Now, two native bark beetle species which have swept the state's forests in an unprecedented outbreak lurk just over the ridge from Big and Little Cottonwood canyons, keeping four resorts and Utah's biggest city on alert.
"It's almost as if it's coming for us," said Maura Olivos, an ecologist for the Alta Environmental Center. She scanned a U.S. Forest Service map of bug-infested trees a limber pine here, five Engelmann spruces there to see an arc of coordinates closing in on the canyons.
The beetles have two things working for them.
One is their sheer volume, the largest in recorded history, having exploded from eruptions of mountain pine beetles and spruce beetles in lower forests that were left old and ripe by fire suppression and lack of logging.
The other is a warming environment. The Forest Service's Rocky Mountain Research Station warns that the bugs are developing faster and thriving at higher elevations.
Olivos is armed with several tools to fight back.
She has planted a seedling orchard and has help from TreeUtah in adding young trees that are not favored by pests. She has Forest Service support throughout the canyons, including aerial surveys and chemical applications on some vulnerable older trees. And she has technology: a panoramic camera system allowing her to zoom in on stricken trees across the mountain, then cut and burn them and the beetles they host.
The Heber Valley is just over the east slope of the Wasatch Range, and trees between there and here are red and dead. Alta's younger trees are perhaps more resilient, but some in the higher elevation have suffered attacks and raised fears.
"We just don't have a lot of trees to begin with," Olivos said. "[Forest composition] is working for us, but we don't have a lot of spruces. If we have an infestation, that will work against us."
Salt Lake City is building contingency plans.
"Alta is for skiers." That slogan is true, but incomplete. These mountains also feed the city's water-treatment plants after the trees do their work. It was a problem a century ago.
"That's why the water in Salt Lake was so bad no trees," TreeUtah Executive Director Amy Collins said during a recent planting at Alta.
A hundred years ago, Salt Lake City Public Utilities Director Jeff Niermeyer explained, Salt Lakers dealt with milky water and the bacteria that it flushed from surface soils into the creeks.
Today, the city boasts modern treatment plants, but they likely would be overwhelmed by spring runoff if not for the trees. Plugged filters and swamped settling ponds could necessitate plant upgrades costing tens of millions of dollars.
Denver knows, Niermeyer said. A forest fire there spilled 50 years of sediment into the system.
A rapid runoff also would mean less water for the city later in the year, unless taxpayers or ratepayers funded construction of storage reservoirs. Under the current configuration, most of the water would gush past and evaporate from the Great Salt Lake.
"We're basically planning that we may have these larger turbidity events because of forest fires and climate change and its impact on vegetation in the canyon," Niermeyer said. But he hopes forest managers can prevent a worst-case scenario.
Hope in the hills
There is reason for that hope. The pace of dying forests here has slowed in the past year or so, largely because most of Utah's mature pines and spruces already have succumbed, said Liz Hebertson, an entomologist in the Forest Service's Ogden office.
Numbers from this summer's aerial surveys aren't yet available, but various beetle species struck about 95,000 acres of Utah forests last year, according to a Forest Service report. That brings the total since the outbreak began in 1997 to roughly 2 million acres an area about the size of Utah County.
Although much of the state's potential damage is done, some areas seeing new attacks this year are highly visible. For instance, Hebertson said, spruce beetles killed about 2,000 trees this summer at eight campgrounds on the Mirror Lake Highway.
"They provide the aesthetics that people have long gone to these campgrounds to enjoy," she said. "These are big, old, mature spruce trees that added a lot of character."
The Uinta-Wasatch-Cache National Forest's response was to shift scarce dollars from the Cottonwood canyons to those campgrounds. The forest had planned this fall to spend $30,000 in Big Cottonwood and $10,000 in Little Cottonwood home to Alta hosing the repellent chemical Carbaryl onto tree trunks around picnic grounds and other recreation zones. Instead, they directed that chemical to at-risk trees on the Mirror Lake Highway.
It means that when beetles fly next July, the trees in Salt Lake City's watershed will be mostly unprotected a calculated risk based on the beetles' recent movements.
The Forest Service will continue removing infested trees in the canyons.
"The hope is we never let the bugs get a real good foothold in the Cottonwoods," Hebertson said. "The concern is that this is just a Band-Aid."
Volunteers planted 1,800 trees at Alta during a TreeUtah outing last month. It was Creighton Hart's sixth such trip up the canyon.
"I'm a 30-year Alta skier," he said while packing the roots under a shin-high Douglas fir something that wasn't widely replanted in Alta's 20th-century reforestation. "It's kind of a sacred home for us here on the Wasatch."
Most of the seedlings came from a nursery that supplies common natives such as firs and spruces. In the future, Olivos hopes, volunteers will use the nursery she started last year under the Albion Lift to restore the rarer limber pines that are threatened atop the mountain.
"They're helpful in soil retention and snow retention," she said of the gnarly species that is particularly stricken above Big Cottonwood Canyon. "But really, they just belong here. It's for a healthy, diverse ecosystem."
Without that, Salt Lake City's water future is murkier.