It’s no secret that 2018 has been much warmer than we’re used to (in January, 5°F warmer than normal across the whole watershed and as high as 7°F above average in Island Park). At the same time, HFF has been reporting SWE (snow-water equivalent) numbers at 96% of average at the end of January (as high as 103% of average this past week), above average streamflow, and near average precipitation across the watershed (not to mention 111% of average Jan. precipitation at Island Park). If you’re looking out your window in Island Park right now, you’re probably wondering, “How is that possible?”
Here’s the deal. This is a good news, bad news situation. The short version is: 1) we’re concerned about early runoff on the Fall and Teton Rivers; but 2) there is a good amount of moisture up high (that SWE number) AND 3) these high winter flows and warm temps are just about the most ideal conditions you could ask for in terms of overwintering survival of juvenile trout (READ: we should have a banner recruitment year in 2019).
Let’s work through this in chronological order. First, it snows (SWE), then trout find creative ways to survive the winter (and Henry’s Fork trout are nothing if not smart), and finally, runoff happens – the timing of which can impact summer fishing conditions.
First, it snows.
This winter we got a bunch of wet snow. Not ideal for skiing and not ideal as a snowpack because it’s already primed and ready to melt. This wet snow is already at risk of melting faster than we’d like. Also, HFF’s data shows that despite watershed-wide SWE being 96% of average at the end of January -- that SWE number we keep reporting is averaged across the entire watershed, everything from Island Park to Grand Targhee -- drill down and you’ll find that lower elevation sites (the areas most of us can see out our window) are quite a bit farther below average. This trend does not exist when we look at precipitation amounts (they’re consistent across elevations), so this is a temperature issue. Luckily, this low elevation snow doesn’t matter as much – our water supply is primarily up high.
So, wet snowpack, bad skiing conditions, risk of melting too soon, but still good water content in the snowpack up high where it matters.
Then, trout find creative ways to survive the winter.
Our trout are as crafty as they come, but this winter they don’t really have to be. Like we said above, so far 2018 has been warmer than average and streamflows have set records. Winter outflows from Island Park Dam are the highest since 2011-2012, averaging over 500 cfs (that’s not including the Buffalo River that adds another 200 cfs or so before the river gets to Last Chance).
So, warm temps and high flows mean plenty of cozy habitat and a pretty optimal winter for survival of juvenile trout.
And finally, runoff happens.
So here’s where we get a little bit nervous (Mother Nature is in charge). The timing of runoff is crucial. Specifically, how long the Fall and Teton River snowpack can hold up. Some might remember HFF going on about how the Fall and Teton River flows carried us through 2017. The timing of runoff on the Fall and Teton Rivers and how long those streamflows last into the summer make a huge difference in holding off the need for delivery from Island Park Reservoir. So, as long as those two rivers can hold on and serve as a natural water supply to meet irrigation demand, we won’t need to deliver storage water from Island Park Reservoir.
Think about summer of 2017 and 2016 as examples. In 2017, runoff peaked 2 weeks later than average (Yay!), and Fall and Teton River flows met irrigation demand all the way to mid-July. In 2016, however, runoff peaked 3 weeks earlier than average, which meant natural water supply ran out and storage delivery from Island Park Reservoir was needed in June, leading to flows closer to the 1,100 – 1,200 range around the Ranch opener.
Another way to think about this is that when the snowpack melts early (early runoff), that water is coming down the river too soon to be used for irrigation, so we lose water that could have met demand in May or June if it had melted later. Early runoff means an earlier switch to storage water delivery from Island Park Reservoir.
Here’s the take-home.
Snowpack is primed to melt early and if the Fall and Teton River snowpack runs off early, storage water will have to be delivered early and less water is left in the reservoir for winter flows next year. On the plus side, we still have good SWE up high and our Henry’s Fork trout are experiencing some pretty prime winter conditions for survival of juvenile trout.
So, what are we hoping for? Well, more snow is always nice, but at this point, colder weather is as important as wet weather.