Napa quake jumpstarts stream flows, though probably only temporarily

Three creeks in Sonoma Valley and two more in Napa and Solano counties have dramatically increased water flows since the Aug. 24 earthquake in Napa County, a phenomenon familiar to scientists for more than a century and well established in Santa Rosa history.

Carriger Creek, a steelhead spawning stream on the city of Sonoma’s west flank, was bone dry - save for shallow, isolated pools of water - before the magnitude-6.0 temblor went off 12 days ago from an epicenter about 9  miles to the east.

Richard Grahman enjoyed the music of crystal-clear water splashing over smooth gray rocks this week in the creek behind his home on Grove Street.

“This is amazing,” said Grahman, a retiree who has lived next to the creek for 14 years. “The sound is delightful.”

Heavy winter rains transform the narrow waterway into a 6-foot-deep torrent that sometimes overruns its banks, with a roar that can be heard a mile away, he said. The melodic flow that now matches a typical April on Carriger Creek is likely caused by seismic shock waves that opened fractures in bedrock, allowing groundwater to flow rapidly into surface streams.

As far back as 1865, a local newspaper described rising streams in the Santa Cruz Mountains following a magnitude-6.5 quake on the San Andreas fault, and a federal government study found the magnitude-6.9 Loma Prieta quake in 1989 squeezed about 23 billion gallons of groundwater from the same mountains.

Flow changes in springs were anecdotally reported in Sonoma County, 124 miles north of the Loma Prieta epicenter, the study said.

“I couldn’t believe my eyes,” Grahman said, when he first spotted the flowing water about three days after the Napa quake. Experts say it won’t last long, and Grahman said it’s already dropped by more than half.

Calabasas and Felder creeks, which also run through Sonoma Valley, have seen similar post-quake flows, said Marcus Trotta, a Sonoma County Water Agency hydrogeologist.

A U.S. Geological Survey gauge on Sonoma Creek downstream from the confluence with Calabasas Creek - and above the other two creeks - measured a 20-fold surge in stream flow from a 0.10 cubic-feet-per-second trickle Aug. 24 to nearly 2.0 cfs Wednesday.

“Holy mackerel,” Grahman said when he heard that statistic. “Over time, that’s a lot of water.”

About 15 miles due east, Eileen Susa marveled over the post-quake invigoration of Tulocay Creek, which flows under Fourth Avenue east of Napa, where the quake struck hard from an epicenter 5 miles to the south.

Dry since April, the creek surprised Susa and her neighbors with flowing water two days after the temblor that wreaked more than $360 million in damage to government property, homes and businesses in Napa County.

“For us it could be a blessing,” said Susa, who lives in a water-poor area outside the city. “We might have struck gold,” she said, if the earthquake “busted open a few springs.” But if it drains away groundwater, the creek would prove to be a thief, Susa said.

When it rains hard, Tulocay Creek is a flood zone, she said, reflecting California’s boom-to-bust relationship with water.

Napa County Flood Control and Water Conservation District crews checked the area, where there are natural springs but no water mains, and found no obvious source for the water in Tulocay Creek, said Rick Thomasser, the district’s operations manager.

“They said it was an impressive flow for this time of year,” he said. “It’s a little bit of a mystery. It’s sort of Mother Nature doing her thing.”

About 5 miles further east, residents of Green Valley, an unincorporated area in Solano County, were surprised by flowing water in previously dry Green Valley Creek four days after the Napa quake. They were also initially a bit concerned because the creek is downstream from two dams that hold water for the city of Vallejo, said Bill Mayben, president of the Green Valley Landowners Association.

“It kind of looks like April,” he said.

The dams were checked and cleared by city, county and state inspectors, said Franz Nestlerode, Vallejo’s assistant public works director for water. But seepage from rock fissures in a small canyon on Wild Horse Creek had increased tenfold, from about 100 gallons to about 1,000 gallons of water per minute, he said.

Wild Horse Creek flows from the city reservoirs and joins Green Valley Creek about 2 miles downhill.

“We think pockets of groundwater opened in rocky material,” Nestlerode said, acknowledging that he is not a geologist. Tests showed the alkalinity of the seeping water was “consistent with groundwater” and not comparable to water in the reservoirs, he said.

Mayben said he has heard that explanation, but said it does not account for flows in Green Valley Creek above the confluence with Wild Horse Creek. “The jury is still out,” he said.

Neither scientists nor Sonoma County historians were surprised by the watery aftermath to the Napa temblor.

“Seismic events have long been known to cause changes in the level of oceans, streams, lakes and the water table,” said a 66-page USGS study of hydrological disturbances from the Loma Prieta quake, including a tsunami in Monterey Bay and increased stream flows in the Santa Cruz Mountains and as far as 55 miles from the epicenter.

Studies in the mountains indicated that “strong shaking increased the permeability of the shallow groundwater system by opening fractures” that sent groundwater into surface streams, said Tom Holzer, a USGS research engineering geologist who coordinated the study published in 1994.

Holzer said in an email that he suspects a “similar phenomenon” boosted stream flows after the Napa quake.

The underground water level in three deep wells in Sonoma Valley monitored by the Sonoma County Water Agency rose as much as 5 feet at 3:20 a.m. Aug. 24, exactly the time of the Napa quake, said Trotta, the agency’s hydrogeologist.

Seismic waves are known to deform the aquifer, pushing water up wells like toothpaste squeezed from a tube, Holzer said.

The two worst earthquakes ever to hit Santa Rosa - in 1906 and 1969 - had an impact on the city’s first tourist attraction, White Sulphur Springs, established on Taylor Mountain in 1862. The popular destination’s run came to an abrupt end in 1906, when the temblor best known for devastating San Francisco also crushed downtown Santa Rosa and shut off the spa’s main mineral spring.

Six decades later, downtown Santa Rosa was again ravaged by ground shaking and the hot spring was temporarily restored. It disappeared again within a year.

Streams in the Santa Cruz Mountains returned to normal six to eight weeks after the Loma Prieta quake, and Holzer said the current flows will probably cease in about the same period.

You can reach Staff Writer Guy Kovner at 521-5457 or guy.kovner@pressdemocrat.com.