Current Henry’s Fork Water Update

As the current water year has developed, it has become even more obvious that the Henry’s Fork Basin is changing. Taking a look back, these past four years on the Henry’s Fork have been four of the driest consecutive years on record since the late 1930s and early 1940s. The Foundation has been sharing statistics on this trend for over 18 months. That is our current reality. It is also not as simple as conditions just being “dry”. The true measure meaningful to growing fish and irrigating crops is water yield into the river. For reasons including lower average snowpack, earlier runoff, maturing forests (forest succession), and longer growing seasons, the Henry’s Fork watershed is not the same as it was in the 1980s and 1990s, much less the 1970s. According to the Bureau of Reclamation, total water yield has been only 64% of normal during the current water year. Water yield is the runoff from the drainage basin, including ground-water outflow that appears in the stream plus ground-water outflow that bypasses the gaging station and leaves the basin underground, or stated differently, water yield is the precipitation minus the evapotranspiration. Sixty-four percent of normal represents a difficult situation for every stakeholder on the river, including anglers. In fact, a look at the records exhibits another startling fact. The Henry’s Fork has experienced only one “great” water year in the last 16 years. It was 2011, and anglers are still living on the last remnants of that year’s bounty in regards to large fish. All of the empirical data indicate that we are experiencing significant impacts on the Henry’s Fork due to a changing climate.

A focused topic this summer has been the condition of Island Park Reservoir. As you probably read, a cyanobacteria bloom, combined with a perfect storm of unfavorable outflow conditions, caused very off-colored water for a good portion of July. A previous statement from HFF covered this topic and the topic of high irrigation delivery earlier in the summer than normal.  We have also dealt with high early-season water temperatures in the watershed, even above Island Park Reservoir. Temperatures now seem to be moderating as the nights get longer.

HFF’s attention has now shifted to the level of Island Park Reservoir at the season’s end.  Understandably, we began receiving calls in late July as the level of the reservoir seemingly rocketed towards empty. At a recent Drought Management Planning meeting, local stakeholders, including HFF and Fremont-Madison Irrigation District, agreed upon a goal of 20,000 acre-feet (15% of capacity) for the minimum pool in Island Park Reservoir. The Bureau of Reclamation is still working to attain that figure as the final level. They must navigate a litany of demands on the Upper Snake reservoir system from Jackson Lake to the Magic Valley. HFF continues to strongly advocate with BOR, and other water managers, the importance of keeping Island Park Reservoir as full as possible this fall. By comparison, Island Park Reservoir most recently bottomed out below 20,000 acre-feet in 2007, 2003, 2002, and 2001. A similar situation was averted the past two years because of uncommon amounts of rain in August 2014 on the lower Snake River Plain and a perfectly timed wet spell basin-wide in May 2015. We have not been so lucky in 2016.  

HFF is also monitoring water quality, specifically the mobilization of mineral sediment from the reservoir downstream into the river. We expected and saw a slight increase once the reservoir dipped below 40,000 acre-feet. Another small, but currently dwindling, cyanobacteria bloom also contributed to a spike in turbidity last week. This week has brought on new conditions. Continual reduction in reservoir outflow over the last 5 days has ushered in much clearer water below the dam, and this has been confirmed by lower turbidity readings on the HFF water quality monitoring sondes. A pattern seems to be developing wherein the closer Island Park outflow (currently around 800 cfs) is to inflow (currently 525 cfs), the less sediment of all types moves into the river. This is a welcome development and one we will continue to monitor. However, HFF still expects higher turbidity in the river as we approach 20,000 acre-feet.

For those travelling to the Henry’s Fork, consistency in angling conditions has been hard to find, and that will continue to be the case for the remainder of 2016. People stopping in the office have indicated that they had the worst trips of their life on the Ranch, while a few anglers reported the best days of fishing they have ever had on the Ranch. Fishing has been very dependent on the exact day and even the time of day folks were on the water.

Upcoming winter flows for 2016-2017 are also on everybody’s mind, since winter flow drives the wild trout population below Island Park Reservoir. Calculations made in the last few days have provided a small amount of hope for this upcoming winter, compared to projections earlier in the week. HFF staff have recently had grave concern that we would see no more than 80 cfs released from Island Park Reservoir all fall and winter based upon a number of water availability constraints. New information suggests that with cooperation through the Drought Management Planning process, we can do a little better. We will provide a detailed look at winter flow in October. A copious amount of fall rain would also help!

It has been suggested that we should shut off flows from Island Park Dam in late October and November to accumulate more water for flows during the coldest part of the winter. The result is that Harriman State Park would have only the 175 cfs from the Buffalo River running through it for 45 days, and the river would be dewatered completely from the dam to the Buffalo River confluence. HFF staff believes in considering every idea and weighing the costs and benefits. However, after considerable discussion and consideration, we believe that a minimum outflow of 80 cfs throughout the fall and winter is a worthy goal that would at least maintain a base level of ecological integrity in the Henry’s Fork. We do not believe that the chance for a few extra fish due to higher juvenile trout survival over the winter would be worth the cost to the entire ecological system of a period of zero outflow this fall.

Hopefully, this update provides the information our HFF members are currently looking for. In the future, look for more information on our blog, Facebook page, and Monthly Hatch about the changing conditions in the Henry’s Fork Basin and across southern Idaho due to both climate change and shifts in water use and management. This summer has provided additional confirmation that Island Park Reservoir and the Henry’s Fork are not operated in a vacuum, and thus HFF must be capable of and willing to engage all water users and managers across the state to determine the best methods to manage water for the benefit of all. Since the vast majority of water rights in the upper Snake River basin—including groundwater on the Snake River Plain--are owned by irrigators, hydroelectric companies, domestic users, and municipalities, HFF will be challenged to continue to provide the best data possible and collaborate strategically with multiple stakeholders to benefit the Henry’s Fork.