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FAQ: Do Low Flows in the Fall Negatively Affect Trout Populations from IP Dam-Riverside?

Updated: Oct 3, 2023



Research done by ISU, MSU, IDFG, and HFF have shown that the single biggest factor limiting the fish population downstream of IP Dam is December-February streamflow. Juvenile trout survival during the winter is higher at higher flows because of better habitat availability. There is no indication that low flows during the late fall have a negative effect on the fishery.


The Strategy


Maximizing winter outflow helps to maximize trout abundance. There is a fixed amount of water to fill the reservoir during the winter and spring. It is optimal to lower the outflow in the fall to store as much water as possible before December. Outflow can then be increased to maximize winter outflow. The winter flow created through this strategy is higher than it would be if the reservoir was filled using a constant outflow all fall/winter.


Additional Benefits: This strategy benefits the hydroelectric power plant, which can generate power throughout the winter when it is more valuable. Water users are also benefitted basin-wide since as much water as possible is stored during the early fall.


Can Low Fall Outflows be Counterproductive?


The above-mentioned researchers have all looked carefully at this issue over the years, and we continue to do so every year. To date, the collective group of researchers has yet to find any statistical evidence relating flows during the fall to the size of the rainbow trout recruit class two years hence.


Here are two reasons why:

  • Trout are more active during the fall and can better avoid predators during this time. Because trout move very slowly when temperatures drop, they are more vulnerable to predators in the winter. To avoid predation, they require cover, allowing them to hide. More streamflow means more cover, which allows a larger number of trout to survive the winter. Temperatures in both the Island Park outflow and the Buffalo River stay above 40 degrees through mid-to-late-November, before temperatures drop into the 30s.


  • River depth in the fall (due to aquatic plants) is the same or greater at 200-300 cfs than at 400 cfs in the winter (when there’s no aquatic plants). River depth is maintained by a combination of aquatic vegetation and streamflow. During the fall, there is enough aquatic vegetation present to create more depth in the river, whereas maintaining that same depth in the winter requires a higher outflow than in the fall.




This means that increasing winter flow is the only way to create more habitat and more trout.


What Happens if there is an Early Cold Spell in November or if Winter Starts Later than December 1st?


In both of these cases, the Drought Management Planning Committee can - and does - make small adjustments to timing and the amount of winter flow as conditions unfold. However, the science has proven to be accurate over the long term and has shown that maximizing December-February outflow (even if that requires low outflow during October and November) will produce the largest trout population.



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