JACKSON, Wyo. — Spring has arrived and it’s time to start thinking about how this year’s snowpack will affect the upcoming whitewater rafting season. Read on to learn how winter snowpack, spring temperatures and the timing of snowmelt all influence the spring runoff, then submit a prediction for the 2020 high mark through Dave Hansen’s High Water Heydey Contest for a chance to win a new raft!
Currently, we can see that snowpack is well above average across the Snake River Basin based on snow-water equivalent measured hourly at numerous locations in the surrounding mountain ranges.
We examined snowpack data from the Rendezvous Bowl Plot at Jackson Hole Mountain Resort (elevation 9,580 feet), stream height and cubic feet per second (CFS) data from the Snake River gauge near Alpine and temperature anomalies across Teton County.
Here is a chart showing the last 10 years of snowpack, temperature and river level data as well as long-term averages from the past 20 years. Take a close look and you may notice several trends that emerge.
Peak winter snow depths relative to average
First, there is the obvious connection between peak snow depths and peak river levels. Over the previous 10 years (not including 2020), snowpack was above average in five years and peak river levels were above average in four of those five years.
The only year in which snowpack was above average and peak river levels were below average was just last year, in 2019. The explanation for the lower stream levels in 2019 is that temperatures were colder than average temperatures in May, which slowed the rate of snowmelt.
Spring temperatures relative to average (particularly in May)
Spring temperatures have a significant influence on snowmelt rates, which in turn influences river levels. Snowmelt rates are at their highest during the month of May, so temperatures relative to average during May are the most highly correlated to river levels.
Years with near or above-average temperatures in May following a winter with above-average snowpack tend to result in the highest river levels, which was the case in 2014, 2017 and 2018.
WHEN snowpack reaches its maximum depth
The other key factor that influences river levels is when the mountain snowpack reaches its peak. Years when the snowpack reaches its peak after April 1 tend to result in higher river levels compared to years when snowpack peaks earlier.
There are two extreme examples in the chart above as well — 2010 and 2011. In both of these years, snowpack did not reach its peak until May, and river levels were higher than average in both of these years despite cooler than average temperatures in May.
This makes sense because there was a huge volume of snow to melt in May and June of both of these years due to the delayed start of the melt season.
Predicting the High Water Mark for 2020
The good news for whitewater enthusiasts is that two of the three criteria for above-average river levels have already been met. Snowpack was above average this year and reached its peak on April 16, which is about two weeks later than average.
The remaining question mark is what happens with temperatures over the next six weeks, and particularly in the month of May. Remember, last year well-below average temperatures in May negated a heavy snow year and contributed to a low runoff season.
Long-range weather forecasting beyond two weeks out is difficult, to say the least. NOAA’s latest long-range outlook is predicting near average temperatures during May, which would favor a higher runoff.
However, the European weather model, which is considered one of the more reliable models, is predicting cooler than average temperatures during May.
Using the above data and based on the average river high mark of 19,218 CFS, what do you think the high mark will be during 2020?
Enter Dave Hansen’s High Water Heydey Contest to guess the highwater mark for 2020 and for a chance to win a NRS Otter 140 Self-Bailing Raft! The deadline to submit entries is May 15.
Here are some resources to follow this spring to track snowmelt and streamflows:
Snake River near Alpine – Current and Historic Flows
JHMR Current and Historical Snow Data
In addition to Jackson Hole Mountain Resort, SNOTEL data is useful to follow to give an overall idea of the snowpack in the surrounding Teton and Gros Ventre Mountains that drain into the Snake River.
SNOTEL Data for Northwest Wyoming
