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.
Natural streamflow is still above average in Fall River and Teton River but below average in the upper Henry's Fork.
Total watershed natural flow is 3,400 cfs, right on average for this time of year. Total watershed diversion is also average, at 3,300 cfs.
Delivery of storage water from Island Park Reservoir began last week; current flow out of Island Park is 1,060 cfs, compared with the long-term average of 1,300 cfs for the date and last year's value of 1,420 cfs.
Island Park Reservoir is currently 96% full, compared with 49% full at this time last year.