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.