Melting Sierra snowpack may pose risk, environmentalists say

SAN FRANCISCO (KTVU) -- The rainy season for the Bay Area has ended but there are new concerns about the massive snowpack that currently covers the Sierras and whether it poses a hazard for some California residents.

Officials with the California Department of Water Resources say as of April 1, 2017 there is 24 million acre feet of frozen water in the form of snow and ice that is currently sitting on the Sierra mountain range. An acre foot covers one acre of land and is one foot deep.

A NASA satellite offers a vivid scene of how the drought in 2015 compares to this year's massive frozen tundra in the Sierra.

"By any measure, the snow pack this year is huge," said Peter Gleick of the Pacific Institute, a leading water think tank. "It's one of the largest on record."

In comparison, the 24 million acre feet of ice currently sitting in the Sierra far exceeds the 20 million acre feet that fills California's nine largest mega dams, which are critical to flood control and are mostly full.

Environmentalists say the best case scenario would be a slow melt.

"A nice slow snow melt that we can manage with the reservoirs that don't overtop the reservoirs or cause serious downstream flooding," Gleick said. "The snow probably won't melt fast enough to cause any intense flooding."

"Snow melt tends to be less intense than these big rainstorms we get," said Roger Bales, a hydrologist and engineer for the University of California at Merced.

But there are risks that the snowpack could melt faster than the reservoirs can handle the runoff.

"The risk, of course, is that we get additional high temperatures which we've seen over the last decade or so and that snow melts much too fast to handle," Gleick said.

The other concern officials say is if Northern California sees additional heavy rains during late spring.

"For intense flooding, the big concern would be is if we get a big rainstorm while the snow is melting and together cause catastrophic flooding," Bales said.

"One late season warm storm (could) dump a lot of warm rain in the mountains that melts a huge amount of snow at once that overwhelms the remaining flood capacity in our reservoirs, which are already pretty full," Gleick said.

Where can we expect flooding?

"You could have flooding in the Sacramento Valley, Sacramento metropolitan area as well as the San Joaquin Valley and the coastal areas," Bales said.

Other areas, including Stockton, Merced, other riverside cities and Yosemite are also on the list because "our levees are not necessarily in the best condition all around the state," says Bales.

Tulare Lake in the southern part of San Joaquin County was once the largest lake west of the Mississippi. But farmers drained it to plants crops and the land keeps subsiding as they pump out more and more groundwater to feed their crops.

Thirty-four years ago, a similar snow pack melted, flooding hundreds of square miles of the valley. Many more residents now live in the region and officials say the land has sunk even more.

"I think it's a legitimate worry to think about flooding over the next month or two," Gleick said.

By KTVU reporter Tom Vacar.