New! Quick reference poster of water quality on the Henry’s Fork

To support our water quality monitoring efforts, we have a new quick reference poster in the office! For now, the poster displays the recent, daily range of dissolved oxygen (averaged over the previous 4 days) and the weekly average temperature from our sampling/sonde sites. We have seven sites that we capture data from; I download data from 3 of the sondes one week, then retrieve data from the remaining 4 sondes the following week.  Therefore half of the displayed data are from the previous week, and the other half of the site data are two weeks old. The poster is hanging in the back of the lobby and displays a map of the Henry’s Fork with the water quality data labeled by location, feel free to stop by and check it out!

We display temperatures in Celsius and dissolved oxygen in mg/L. To help you convert Celsius to Fahrenheit, recall that 20˚C = 68˚F; 10˚C = 50˚F; and 0˚C = 32˚F. It may also be useful to know that optimal temperature for trout growth and activity are between 10–20˚C (50–68˚F). Interpreting dissolved oxygen levels generally relies on understanding how much oxygen trout need at different life-history stages. Average optimal levels of dissolved oxygen are 7mg/L or greater and levels below 3mg/L are very stressful for trout and may even be lethal if trout cannot find better habitat. Dissolved oxygen levels of between 4–6 mg/L mark a threshold between stressful and optimal since oxygen requirements depend on water temperature, where trout require higher levels of dissolved oxygen at higher temperatures. Trout eggs require much higher levels of dissolved oxygen as less than 11mg/L can delay hatching and less than 8mg/L can impair growth and survival. A visual scale with this information on how different ranges of dissolved oxygen and temperature impact trout will be coming soon!

I attached a picture of the poster with the current data labeled. You can see that river temps are slightly cooler north of Ashton Dam, with the exception of below IP Dam, where water has a chance to interact with the air and warm up during its slow transit through the reservoir. As the weather turns cooler, we’ll see this warming effect disappear.

Last week we saw the impact of turbidity mixing air into the water at the outflow points of Island Park Reservoir and at Mesa Falls so that the IP Dam and Marysville sites had higher levels of dissolved oxygen, between 10–14 mg/L. The outflow point for IP Dam was temporarily transferred from the power plant to the gates side over the last couple of weeks while the power plant was undergoing work. Outflow from the gates side mixes oxygen into the water as water gushes out of it, but once outflow returns to the power plant side, we expected dissolved oxygen to decrease downstream of IP Dam. That’s what we see this week: average, daily range of dissolved oxygen decreased from 10–14mg/L last week to 9–11mg/L this week. We’re working with Fall River Electric to see what can be done to increase oxygen levels in the outflow from the reservoir through the power plant.


Raleigh, R.F., T. Hickman, R.C. Solomon, and P. C.Nelson. 1984. Habitat suitability information: Rainbow trout. U.S. Fish Wildl. Serv. FWS/OBS-82/10.60. 64 pp