Runoff forecast to be steady

By Emily Wolfe Explore Big Sky Managing Editor

Streamflow forecasts are well above average in much of Montana this year, with the statewide snowpack at 152 percent of normal and 241 percent of last year as of May 22, according to data from the Natural Resources Conservation Service in Bozeman.

This is due largely to the heavy snowfall that blanketed most of the state in February and March, followed by cool spring weather, said NRCS hydrologist Lucas Zukiewicz.

“We’ve really seen the most ideal scenario play out this year, where we see warm sunny days followed by a period of cool and cloudy days,” Zukiewicz said, referring to the weather in April and May, which delayed the start of runoff at higher elevations.

Locally, the snow-water equivalent in the Gallatin River Basin peaked in April, and while it has seen some snowmelt at the middle and lower elevations since then, it accumulated at the upper elevations simultaneously.

“The Gallatin is kind of an anomaly right now in the state, melting a lot slower than many other basins,” Zukiewicz said.

Driven by meltwater from the southern Gallatin Range and the east side of the Madison Range, the Gallatin is largely unrestricted as it flows down the canyon. It began rising visibly during the week of May 19, with the river running at 3,870 cfs at the gauge near the north end of the canyon on May 23 .

“Compartmentalizing snowmelt is better,” Zukiewicz said, explaining that when the low- and mid-elevation snow melts out in phases, instead of all at once, it brings waves of moisture down through the water system.

“Considering we had so much snowfall all at once this year, [that] keeps it from being one big wave of water all at once, which is what happened in 2011, because [runoff] was so delayed.”

Zukiewicz says excessive spring precipitation that year delayed runoff and built up the snowpack, so when it did melt, it did so in a spike, causing rivers statewide to overflow their banks in May and June and incite widespread flooding.

“That was way out of character for our typical weather patterns,” he says. “We haven’t seen near those conditions this year.”

As of May 1, streamflow prospects remained at 146 percent of average statewide, and 152 percent of last year, according to the Water Supply Outlook Report. Zukiewicz’s Montana Snow Survey team uses data from the automated SNOTEL site, and from the manual snow course sites measuring snow water equivalent at select locations in the mountains, to compile the report, which projects total water amounts released from major river basins statewide from May through July.

In the Gallatin Basin as whole, the streamflow is forecast to be 136 percent of average, and 189 percent of last year. The Gallatin River at Gateway is forecasted to receive 130 percent of average flows, and 163 percent of last year during the May-July time period.

The extreme headwaters of the Jefferson River Basin had the lowest forecasts in the state, indicating 72 percent of average May through July flows for Lima Reservoir Inflow and 80 percent for Clark Canyon Inflow.