Rain, high lake level could make for dicey flood season | SierraSun.com

Rain, high lake level could make for dicey flood season

David Bunker
Sierra Sun

For the last six years, the Truckee River has had a 193-square-mile buffer against flood-causing rains.

Lake Tahoe’s low lake levels over the last six years have been able to easily absorb much of the winter rain and spring runoff.

This year, however, Tahoe retains much of the water that pushed it to its highest elevation in six years, and water regulators are preparing to stay one step ahead of the volatile Sierra Nevada weather to prevent damaging floods on the Truckee River.

“We are a little high, higher than we’ve been in a number of years,” said Chad Blanchard, chief hydrologist with the Federal Watermaster’s office in Reno, which controls the outlet of Lake Tahoe. Lake Tahoe, after reaching its maximum limit this summer, is only about 1.5 feet from its brim.

Heavy rains like those experienced last New Year’s weekend might not only engorge the Truckee River, but could force water operators to dump water from Lake Tahoe to keep the lake from exceeding its limit.

The weather forecast so far has not given any clues about what to expect in the coming winter months.

The area will experience a mild El Nino pattern, a weather cycle produced by warm Pacific Ocean temperatures. And while El Nino is often associated with flooding rains, this is typically only true for the southwest United States.

Around Tahoe, an El Nino pattern basically means anything could happen.

“Right now we are in a weak El Nino going into a moderate El Nino, and what that means for us is unclear,” said Rhett Milne, a meteorologist with the National Weather Service in Reno.

In 1997, El Nino meant torrential, warm rains that produced a raging Truckee River and led to more than $1 billion in flood damage to Reno and Northern Nevada. Downtown Truckee and businesses and homes along the Truckee River also flooded in the storm.

While the Truckee River reached flood stage on its own, the flooding was exacerbated by the watermaster opening all of the Lake Tahoe dam’s gates to keep Tahoe from going over its legal limit.

In a region where one warm storm can spell disaster, forecasters did not see that flood a normal byproduct of an El Nino pattern in Tahoe.

“It can happen any winter, anytime,” said Milne. “All it really takes is one pattern, or one major storm.”

These warm, tropical storms that drop a deluge of rain and often melt off some of the Sierra snowpack are often called “pineapple expresses” because they come from the Pacific Ocean near Hawaii.

Milne said last year’s flood-causing rains came from Indonesia, and were a rare-weather event.

“We called it the ‘mango connection,'” Milne said.

Knowing that Tahoe is at a higher level than it has been over the last six years, Milne said the National Weather Service will talk with the Federal Watermaster’s office during the winter.

The watermaster’s office controls not only the outlet of Tahoe, but also dams on Prosser, Boca and Stampede reservoirs.

“We’re in contact with them,” said Milne. “If we see a major rain event coming, they will begin releasing water.”

For Blanchard, whose decisions at the watermaster’s office are mostly dictated by federal water law, the only thing to do is see how the winter weather plays out.

“All we can do now is sit and wait,” said Blanchard.