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After Heavy Snowfall, a Massive Rainstorm Hits California: What You Need to Know

March 20, 2018
by Glen Martin

Water managers and hydrologists are a mite worried. The good news is that the recent cold storms dumped a lot of new snow in the Sierra. That will help keep reservoirs charged and Californians adequately hydrated through the coming year. But there’s a literal dark cloud counterbalancing that silver lining—a massive “atmospheric river,” aka the Pineapple Express, now poised to wallop California.

Billed as the largest storm of the season, it’s expected to last through Thursday and drop up to five or six inches of water in the central and southern portions of the state. That could present significant public safety risks, particularly in those parts of SoCal that were ravaged by wildfires last year.  Santa Barbara County and Ventura County officials, concerned about a reprise of the mudslides that killed 21 people in the Montecito area, have ordered mandatory evacuations of vulnerable areas.

These “rain-on-snowpack” events are typically responsible for the worst flooding in the state.

“This [incoming storm] is aimed at the area between Monterey and Santa Barbara and there’s a real possibility of localized flooding and more landslides,” says climate change and water resources authority Peter Gleick, (PhD, ‘86), the president emeritus of the Pacific Institute.

Moreover, as foreshadowed by the storm’s tropical fruit-associated moniker, this precipitation will fall as rain, not snow, even at higher elevations—i.e., the Sierra Nevada. That has long-term implications that are even more worrisome than the immediate threat. These “rain-on-snowpack” events are typically responsible for the worst flooding in the state, says Gleick, especially in the Sacramento and San Joaquin Valleys.

Indeed, such occurrences figure prominently in the apocalyptic scenario that has long been feared: a huge snowpack suddenly melted by warm and torrential late winter or early spring storms, overwhelming already full reservoirs, ultimately breaching dams and levees from Lake Shasta at the north end of the Sacramento Valley to Friant Dam in the southern San Joaquin Valley, turning the interior of the state into a vast inland sea.

While not impossible, that’s unlikely this year, says Gleick. Though reservoirs mostly were filled by last year’s extremely wet winter, the most problematic reservoir, Lake Oroville, was drawn down to allow for repairs to its gigantic earthen dam, which was perilously close to failure following a series of big storms.

The data over the years have shown a clear trend towards more extreme weather…such shifts were predicted in the mid-1980s and those changes are now happening.

“Still, there has been a huge increase in the snowpack over the last couple of weeks,” says Gleick. “If it melts slowly, great —but if we get a lot of warm rain on it and rapid melting, we could have flooding. We always worry about these warm storms.”

Moreover, says Gleick, the current storm “needs to be superimposed on top of a very clear long-term trend in California hydrology that includes less snow and more rain. The data over the years have also shown a clear trend to more extreme weather, both in terms of dry spells and [the violence and duration] of storms.”

Such shifts were predicted in the mid-1980s, says Gleick, and “those changes are now happening.”

That’s deeply problematic for California’s water storage and distribution system because it was designed for a very different regime, one predicated on storing snowmelt.  Under the original plan, snow fell in the Sierra during the winter and melted slowly in the spring and early summer, yielding water at a steady and predictable rate to waiting reservoirs. Managers had little difficulty in controlling the level of the reservoirs for both flood control and water supply. But a warming planet has disrupted that strategy. Big, warm storm fronts can melt accumulated snowpack with great rapidity, swelling reservoirs and threatening to overtop dams. That means managers have to operate reservoirs as flood control structures first, releasing massive quantities of water to maintain dam integrity, sometimes at the expense of water supplies later in the year.

And even then that may not be enough: dams and reservoirs constructed in the 1950s may be inadequate for the geophysical realities of the 21st Century.  Further, new dams won’t solve the water supply problem. Most of the Sierra’s rivers have one or more dams on them already and new dams would offer minimal additional storage. Two major coastal river systems, the Eel and the Klamath, could be dammed, but the environmental impacts would be profound and political opposition would be fierce. The projects also would be astronomically expensive, likely negating many of the benefits of additional storage.

“We’ll never be able to replace lost snowpack with new surface storage,” says Gleick. “We have to manage our existing reservoirs for a changing climate, and we need to pursue other options available to us. That includes groundwater storage, water recycling and increased efficiencies in use. Yesterday’s climate is gone, and it’s not coming back.”

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