As flood risk looms, inside the scientific guesswork that keeps Utah water managers up at night

Utahns rely on spring runoff to fill reservoirs that provide communities with the water they need for drinking, washing and working. For local water managers, that’s been a challenge in recent years amid extreme drought.

But when there’s an abundance of snow in the mountains — Utah just broke a record set in 1952 for snow-to-water equivalency this month with more than 29 inches — water managers must make the opposite calculation. How much water should they release to make room for the melting snowpack and prevent flooding, like the state saw in 1983?

Those decisions keep some water managers up at night. The Salt Lake Tribune talked with three organizations currently doing this work to better understand the process.

‘Pushing the limits’

To decide how much water to release from reservoirs — and when — water managers must analyze data from multiple sources, according to Jesse Stewart, deputy director of Salt Lake City Public Utilities. Salt Lake City Public Utilities oversees Little and Mountain Dell reservoirs in Parleys Canyon to provide the capital city’s drinking water and manage flooding.

Those sources include stream-gauge data that shows current water levels and flow, data they collect from snow cores, and forecasts and conditions from the Colorado Basin River Forecast Center, like snow-to-water-equivalents at “SNOTEL” measurement sites in the mountains.

Water managers also pay attention to soil moisture, or how much the soil is saturated with water. The higher the soil moisture, the less snowmelt it absorbs and the more water makes its way down to reservoirs. This year, according to a March report from the Department of Natural Resources, soil moisture is at 56% — about 4% more than normal for that time of year.

Salt Lake City Public Utilities has been releasing water from its reservoirs, primarily Little Dell, for the last several months after officials saw the snowpack was building, said Laura Briefer, the department’s director. While it’s normal to release water, Stewart said this year they “started earlier and will probably go a little larger and longer.”

Water storage capacity in Parleys Canyon is about 10,000 acre feet, but estimates on April 3 showed around 18,000 acre feet in this year’s snowpack — “and it seems to keep growing,” Briefer said. This year, she said water managers may release more water from Little Dell than normal because of anticipated heavy runoff.

Briefer said her staff make these decisions through a “data-centered process,” with frequent monitoring from people both in the office and in the field.

“We don’t have control over what the weather is going to do, and so we’re constantly making adjustments in terms of how much space we feel like we need to have for flood control in the reservoir,” she said. They ask questions like, “How much should we be releasing at any one time? What’s happening downstream from those releases that we need to be aware of?”

Water managers at the Weber Basin Water Conservancy District have also been letting water out of their reservoirs since January, with the biggest releases coming from Pineview and Echo reservoirs, general manager Scott Paxman said.

Paxman said as of April 3, officials are releasing 1,300 cubic feet per second of water out of Pineview, which can safely send out about 1,600 cubic feet per second into the Ogden River. By the end of the week, Paxman said they would be releasing around 1,500 cubic feet per second — “really pushing the limits.”

He said Echo Reservoir will have similar release levels, but their safe channel capacity is higher, around 2,250 cubic feet per second.

If spring runoff pushes water through the system beyond that safe channel capacity, Paxman said water managers can’t close the release valves to stop the flow. If they did, the reservoirs would overflow, and they would lose what control they have over the flows.

At that point, they would start working with emergency managers to notify folks living in flood zones of the imminent flooding.

“We’re really worried about the Lower Weber, of course, for flooding,” Paxman said, “because all this water has to go through there.”

Different calculations in southern Utah

Water management in the southwest part of the state looks a little different, because they have fewer diversion options, said Karry Rathje, spokesperson for the Washington County Water Conservancy District.

The district manages two major off-stream reservoirs that supply water from the Virgin River Basin for its cities, but those facilities can’t be used for flood control.

Water from that river is diverted at the Quail Creek Diversion Dam, built in the 1980s for snowmelt, but they can only move water from this system when there’s enough water to capture and, even then, only after more senior water rights are fulfilled.

“During high flood events, we’re not able to divert water into our pipeline — it would be like trying to drink water from a fire hose. Flood water flows downstream,” Rathje said. “We do not have any infrastructure to stop flooding events or move water in and out of reservoirs to control river flows.”

While they do have a few smaller on-stream reservoirs, once they fill, excess water would just go over the spillway or through a discharge valve.

That’s why the falls at Gunlock State Park have been running, Rathje said. The reservoir is at 106% capacity, according to the Utah Division of Natural Resources.

‘Plenty stressed’

While much of these decisions are made based on data, weather is not an exact science. It changes frequently, sometimes in ways forecasters don’t predict, and water managers must adjust based on its impulses.

And Mother Nature doesn’t work 9-to-5.

Briefer said Steward will “get up in the middle of the night with data running through his mind, and taking notes so that he doesn’t forget it” when he gets to work in the morning. Paxman said the staffer making those calculations for his water district is also “plenty stressed.”

Because despite all that work, even the best guesses can be wrong, and wrong guesses can have catastrophic consequences.

That’s why Salt Lake City officials have also been looking at a less quantitative data source to prepare for spring — footage from and explanations of what happened during the 1983 flood.

“Because there’s nothing that is better than looking at how the system behaved in real life,” Briefer said.

Leading into mid-May 1983, winter brought historic snowfall and a colder-than-normal spring. Then one weekend, temperatures soared in the 90s.

Water ran fast off the mountains and into downtown Salt Lake City, where residents kayaked and fished along newly formed waterways. Flooding damaged houses and washed away backyards as residents scrambled to fill and distribute sandbags, and workers built diversions to move the water out of the city and protect property.

With all that in mind, Briefer said this year, her team is going to err on the side of flood control with its reservoirs, instead of water supply, ”even if it means that we might underfill Little Dell reservoir at the end of the day a little bit,” she said, adding that “we still want to get it pretty close.”

As Salt Lake City and the Wasatch Range were hit with a multi-day storm last week that brought more snow in the first days of April than Utah had seen in the entire month since 1984 — and the coldest temperatures seen since 1922 — that caution may well pay off.