California's big snowpack brings relief -- and flooding worries

california snow pack years compared elam pkg_00012728
california snow pack years compared elam pkg_00012728

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    California drought relieved by winter storm

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California drought relieved by winter storm 02:29

Story highlights

  • Scientists, skiers wowed by deep snowpack in California mountain's
  • While it's great for water supply, too much melt will bring flooding

Mammoth Lakes, California (CNN)After years of drought, this winter finally brought relief to California as storm after storm walloped the state, leaving it drenched with rain at lower elevations and covered in snow in the mountains.

While there certainly was a seasonal downside -- including flooding and concerns that a dam might fail at Lake Oroville -- residents are heartened by the highest snowpack level since 2011.
Why is that so important?
    That snow is key to California's water needs. The pack is made up of layers of snow that accumulate in the winter and spring in the Sierra Nevada. During the warmer months, the snow begins to melt, filtering its way down the mountains into creeks, streams and rivers and eventually into the Golden State's water system.
    About 80% of California's annual precipitation comes as snow.
    This season's snow is particularly dense and full of water.
    That's the good news -- and the bad. Officials are worried that the gush of water will put stress on infrastructure.
    "California winters are very changeable. You can go like we (had) two years ago, driest on record, to this year flirting for a while with maximum on record," said Frank Gehrke, chief of the California Cooperative Snow Surveys program for the Department of Water Resources. "Right now, we are looking at probably around 160% of average for this time of the year."
    A review of state data found that in 2016, the snowpack was up from 2015, but still below average (88%). In 2015, it was at 5% of the average.

    Skiers love it, and levels are staggering

    Despite some melting occurring in lower elevations, there is now so much snow at about 9,000 feet and higher in the central Sierras that CNN couldn't even return to a spot that could be reached two years ago.
    About this time in 2015, the snowpack measured a paltry 4 inches of snow. There was no need for a big coat and there wasn't any snow on the road.
    Fast forward two years. In the same location near Tioga Pass, the snowpack is more than 200 inches deep.
    "We are looking at probably not having the road open until maybe mid-July," Gehrke said.
    Now you see it (a Tioga Pass kiosk in 2015) and now you don't, except for an antenna.
    To further that point, just up the road from the closed gate, one can see there's been a rockslide. A giant boulder has settled in the middle of the temporarily closed highway.
    Rangers in Yosemite National Park recently trekked out to the kiosk at the top of the Tioga Pass. Their pictures show a drastic change from two years ago, when a visitor could walk right up to the building.
    Now the structure is completely covered in snow. "Except for the flagpole and the antenna, you would not know that there is a building there," Gehrke said.
    For skiers and snowboarders, the snow is a boon. At Mammoth Mountain Ski Area, nearly 250 inches of snow fell in January -- the most snow to fall in a single month on record.
    To determine just how much snow is in the pack, Tom Painter, principal investigator of NASA Jet Propulsion Laboratory's Airborne Snow Observatory, heads to the sky.
    "In 2013, we started flying the Airborne Snow Observatory essentially to take laser and spectrometers to map all the snow to be able to know the exact distribution across the landscape," Painter said, noting some of the remarkable depths of snow around Mammoth Mountain.
    "Some of the snow we are looking at out here, some of it goes deeper than 75 feet. And there are a lot of places that are much deeper than 50 feet all across the landscape."

    Great surface water levels, but groundwater needs to recover

    So does all of this snow mean California is done with the drought? Not quite.
    "We are not out of the drought, but we are out of the surface water drought," Painter said. "The groundwater that we have pumped so massively over the last several years during the drought to compensate for the lack of snowfall? It takes a long time for that to replenish. And so we need a lot of years like this to replenish that groundwater."
    A series of winter storms erased much of the drought that plagued California for nearly five years. The US Drought Monitor now shows no areas of extreme or exceptional drought, and moderate drought is mapped in only a handful of counties in Southern California.
    At its height, the "exceptional drought" area covered nearly 2/3 of the state and as recently as Christmas 2016, approximately 20% of the state was still in the worst of the drought.

    Rising concerns about spring flooding

    While millions of residents are happy for the rain and snow, the state decided to keep emergency drought rules in effect, at least through the spring.
    That decision came as officials grew concerned about the potential for flooding once the snow begins melting. California's aging infrastructure may be further taxed.
    Knowing just how much water will be coming out of the snowpack is key for preparation.
    "It's a matter or managing a big snowpack with full reservoirs and making sure there's not any downstream damage from high releases," Gehrke said.
    "We've gotten so much snow and rain in the lower elevations that the reservoirs have filled up. We know we have a ton of snow still sitting up there. Flooding is coming. It's going to be tough on the water managers this year to capture that water and lose a bunch of it," Painter said.
    To that end, Los Angeles Mayor Eric Garcetti declared a state of emergency over concerns of flooding homes and damage to the Los Angeles aqueduct as the snowpack in the eastern Sierra Nevada begins to runoff.
    The aqueduct is crucial for getting water to the Southern California municipality.
    Not all the snow in the Sierra Nevadas will melt this summer. But there's no way of knowing when another snow-rich winter will happen in California.
    "The snowpack is the critical resource in the western United States," Painter said. "Civilization is built around it existing and replenishing groundwater and making that water supply available. So we have to take care of it and we have to understand how it is changing if we are going to survive out here."