Six weeks into the new water year, most measures of water supply in the Henry’s Fork watershed are average or better:

1. Snow accumulation in the mountains is well above average for early November.
2. Watershed-wide natural flow has been above average since October 1.
3. Island Park Reservoir is already at its April-1 average content.
4. Medium- and long-term weather forecasts call for above-average precipitation.

Snow Accumulation Above Average So Far

After well above-average precipitation during late September and early October, the first six weeks of the water year have generally been on the dry side. Water-year 2018 precipitation to date is below average in the upper Henry’s Fork subwatershed and in the valleys. Due to good precipitationlast week, accumulated precipitation to date has risen above average in the Fall River and Teton River subwatersheds, continuing a trend that we observed during most of water year 2017. Accumulated precipitation, as a percent of average, is shown in the graph below.

Meanwhile, temperature has been well below average since the middle of September, as shown in the graph below.

As a result of cold temperatures, snowpack has been accumulating at the highest elevations in the watershed since September, and even low- and mid-elevation areas have been accumulating snowpack over the past month. Although it is too early in the snow accumulation season for percent-of-average figures to be meaningful, the graph below shows that watershed-averaged snow-water-equivalent accumulation is well above average for the date and equal to the average accumulation on November 19. In other words, we are already 10 days ahead in snow accumulation, with more on the way.

Streamflow Above Average

Total watershed-wide natural streamflow has been above average since October 1. Even at the daily scale, streamflow has been above average on most days, falling to average only a few times so far in the young water year. Streamflow in Fall River and Teton River is above average, whereas that in the upper Henry’s Fork has been below average, continuing the pattern we have observed since last May. However, upper Henry’s Fork streamflow has improved substantially in the past year, from about 75% of average in late summer of 2016 to 90-95% of average so far this fall.

Island Park Reservoir 83% Full

As a result of well above-average streamflow in Fall River and Teton River all summer, very little storage delivery was needed from Island Park Reservoir. Reservoir content bottomed out at 109,410 acre-feet (81% of capacity) on September 29 and has filled slowly since then. As of this morning, reservoir content was 112,526 acre-feet (83% of capacity). That is higher than the 1978-2017 average content on April 1. In other words, the reservoir is already at its typical April 1 content, as can be seen in the graph below.

Essentially no water needs to be stored in the reservoir between now and the end of the winter, which means that outflow will be very close to inflow. Mean inflow over the winter is projected to be 415 cfs, so outflow will also be around 415 cfs, which is higher than winter outflow has been since the 2011-2012 winter. So far this fall, outflow has been much higher than average, as shown in the graph below.

Winter Forecast

The latest medium- and long-term weather forecasts call for a better-than-even chance of above-average precipitation through January. If that forecast proves correct, snow accumulation will be above average by the end of January, given our above-average starting point right now. As mentioned above, Island Park Reservoir is already at its April-1 content, so it will fill easily with above-average outflow during the winter and early spring. I anticipate that watershed-wide natural streamflow will fall below average once temperatures get really cold, since baseflow from the springs in the upper Henry’s Fork provides the majority of total watershed streamflow during the middle of the winter. That upper Henry’s Fork contribution will remain at 90-95% of average most of the winter and drag the watershed total down below average once Fall and Teton rivers recede from the effect of autumn rains.

All in all, however, water year 2018 is off to a good start.