Fall-Winter River Flows and Drought Management Planning Update

What flows can we expect out of Island Park Reservoir this fall and winter?

At last week’s Drought Management Planning meeting, stakeholders agreed to a reduction in flows out of Island Park Dam, with a goal of 60 cfs outflow by mid-October. These stakeholders include Fremont-Madison Irrigation District, the Henry’s Fork Foundation (HFF), Bureau of Reclamation, Water District 01, Fall River Rural Electric Cooperative, North Fork Reservoir Company, Idaho Department of Fish and Game (IDFG), the Nature Conservancy, Trout Unlimited, and representation from the outfitting community as well as HFF’s Board of Directors. This process began over the weekend with a reduction in outflow by 200 cfs, and additional reductions of approximately 50 cfs will occur each week after that.

As IDFG Regional Fisheries Manager Dan Garren explains, reducing flows in the fall before temperatures drop allows us to store substantially more water in Island Park when it is less critical for fish, and then release more water when the really cold temperatures reach us. IDFG research shows that higher flows during the coldest parts of the winter benefit our young trout, increasing survival and making fishing better in following years. Although the water outlook is not great for the Henrys Fork this year, the recent agreements between the different entities at the DMP meeting provide the best case scenario for using what little water is available to benefit both fish and other water users.

Projections indicate that based on the current reductions in outflow, we have a good chance of 120 cfs outflow in mid-winter, nearly equal to that of last year. While this is not optimal, it is quite an achievement considering how low the reservoir is compared to last year.

On another positive note, winter inflows will be 20 cfs higher this year than last year thanks to a better snowpack in 2016. That improved snowpack may not have been evident in the river this summer, but it did soak into the ground and will reemerge as flows out of Big Springs this fall and winter. NOTE: Some might wonder why the goal was set at 60 cfs, rather than the 80 cfs value we have become accustomed to hearing about as the “minimum” possible release out the reservoir. The answer has to do with the minimum opening setting of the gates on the dam and the elevation of Island Park Reservoir. This year, the minimum volume at Island Park Reservoir reached 20,436 acre-feet [on Sept 14th at 10:45 am]. For comparison, this time last year, the reservoir reached a minimum volume of 38,621 acre-feet. The gates at Island Park Dam have a threshold setting at which they are either closed or set at a minimum opening. So, last year when the reservoir was at a level of 38,621 acre-feet, setting the gates at this minimum opening resulted in a minimum flow of 80 cfs. This year, there is less water in the reservoir and therefore less pressure above the gates, so setting the gates at that same opening will result in a flow of around 60 cfs.