out to be about average in temperature, as warm weather in the middle of the month was offset by cold periods at the start and end of the month. However, precipitation for the month was only 57% of average. Monthly precipitation was above average in the valley areas, and Ashton led the way at 119% of average. Precipitation was below average at all nine SnoTel sites. By subwatershed, monthly precipitation was 63% of average in Teton, 56% of average in Fall River, and 47% of average in upper Henry’s. As a result, water-year precipitation is only 84% of average, roughly 100% of average in valleys, 90% in Teton, 80% in Fall River and 75% in upper Henry’s. Ashton and Grand Targhee are the only two stations with above-average precipitation, at 115% and 102%, respectively. Watershed-wide precipitation deficit is almost 2 inches for the water year.

Snow water equivalent (SWE) follows the same pattern. December SWE accumulation was only 47% of average: 57% of average in Teton, 43% of average in Fall River, and 41% of average in upper Henry’s. Current SWE is 70% of average: roughly 80% in Teton, 70% in Fall River, and 60% in upper Henry’s. SWE is not recorded at the valley stations, but if it were, SWE would be above average in Rexburg and Ashton and around average in Teton Valley. Watershed-wide deficit is 3 inches for the water year.

Streamflow and reservoir content remain strong

The good news is that streamflow and reservoir contents continue to reflect a long period of above-average precipitation that lasted from September of 2016 through June of 2018. Although ice is currently affecting many of the stream gages in the watershed, the available data show that so far this water year, natural streamflow has been just a hair below average in the upper Henry’s Fork, a hair above average in Fall River, and close to average in Teton River. Thanks to precision management of the Henry’s Fork irrigation system last summer and agressive reservoir fill during the fall, Island Park Reservoir started the winter already at its April-1 target volume. That means that outflow will roughly equal inflow all winter for the second year in a row. In fact, the reservoir got a little too full two weeks ago, and outflow had to be increased a little to keep the reservoir’s ice cover from reaching the brackets that attach the rubber spillway collar to the concrete. The resulting outflow was actually greater than the river’s flow would be under natural conditions. Outflow has averaged 516 cfs since December 1, compared with last year’s average of 504 cfs over the whole winter and with the 1978-2018 winter average of 351 cfs. Small adjustments to outflow will be made as needed throughout the winter to keep the reservoir in the range of 119,000-120,000 ac-ft, or 88-89% full.