Runoff, already?

With increased natural flow, elevated turbidity levels, and the disappearance of snowy horizons, runoff has signaled its arrival throughout much of the Henry’s Fork basin. Using Snow Telemetry (SnoTel) data, this blog will discuss how the current status of snowmelt and runoff in the Henry’s Fork headwaters compare to historic data and what this means for streamflow during the prime fishing time of mid- to late-June.

Snowmelt: Magnitude and Timing

As Rob mentioned in his post from February, there are four SnoTel sites that serve as the primary predictors for water supply in the upper Henry’s Fork: 1) Island Park, elevation 6290 feet, located near Ponds Lodge in Island Park, 2) Crab Creek, elevation 6860 feet, located in the Centennial Mountains just to the east of Interstate 15, 3) White Elephant, elevation 7710 feet, located on the east side of Mt. Sawtelle in Island Park, and 4) Black Bear, elevation 8170 feet, located just over the continental divide in Montana, between Big Springs and West Yellowstone. We can use the SnoTel data from these sites to anticipate how much water the snow will contribute to the basin when it melts – a time period referred to as “runoff.” Snowpack data is an integral piece of spring-time water management, specifically regarding dam outflow and the maintenance of reservoir levels for flood control.

So how are we faring compared to years past? The left panel of Figure 1 shows historic Snow Water Equivalent (SWE) data in inches compared to water years 2015 and 2016. Both of the lower elevation locations, Island Park and Crab Creek, tracked their respective 30-year means until April 1. However, there was a bit more variability in snow accumulation at the higher elevation locations. White Elephant fell below average in February and fluctuated between the 25th and 50th percentile until it reached its near-average peak on April 1, while Black Bear sat below average at the 25th percentile for much of the water-year. Fortunately, all four of the SnoTel sites have a larger snowpack than 2015 – the driest water-year since the early 1940s. Unfortunately, this snow is melting earlier at most of the sites.

This is evident in both panels of Figure 1. On the left panel, 2016 SWE at Island Park, Crab Creek, and White Elephant, began decreasing around April 1. There was a short melting reprieve and even a small gain in SWE due to a spring storm in early-April, but the general trend is downward. In fact, current SWE at the Island Park site is almost where it was at this time last year. Black Bear is remaining stable, possibly due to its high elevation, and has yet to decrease significantly.

While the left-side panel shows the magnitude, or amount, of SWE at each site, the right-side panel demonstrates timing of gains and losses, or accumulation and melting, in SWE relative to the water-year peak. Notice that the y-axis goes from 0 to 1. When the line reaches 1, the snowpack has reached its peak for the water year – it has accumulated 100% of its total snow-water-equivalent for that year. When the line is downward sloping and reaches 0.5, half of the total accumulated snow has melted, and runoff is in full swing. Both Island Park and Crab Creek have already lost more than half of their accumulated snow. Snow is currently melting about two weeks earlier than usual at Island Park and Crab Creek and four weeks earlier than usual at White Elephant– the current fraction of snow melt at these three sites is not usually seen until early to mid-May, depending on location. In fact, despite having more snow than 2015, we are melting at the same rate as 2015 at all locations excluding Black Bear. In other words, the fraction of snow currently left in the system is equal the fraction of snow left in the system at this time last year – a record drought year.

Translating Snowmelt to Streamflow

Figure 2 shows how snowmelt translates to streamflow. The left y-axis and associated gray lines depict cumulative snowmelt to date at Crab Creek while the right y-axis and blue lines depict watershed input, or streamflow, between Henrys Lake and Island Park Dam.

Streamflow between March 24 and April 1 of this year was about 150 cfs below average. This was due to the four preceding drought years, which contributed only a small amount of recharge to the groundwater springs that generate baseflow for the river reach between Henrys Lake and Island Park Dam. Substantial snowmelt at Crab Creek between April 1 and April 15 augmented these baseflows, leading to above-average streamflow for this time of year. While above-average streamflow is usually considered a blessing, it is not in this case. The current rate of increase in snowmelt and streamflow is not normal. In fact, 15% of the total April-to-June runoff typically occurs between April 1 and April 21, but over that time period this year, 21% of the total springtime runoff has already occurred.

When snow accumulation reaches its peak, the water that will melt from that snowpack is a fixed volume. Once it’s gone, it’s gone. The rate at which it is input into the system is important. Snow that melts early “borrows” water that would have been in the river later in the season. The above-average flows between Henrys Lake and Island Park Dam are a result of front-loaded snowmelt. Above-average flows now mean below-average flows between Memorial Day Weekend and the end of June.

Moving Forward

Moving forward, even if everything runs perfectly – if irrigation demand does not start until July, Island Park Reservoir fills, and the Bureau of Reclamation only passes natural inflow out of Island Park Dam – flows out of Island Park will be about 150 cfs lower  from June 10 to June 30 than if the snow had melted at its average rate. This means that instead of 500-600 cfs during this time of year, be prepared for flows as low as 400 cfs. At this point, rain will contribute more to plant evapotranspiration than to runoff, but it will also delay the need for storage water to meet irrigation demand until later into the summer.

With lower temperatures, cloudy skies, and showers forecast for the weekend in Island Park, we may get a short reprieve in the accelerated snowmelt and be able to stretch our runoff supply just a few days longer, increasing those mid-June flows a little.