Henry’s Fork Basin End of Season Recap 2017

Henry’s Fork Basin End of Season Recap 2017

As you may know, the 2017 season started with lots of snow and a surplus of water around the greater Jackson Hole and Teton Valley region. Our friends at the Henry’s Fork Foundation have compiled some great information about the Henry’s Fork Basin from the 2017 season. Below is the Henry’s Fork Basin End of the Season Recap for 2017. For more information please reach out to the Henry’s Fork Foundation!

Water Supply – 

The Teton and Fall Rivers carried us through this year, meeting most of the irrigation demand and limiting the need for irrigation delivery from Island Park. As a result, Island Park Reservoir ended the water year at 81% full. Predicted winter flows = 400 cfs outflow from Island Park Dam + ~200 cfs from the Buffalo River = about 600 cfs through Box Canyon.

For the 2017 water-year, mean natural flow on the Teton River was 134% of average, Fall River was 120% of average, and the Upper Henry’s Fork was 91% of average. Compare that to water-year 2016 when the Teton was 76% of average, Fall River was 73% of average, and the Upper Henry’s Fork was 70% of average.

Water-year total precipitation for the Teton River was 140% of average, Fall River = 144% of avg, the Upper Henry’s Fork = 134% of avg, and precipitation in the valleys was about 133% of average. Those four together give us an average for the whole Henry’s Fork Watershed of 138% of average (water-year total precipitation).

See PDF attached for more information on water-year total precipitation and water-year mean natural flow.

Henry's Fork Basin End of Season Recap 2017

Idaho Dept of Fish and Game (IDFG) –

IDFG is conducting electrofishing surveys near St. Anthony this week (Railroad Bridge to Red Road). Preliminary estimates indicate the species composition is about 96% Brown Trout and 4% Rainbow Trout.

Water Quality (Temperatures) – 

There were concerns this summer about water temperatures through the Ranch and it was asked if increased outflow from Island Park Dam (to increase flows through the Ranch) would cool the water/decrease water temps. It’s a good question, but our water temperature data from Pinehaven showed that water temperature during summer high flows last year were no cooler than temperatures at the same time this year when flows were much lower. Essentially, higher flows in 2016 did not bring cooler temperatures.

Also, we’ve learned that keeping Island Park Reservoir full has the benefit of cooler water being released from the dam so, as the summer progresses and higher flows lead to lower reservoir contents, we end up with in higher water temperatures at the dam.

 Henry's Fork Basin End of Season Recap 2017


1. What is the approximate outflow from Island Park Dam as of today? (10/3/17)

500 cfs  — about 530 cfs — decay of aquatic plants in the stream channel have caused the stream gage to read lower than actual outflow

NOTE: Rob provides daily water supply updates throughout the year and calculates actual outflow when the stream gage is reading high or low. Anyone can be added to those daily emails. Simply email rob@henrysfork.org and let him know you’d like to be added to that list.

2. When is the official start of the water year?

October 1st

 3. How did July – September air temperature in the watershed this summer compare with that in 2016?

 It was warmer by about 0.9 degrees F

 4. In how many of the past 40 years was natural streamflow in the upper Henry’s Fork Watershed above average? NOTE: upper Henry’s Fork = Henry’s Fork above Ashton

15 of the past 40 years were above average.

Between 1978 and 2017, the average natural flow (over the whole year) was 1,596 cfs. During that time, 15 years were above that average with the maximum being 1997 at 2,305 cfs. 2017 was 25th with and average natural streamflow of 1,475 cfs.

Looking even more long-term…

…you might notice a consistent period of above average natural flow from about 1970 to the mid 1980s (with the exception of one below average year in the mid 70s). More recently, you’ll notice 2011 and 2012 were above average water years, but 2013-2016 were well below average and even this year (2017) was just shy of average. Remember this chart depicts natural inflow above Ashton, not the entire watershed.

5. What minimum level did Island Park Reservoir reach this year?

81% full

In 2016, the reservoir reached a minimum of 15% full and in 2015 reached a minimum of 30% full.

6. Over the last 40 years, how much has average spring-time air temperature increased (in degrees F)?

5.5 degrees F

7. What is this year’s predicted winter flow through Box Canyon?

600 cfs

Outflow from the dam (400 cfs) + about 200 cfs from the Buffalo River = about 600 cfs through Box Canyon.

8. Keeping Island Park Reservoir as full as possible provides which of the following benefits for the fishery?

(All of the above) Better water clarity, Cooler water released from the dam, Better winter flows, AND Less sediment delivery from the reservoir.

Thanks again and please feel free to reach out with any questions or if we can provide any additional information.

Hope everyone has a great fall/winter and we’ll see you all next year!


Jamie Laatsch
Conservation and Outreach Coordinator
Henry’s Fork Foundation

P.O. Box 550

Ashton, ID 83420

office: 208-652-3567

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