HFF Blog

Yellowstone Cutthroat Trout Management on the South Fork Snake River


  • Fall IDFG surveys estimated Rainbow Trout and Rainbow x Yellowstone Cutthroat Trout Hybrid abundances nearly doubled in 2018 and set a record high of 3,073 fish/mile in the Conant index reach.
  • If left unmanaged, Rainbow Trout would likely hybridize and out-compete native Yellowstone Cutthroat Trout in the South Fork Snake River.

Fish of the Month: Year Four

Photo of Brown Trout.

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

Photo of Brown Trout.
  • 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.

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

Photo of Brown Trout.
  • 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

Photo of Brown Trout.
  • 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.

Reservoir Remains Nearly Full Despite Dry July

Photo of clouds over Fall River
  • 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.


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