Despite a forecast for conditions that could have resulted in large loss of snowpack yesterday, atmospheric conditions lined up just right to not only avoid the loss but actually gain a very large amount of snow-water-equivalent (SWE). Read on for the details, as well as for an example from the spring of 2010 that illustrates a very large rain-on-snow event.
Over the month of January, water-year precipitation increased from 87% of average to 93% of average, and snow-water-equivalent increased from 89% of average to 96% of average.
Streamflow throughout the watershed was generally above average, including outflow from Island Park Reservoir, which, at 508 cfs, was 134% of average and the highest since the winter of 2011-2012.
The only negative aspect of the current water situation is that January was much warmer than average, limiting snow accumulation at lower elevations and setting up the snowpack to melt early and rapidly.
As the climate warms and water availability becomes more uncertain, the future of wild trout fisheries in the Henry’s Fork depends critically on science-based management that ensures favorable streamflow, good water quality, and a positive fishing experience.
December precipitation was only 51% of average across the Henry's Fork watershed. However, water year-to-date precipitation and snow-water-equivalent are at 87% and 89% of average, respectively. And, thanks to above-average precipitation during water year 2017, the upper Snake River reservoir system is 87% full, and winter streamflow is above average. Long-range forecasts call for average to above-average precipitation for the remainder of the winter, so the overall water-supply outlook remains good as we head into the New Year.
Each of the last two years at this time, I have reported on the resurrection of “fish of the month,” a tradition that Henry’s Fork Anglers guide Tom Grimes and I started many years ago. The goal is to catch at least one wild trout every month of the year, on a fly, in our local Idaho, Montana, and Wyoming waters. My longest fish-of-the-month streak lasted 55 months, from July 2004 through January 2009. On December 4, I added month 36 to the current streak.
Water year 2017 was above average in both precipitation and streamflow. Averaged over the whole watershed, precipitation was 138% of average, led by the Fall River watershed, at 144% of average. Due to the cumulative effect of four years of drought, especially in the groundwater-dominated subwatersheds, natural streamflow was only 109% of average, led by the Teton River at 134% of average. Streamflow in the upper Henry's Fork, which is dominated by groundwater, was only 91% of average.