Where are the fish in Henrys Lake? Henrys Lake Quarterly Newsletter - September 2020

Henrys Lake Quarterly Newsletter
September 2020


Many of you may have noticed that the hatchery has been closed to the public and our IDFG presence has been reduced around the lake this season. In response to guidance from Idaho Department of Health and Welfare, all IDFG hatcheries remain closed across the state to limit the risk of transmitting the COVID-19 virus to both department staff and local residents until further notice. Our goal is to continue to serve our sportsmen and women while keeping safety at the forefront of our work.


Henrys Lake Graduate Study
by Darcy McCarrick University of Idaho

Creative Commons Licence
Darcy McCarrick

The 2020 field season has been very productive. From June to August we tracked movements and habitat use of Yellowstone Cutthroat Trout Oncorhynchus clarkii bouvieri and Utah Chub Gila atraria by boat and plane. In June, Yellowstone Cutthroat Trout and Utah Chub were both concentrated near shore (Figure 1). As water temperatures increased in July, we started seeing Utah Chub move into deeper water and Yellowstone Cutthroat Trout become more concentrated in cold water sources (e.g., mouth of Targhee Creek, Staley Springs; Figure 1). Utah Chub were consistently observed schooled together (presumably for spawning) in July and August. We documented schools near the Idaho Department of Fish and Game (IDFG) hatchery, Henrys Lake State Park, and most frequently down the outlet (Figure 1).


Figure 1. Location maps for Utah Chub (black dots) and Yellowstone Cutthroat Trout (stars) in Henrys Lake, Idaho (June to August 2020). Forty-five Utah Chub and 44 Yellowstone Cutthroat Trout were surgically implanted with radio transmitters and tracked via boat and plane.

Are fish traveling up the creeks?

Yellowstone Cutthroat Trout were detected in Duck, Howard, Targhee, and Timber creeks during June (Table 1). In July and August, Yellowstone Cutthroat Trout were also detected in Targhee Creek. Trout are likely using Targhee Creek as thermal refuge during warm summer temperatures.


Table 1. Yellowstone Cutthroat Trout (YCT) detected in four tributaries at Henrys Lake from June to August 2020.

Creative Commons Licence
James Brower Idaho Fish and Game
Creative Commons Licence
James Brower Idaho Fish and Game
Creative Commons Licence
James Brower Idaho Fish and Game



What's next?

Next week, we will conduct our fall gillnetting to complete data collection for the gillnet selectivity analysis. Sampling will begin September 20th and continue until September 26th. A combination of historic and random sites will be used. Once data collection is complete, we will be able to perform the selectivity analysis for the different mesh sizes, compare IDFG gillnet and customized AFS gillnet catch rates (from the spring sample), and compare seasonal differences between spring, summer, and fall. Gillnet selectivity will provide insight into how effective the each type of gillnet is at sampling different sizes and species of fish in Henrys Lake.


Upcoming Work - September to December 2020

1. Riparian fences and fish screens: Our crews have been maintaining over 100 miles of electric and jack fences on the tributaries of Henrys Lake this summer. These fences help to protect riparian areas and allow for the rehabilitation of these important stream corridors.

Creative Commons Licence
Thacia Schmidt, IDFG

This photo shows a section of electric fence that we maintain on Howard Creek. A solar panel charges a car battery which in turn provides power to a blitzer which sends electricity to the fence. Cattle are able to graze on the left side of this electric fence while the creek and riparian areas are left untouched on the right. Although it is quite smokey around the lake right now, you can make out Henrys Lake and the centennial mountains in the background of the photo.

In addition, we maintain 11 fish screens around the lake which keep fish out of irrigation canals. Due to the excellent collaboration with multiple landowners around the lake we are able to protect important trout spawning and rearing grounds.

Creative Commons Licence
Jenn Vincent, IDFG

The photo above depicts one of our irrigation screens. An open head-gate allows water to travel from the creek into the irrigation canal. A water powered paddle wheel controls a debris screen that keeps debris out of the irrigation canal. A bypass pipe is available on the upper side of the screen. This allows any fish that travels through the head-gate into the canal to travel through the underground pipe back to the creek.

2. Stocking: Approximately 1 million Yellowstone Cutthroat Trout and 150,000 sterile Brook Trout are scheduled to be stocked in Henrys Lake during the last week of September 2020.

Creative Commons Licence
Jenn Vincent, IDFG

3. Graduate study: Darcy, a graduate student with the University of Idaho will be conducting another round of gill netting on the lake this September to understand gillnet selectivity in the spring, summer and fall. Sampling will occur during the week of September 20th, 2020. Photo credit: Darcy McCarrick

Creative Commons Licence
Darcy McCarrick

4. Rock Creek diversion: This October we will complete the installation of the diversion dams constructed on Rock Creek in October 2019. This year we will install the flumes which will monitor flows and reduce erosion in the stream.
Below you can see the diversion structures we completed last October. These structures divert water from Rock Creek into an irrigation canal.


Ask a Biologist
At the end of each newsletter we will have a section addressing any questions you may have. Please send any new questions to

Q: How can I find out how full Henrys Lake is?

A: In 1924, the North Fork Reservoir Company constructed a dam on the outlet of Henrys Lake with the aim to increase water storage capacity of the lake to supply irrigation water to the St. Anthony area. This dam was rebuilt in 1964 with the total capacity of the lake at around 90,000 acre-feet today. The United States Geological Survey (USGS) maintains monitoring stations throughout the state to provide real-time/ recent surface-water, groundwater, or water-quality data. In cooperation with Water District No.1, the USGS maintains a monitoring station on the Henrys Lake outlet which tracks the current reservoir storage level (acre-feet) of Henrys Lake. All information and data can be accessed here for the Henrys Lake monitoring site:

The public is able to access the current and past water storage levels of the lake using the above website. You are able to look at the raw reservoir storage amount recorded every 15 minutes and graph the data to see how water levels have varied throughout the year and from year to year. Here you can see the storage (acre-feet) of the lake from January 2017 through to September 2020.
The water stored in Henrys Lake is utilized primarily for irrigation needs both around Henrys Lake and downstream. It is important to keep water high in the sub-basin as long as possible through the summer in order to supplement water needs in the lower reaches of the sub-basin, especially during years of drought. As such you can notice water levels begin to rise through the winter and spring as there is minimal draw down via irrigation waters and spring runoff begins. This continues through the beginning of summer until water needs increase throughout the sub-basin into the fall.

Creative Commons Licence

To specifically find out how full the lake is on a specific day, you can take the daily average reservoir storage and divide it by the total capacity of the lake (90,420 acre-feet). This value is also reported for Henrys Lake and various other bodies of water around Island Park by Island Park news each week.

You can access the June 2020 edition of our newsletter here:

Our next quarterly newsletter will be sent out in December 2020 so stay tuned!

Follow us on Facebook @ Idaho Fish and Game Upper Snake