Headlines: Water temperatures through the Ranch have been on the warm side. Temperature has climbed above 71˚F for a couple of hours each day over the last week. However, daily maximum temperatures have not been deadly for rainbow trout, and only a few daily averages have been at the lowest levels that would stress fish.  This is the same pattern we saw in both 2015 and 2016, when streamflows were about 800 cfs higher than current flows. Thus, releasing additional water from IP dam does not result in significantly cooler summertime water temperatures, but we do know that increasing flow out of Island Park Dam decreases water clarity and will reduce winter flows, which reduces survival of juvenile trout next winter and hence recruitment of 2-year old fish into the population in 2019.

There have been some concerns expressed to us about water temperatures being too high through the Ranch section of Harriman State Park. This blog discusses what our water quality monitoring data collected from Pinehaven (just below the Ranch reach) can tell us about current conditions in the context of trout habitat requirements and recent in-house statistical analyses.

Figure 1 displays water temperature data taken every 15 minutes by our Pinehaven sonde from 2015, 2016, and 2017. Daily temperature fluctuations are clearly visible. The background colors provide habitat requirements based on thresholds of lethal (red), stressful (yellow), good/sub-optimal (light green), and optimal (green) temperature conditions for Rainbow trout (references below). Thresholds that apply to development and survival of eggs and sac fry are plotted from April 1 through June 1 and include the period from spawning to emergence for Rainbows. Thresholds shown during the remainder of each year apply to adult and juvenile requirements for survival, growth, and activity.

First of all, notice the temperature data are remarkably similar across years. Each June through July since 2015 we have observed days when the daily maximum temperature exceeds 70˚F (the lower bound of the yellow region in Figure 1).

Over the last three years average water temperatures have been very good for trout during the summer months. Notice all the time spent in the optimal range for growth and activity (bright green area, Figure 1). Water temps do get a little on the warm side for a few days each summer and recently temperatures have certainly been getting up to 70-72 ˚F during the warmest hours of the day. We see this has happened every year since 2015. So far, this summer looks almost identical to the previous two years, as far as water temps go.

Discharge out of Island Park (IP) reservoir, on the other hand, has been drastically different during late June and early July this year compared to the last two years; streamflow below IP dam has been less than 700 cfs this week compared to streamflow of over 1500 cfs during the same time period over the last two years (Figure 2).


That means that water temps this year have been almost identical to what we’ve observed over the last two years even though streamflow has been 800 cfs lower this year. Thus, this provides poor evidence for increasing reservoir discharge in order to potentially cool water temperature through the summer.

Figure 2 note: the dashed line is Idaho D.E.Q.’s regulation maximum daily average temperature for cold water species (reference below).

Increasing flows now will, however, negatively impact the fishery in four key ways: high flows lead to low reservoir volume and higher turbidity (when outflow goes through the USBR gates); low summertime reservoir volume leads to higher turbidity and suspended sediment, and lower trout population size. The former conclusions regarding water quality are the result of statistical analysis using our water quality monitoring data going back to 2013 (forgive me, I’ll post more about my analyses soon, but if you’ve caught any of my presentations in the last 12 months, these results sound familiar). The latter conclusion about population size refers to our result, using XX years of data, that increased winter flows result in increased juvenile habitat, which supports better over-winter survival of these fish. These juveniles then recruit into the fishable population two years later. Bryce has mentioned these results here (see Figure 2 in that blog) and will also post more soon. For example, if outflow were increased by 800 cfs for the next 2 weeks, similar to the flows we saw last year at this time, that would reduce flows during the three coldest winter months by about 124 cfs! That is a huge amount of lost wintertime flow for trout especially considering flows out of the reservoir were approximately 174 cfs last winter.

HFF’s mission is to promote wild trout population numbers and to protect and conserve this unique and beautiful fishery. Our data and research show that current average water temperature through Harriman and Pinehaven, while on the warm side, are well within the tolerable range for Rainbow trout and no warmer than what we’ve observed over the past two summers when Island Park outflow was more than double that over the past two weeks. What’s more, releasing additional water from IP reservoir may not appreciably reduce summertime temperatures, but it will increase turbidity and suspended sediment, and reduce survival of juvenile trout next winter and hence the number of 2-year old fish joining the population in 2019. If you are personally concerned about the effects of current water temperatures on trout survival, one option would be to fish during the morning and early afternoon, and avoid fishing during the time of warmest water temperatures, which is between 6 p.m. and dark.

As part of our on-going water quality monitoring and research, as well as research being done in collaboration with Jack McLaren—a graduate student from Indiana State University—we are already working on further understanding the relationship between volume and outflow from IP reservoir and downstream water temperature and will be providing additional information in the forth-coming months. Stay tuned!


Raleigh, R.F., L. D. Zuckerman, and P. C.Nelson. 1986. Habitat suitability index models and instream flow suitability curves: Brown trout, revised. U.S. Fish Wildl. Serv. Biol. Rep. 82(10.124). 65 pp.

Idaho D.E.Q. (2015, October). Stream Temperature Standards. Retrieved from http://deq.idaho.gov/water-quality/surface-water/temperature.aspx