Fish of the Month: Year Four

In January of 2015, I started a new run of "fish of the month," a tradition I started years ago with long-time friend Tom Grimes, who is a guide at Henry's Fork Anglers. The idea is to catch a wild trout or whitefish every month of the year in our local waters, the streams and lakes of Montana, Wyoming, and Idaho in the Yellowstone region. My previous record was 55 months, from July 2004 to January 2009. Five academic years spent in California broke that streak, but I'm four years into the current one. How did 2018 turn out?

After dry December, snowpack only 70% of average

Precipitation for the month of December was only 57% of average.
As of January 1, water-year precipitation was 84% of average, and snow water equivalent was 71% of average.
However, because of above-average precipitation from September 2016 through June 2018, natural streamflow is near average throughout the watershed.
Winter flow out of Island Park Dam has averaged 516 cfs since December 1, compared with the 1978-2018 average of 351 cfs.

The Paradox of Irrigation Efficiency

Diversion from the Henry's Fork has decreased 30% over the past 40 years, yet consumptive use of water by crops has increased, there is less water in the river during the middle of irrigation season, and use of storage water has increased, all other things being equal. How could this be?

Winter Starts Dry, but Flow at IP Dam is 143% of Average

As of December 10, water-year precipitation is 87% of average, and snow-water-equivalent is only 75% of average.
Precipitation deficit since July 1 is over 5 inches.
Natural streamflow has been around 95% of average since early October.
Outflow from Island Park Dam has averaged 502 cfs since December 1, 143% of average.

Water Year 2018: Average supply but Above-average Reservoir Carryover

Water-year 2018 ended up close to average, at 102% of average precipitation and 105% of average streamflow.
Upper Henry's Fork subwatershed was below average in precipitation and streamflow, vs. above-average values in Fall and Teton rivers.
However, Upper Henry's Fork water supply improved from 70% of average in 2016 to 91% in 2017 and 94% in 2018, indicating recovery of deep aquifers from 2013-2016 drought.
May and June rain compensated for early snowmelt and resulted in below-average irrigation diversion.
Despite very dry conditions and below-average streamflow during July, August and September, Island Park Reservoir ended the water year at 73% full, compared with 43% full on average, thanks to careful and precise water management.

August Brings Relief from Hot, Dry Conditions

Temperatures and rainfall were near average in August.
Natural streamflow and diversion were both around 90% of average for the month.
As of September 10, Island Park Reservoir is 73% full, compared with 46% full on average.

Reservoir Remains Nearly Full Despite Dry July

The month of July was a little warmer than average but very dry.
Natural flow and diversion were both around 90% of average for most of the month.
Moisture from May and June rain has kept total diversion for the season about 100,000 ac-ft below average.
As of August 3, Island Park Reservoir is still 82% full, compared with an average of 70% full at this time of year.

June Closes Out Warm, Wet Spring

Watershed temperature over the month of June was 1 degree F above average.
June precipitation was 135% of average, but average natural streamflow at Island Park for the month of June was only 92% of average.
As of July 6, watershed-wide natural flow has dropped to 89% of average.
Delivery of Island Park Reservoir storage water began on July 3.
Watershed temperature over the April-June period was 2 degrees F above average, continuing the 40-year trend of increasing springtime temperature.
April-June precipitation was 134% of average.

May was Warm and Wet; Very Little Snowpack Left

May 2018 saw warm temperatures, heavy rain, and above-average snowmelt.
Only 30% of this year’s peak snow-water-equivalent (SWE) remains, compared with an average of 43% remaining on June 1.
Water-year precipitation stands at 108% of average, but SWE has dropped from 117% of average at its peak in April to 82% of average on June 1.
Lack of snow means that streamflow will drop rapidly once the current wet weather ends.

Heavy Rain Produces High Streamflow: How high is high?

May 22-24 precipitation totals were over 1 inch at most locations; water-year precipitation jumped from 105% of average to 109%.
Snowmelt continues at average rates, and SWE remains at 102% of average.
Watershed-total natural flow has increased to its highest level so far this year and higher than last year’s peak.
Inflow to Island Park Reservoir is around 1,600 cfs, and outflow is currently just a hair over inflow, allowing the reservoir to drop very slowly. Current reservoir content is a little higher than full pool.

Henry's Fork Foundation

Dr. Rob Van Kirk's blog