Santa Rosa earthquake unleashed higher flow in Mark West Creek

The earthquake that jolted Santa Rosa last week apparently shook loose a water source beneath Mark West Creek, boosting the drought-depleted stream level by about six inches in the hours following the temblor and bumping the current flow to more than seven times its previous rate.

John Macaulay witnessed it happening through a window from the second floor of his home in the Larkfield-Wikiup area north of town. He walked down to the creek behind his house, where he was surprised to see the creek swell as he looked on.

What had been a shallow stretch of stagnant water was surging, the water rushing over a chain of rocks slightly upstream like waves coming ashore on the coast, he said.

“I actually saw it moving up,” said Macaulay, whose home stands well above the creek bed across from John Riebli School, about 30 yards away from the water’s edge.

A chunk of tree limb that had lain in the creek bed all summer, exposed and dry, is now under about 2 inches of water. Submerged as well were some flat rocks on the far bank that had been a frequent resting place for a fox family with four kits that Macaulay and his wife, Suzie Dickinson, have watched grow.

Neighbors and others who are as far as 9 miles upstream, where the creek follows St. Helena Road down the west slope of the Mayacamas Mountains, also saw the creek had risen about 6 inches in the aftermath of the Sept. 13 quake, reaching a level it’s maintained since.

Among them was Carlos Diaz, a principal engineer with Sonoma Water, the county agency, who lives on Londonberry Drive, about 1,500 feet from Macaulay.

“I recall this exact same thing occurring during the (2014) Napa earthquake,” Diaz said. “My girls and I had gotten used to skipping across the creek on some steppingstones during that summer, and they all vanished underwater after that quake.”

This time it makes more sense, he said, given the proximity of the Rodgers Creek fault, which triggered the 4.4 quake and 4.3 aftershock last week. The fault line runs through Santa Rosa and Larkfield, where it damaged numerous homes.

The U.S. Geological Survey describes ground- and surface-water sources as features of an intricate “plumbing system” set within the earth, where shifting rocks and sediments can spur hydrogeologic changes even thousands of miles from an earthquake’s epicenter.

The effect of an earthquake on a region’s hydrology also can take many forms. In some cases, streams, springs or wells discharge more water than they did previously. They also may dry up. Or there may be sudden turbidity that renders a water source unusable.

Diaz said a friend just told him of a cousin in Humboldt County who had been reliant on a spring that stopped producing after an earthquake.

The Aug. 24, 2014, Napa quake was a magnitude 6.0 and dramatically increased flows in at least six creeks, including three in Sonoma Valley, two in Napa and Solano counties and Mark West Creek — not all of them for the first time.

What the creek looked like before the earthquake:

In the Mayacamas, the uplift that formed the range, as with other coastal ranges, resulted in fractures in the bedrock that allow water to move around within it. Mark West Creek, a key salmon and steelhead trout stream that feeds into the Russian River, has many seeps and springs that contribute to the base flow year-round, according to studies.

All the shaking last week either loosened up microfractures that normally hold water or changed the pressure gradient somewhere, allowing water to flow more freely into the creek, said hydrogeologist Matt O’Connor, whose firm, O’Connor Environmental Inc., conducted a recent creek flow study under the direction of the Coast Range Watershed Institute.

Those who live along the creek are accustomed to observing its response to changes in the environment. Many say they generally notice increased flow beginning around late September, when autumn sets in and the surrounding trees are using less water.

After the 2020 Glass Fire swept through the St. Helena Road corridor, decimating the area and killing thousands of trees, the flow rose, as well.

And after the Napa quake and the Santa Rosa temblors last week, several residents said they were watching to see what happened to the water levels.

“I checked it the next day,” said Marcel Siegle, who lives on the lower part of St. Helena Road about a half mile from Calistoga Road. “One thing we’ve figured out is that we can hear the creek better.”

The Mark West Creek water gauge near Michele Way, slightly east of Mark West Springs Road, showed the creek’s flow rate at about 0.40 cubic feet per second in the days before the Sept. 13 earthquake.

Afterward, it rose to about 2.87 cubic feet per second where it stayed, spiking slightly during the weekend rain before returning to 2.87.

Macaulay and Dickinson built their house with the creek as a focal point after losing what had been their home of three months to the Tubbs Fire in 2017. The newly rippling stream adds to the serenity of their natural surroundings, including a creekside spot beneath the trees from which Macaulay recently watched river otters play.

“The last couple weeks before the earthquake,” Dickinson said, “the algae was so bad here, until the earthquake took care of it.”

“There was hardly any flow,” Macaulay said.

You can reach Staff Writer Mary Callahan at 707-521-5249 or mary.callahan@pressdemocrat.com. On Twitter @MaryCallahanB.