As snow keeps coming and water riches keep rising, Utah flood fears keep growing

Spring has barely begun, but water is thundering out of the Wasatch Mountains’ reservoirs as dam operators clear room to receive the liquid assets stored in a snowpack that is up to twice its normal size.

In coming weeks, that snow will turn to water and pour down canyons, bringing both the promise of increased supplies and the peril of flooding.

Water managers like to see the snow stay in the mountains as long as possible, but that can lead to floods if the weather quickly warms. Utah’s big snowpacks will continue growing at the upper elevations under a weather pattern that is expected to bring more snow and cooler temperatures through the end of April, according to speakers at Tuesday’s monthly water supply meeting hosted by the National Weather Service in Salt Lake City.

“High-elevation sites are going to continue to gain and the lower-elevation sites are really starting to lose,” said Troy Brosten, a hydrologist with the Natural Resources Conservation Service. “Now that we have the snowpack, we are worried about how it might come off.”

(Christopher Cherrington | The Salt Lake Tribune)

Coming after Utah’s driest year on record, the snow loads around the state cheer water managers like Scott Paxman, assistant general manager of the Weber Basin Water Conservancy District. But they will have a tricky balancing act in dam operations this spring, holding back sufficient water to ensure full reservoirs deep into summer, when water demand is highest.

“It’s a fine line,” said Paxman, whose district operates a string of reservoirs on the Weber and Ogden rivers, including Pineview at the top of Ogden Canyon. That drainage is expected to produce stream flows 163 percent of average this year, according to weather service hydrologist Brian McInerney.

“Right now, there is well over the volume in the snowpack that would fill that reservoir, even if it was empty,” Paxman said. “We try to forecast what releases we need to do to fill the reservoir, but we don’t want to flood [Ogden] Canyon either.”

In other words, the district wants to maintain enough room in Pineview so that it will not be forced to discharge dangerously high flows down the narrow canyon filled with homes. The reservoir, with capacity for 110,000 acre-feet of water, is about 60 percent full, and the district is releasing about 800 cubic feet per second, trying to match the volume coming in from the Ogden River’s headwaters. Paxman expects that flow to be increased in the coming days.

“We like to see the warm and cool because it prolongs the runoffs and decreases the chances of flooding,” Paxman said. “A lot you can’t anticipate. That’s why we have to keep room in the reservoirs so we can control those flows. We anticipate all our reservoirs to fill. It was a great year.”

Utah water managers remain concerned about the possibility of a chaotic runoff, like the one that occurred in 1983. That May, sandbag-channeled floodwaters rolled down Salt Lake City’s State Street and 1300 South.

“How long is the stormy weather going to continue and how is that going to affect peak flows?” McInerney asked. “If you look at the big flood years, it was due to a wet spring that just kept going. If you look at 1983, it was an average year, but they went all the way to the third week of May, and then it started melting. Could than happen? Probably not, but it’s a possibility.”

The threat of flooding is worth the trouble, considering the dire need for a fresh infusion of water into the state’s drought-stressed supplies.

On average, the state’s reservoirs are 65 percent full, and all but a few are expected to completely fill this spring, according to Gary Henrie of the U.S. Bureau of Reclamation. Red Fleet and Steinaker, both fed by streams off the Uinta Mountains, won’t fill, nor will Utah’s largest reservoir, the seriously depleted Lake Powell. It will bottom out at 36 percent of its capacity this month, but rebound to 48 percent this spring.

“It will take 10 or 15 good years for us to top it off,” Henrie said.

The 2018-19 water year will go into the record books as one of Utah’s best, nearly on par with 2010-11, but the dry soils, a vestige of last year’s scarce precipitation, will temper the runoff picture.

“February and March were phenomenal,” Brosten said. Across the state’s hydraulic basins, each of those months added 6 inches of water equivalence to the snowpack, up to 8 inches in some places.

“We have double the snowpack from this time last year. Across the state, we are 139 percent [of the 30-year average] with 21.1 inches of water,” Brosten said, noting that "2011 kept climbing through April and peaked in May. We have more wet weather coming. We could keep climbing and have a similar year.”

Not only is the wet weather continuing, but it also is cooling off as well, according to meteorologist Glen Merrill of the weather service.

“We are going to see snow drop down to the valley floors by tomorrow [Wednesday]. Midelevations could do decent as well. It’s pretty much a statewide event,” Merrill said. The current storm marks a pattern shift that could keep the snow in the mountains even longer.

“The bottom line is the temperatures are going to remain cool, if not very cool for the next 10 days and likely thereafter with periodic storm activity,” Merrill said. "Over the last month, in between storms, we warmed up quite a bit. In this scenario, there’s going to be a higher potential that we won’t warm as much between storm events. At the higher elevations, I don’t see much potential for those sites to lose ground. If anything, they are going to keep climbing.”

So with April’s expected activity, this year’s water story still has another chapter. While the prognostications are good, no one is sure how it will end.