The earthquake that shook Santa Rosa last week appears to have shaken the water source below Mark West Creek, raising the drought-depleted stream by about six inches in the hours after the quake and increasing the current flow more than sevenfold in compared to the previous speed.
John McAuley witnessed it through the second floor window of his home in the Larkfield-Wikiap neighborhood north of the city. He went down to the stream behind his house, where he was surprised to see that the stream was growing.
What used to be a shallow area of stagnant water was rising, the water rushing over a chain of rocks just upstream like waves coming ashore, he said.
“I actually saw it moving up,” said McAuley, whose home sits well above the creek bed across from John Ribley School, about 30 yards from the water’s edge.
A piece of wood that had been lying in the creek bed all summer, exposed and dry, is now under water at a depth of about 2 inches. Flat rocks on the far bank were also submerged, a frequent resting place for a family of foxes with four chicks, which Macaulay and his wife, Susie Dickinson, watched grow.
Neighbors and others 9 miles upstream, where the creek follows St. Helena Road down the west slope of the Mayacamas Mountains, also saw the creek rise about 6 inches after the Sept. 13 earthquake, reaching a level it has maintained since those times.
Among them was Carlos Diaz, chief engineer for the Sonoma Water District Agency, who lives on Londonberry Drive, about 1,500 feet from McAuley.
“I remember the same thing happened during the (2014) Napa earthquake,” Diaz said. “That summer, the girls and I used to jump across the creek on the steps, and they all disappeared under the water after that earthquake.”
It makes more sense this time, he said, given the proximity of the Rogers Creek fault, which triggered the 4.4 earthquake and 4.3 aftershock last week. The fault line runs through Santa Rosa and Larkfield, where it damaged numerous homes.
The US Geological Survey describes underground and surface water sources as features of an intricate “conduction system” set in the ground, where shifts in rock and sediment can cause hydrogeological changes even thousands of miles from the epicenter of an earthquake.
The impact of an earthquake on the hydrology of a region can also take many forms. In some cases, streams, springs or wells release more water than before. They can also dry out. Or it may suddenly turn cloudy, making the water source unusable.
Diaz said a friend had just told him about a cousin in Humboldt County who depended on a spring that stopped working after the earthquake.
The Aug. 24, 2014, magnitude 6.0 Napa earthquake 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 for the first time.
In the Mayacamas, the uplift that formed the ridge, as in other coastal ranges, caused fractures in the bedrock that allow water to move along it. According to research, Mark West Creek, a key salmon and trout stream that flows into the Rushen River, has many tributaries and springs that contribute to base flow year-round.
All of the tremors last week either weakened micro-faults 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 study of the creek’s flow. under the leadership of the Coast Range Watershed Institute.
Those who live along the stream are accustomed to watching its response to environmental changes. Many say they usually notice an increase in flow around the end of September, when fall sets in and surrounding trees use less water.
After the Glass Fire swept through the St. Helena Road corridor in 2020, destroying the area and killing thousands of trees, the flow also increased.
And after the Napa earthquake and last week’s Santa Rosa earthquakes, several residents said they’ve been watching what’s happening with water levels.
“I checked it out the next day,” said Marcel Ziegl, who lives on lower St. Helena Street, about a half-mile from Calistoga Road. “One thing we found out is that we can hear the stream better.”
A Mark West Creek water gauge near Michele Way, just east of Mark West Springs Road, showed the creek flowing at about 0.40 cubic feet per second in the days before the Sept. 13 earthquake.
After that, it rose to about 2.87 cubic feet per second, where it stayed, bouncing a bit during the weekend rain before settling back to 2.87.
McAuley and Dickinson built their home with the creek as a focal point after losing what was their home for three months in the 2017 Tubbs Fire. The new rippling stream adds serenity to their natural environment, including a creekside spot under trees from which Macauley recently watched river otters play.
“The last couple of weeks before the earthquake,” Dickinson said, “the algae here was so bad until the earthquake took care of it.”
“There was almost no flow,” McAuley said.