Every decade or so, a massive storm hits Petaluma, causing creeks to jump their banks and flooding low-lying areas of the city. But the problem may be made worse during intense storms, known as atmospheric rivers, as water that is destined for Rohnert Park ends up further inundating Petaluma.
The issue stems from complicated hydrology in the area where Sonoma County’s two main watersheds meet. A multi-agency task force is studying the problem, but with sensitive environmental concerns, a solution could take years.
Water officials have identified Copeland Creek as a potential culprit for adding additional water to Petaluma during harsh winter storms. The stream originates on the slopes of Sonoma Mountain and skirts Penngrove to the north on its way through Rohnert Park, emptying into the Laguna de Santa Rosa and ultimately the Russian River.
Copeland Creek is at the extreme southern edge of the Russian River watershed, just feet from the Petaluma River watershed. During heavy storms, water jumps the banks of Copeland Creek and heads into Petaluma, adding water from an outside watershed to the already overstretched Petaluma River hydrological system.
The water spills down from Penngrove, flooding mobile home parks and sloshing down McDowell Avenue. It floods the Petaluma Auto Mall before dumping into the already storm-choked Petaluma River. Such memorable floods happened in 1982 and 2006.
It also happened this past January. Rick Savel, a Penngrove resident and former member of the regional flood advisory board, witnessed the January flooding along Lichau Road.
“It’s a tale of two watersheds,” Savel said. “Lichau Road divides the two watersheds. When we start introducing water from outside the watershed, we don’t have the capacity to deal with that.”
Savel said that county supervisors David Rabbitt and Susan Gorin scrambled work crews to temporarily patch up Copeland Creek and keep the water within the banks, but a long term solution is still years away.
“My worry is what do we do in the interim if this is going to happen again?” he said.
The Sonoma County Water Agency is leading a task force, including officials from Rohnert Park, Petaluma, various county agencies and Sonoma State University. In an email, Michael Thompson, interim water agency general manager, said Copeland Creek flooding has been a recurring problem for generations.
Indeed, the state public works department studied the issue more than a century ago, according to a report from 1896.
“At times of greatest rainfall, Copeland Creek becomes engorged to such an extent that a portion of its waters separates from the main channel, and instead of flowing into the Russian River, take a southeasterly course to join the waters of Petaluma Creek,” the report says. “This can readily be prevented by a very moderate expenditure at the point of separation of the waters of Copeland Creek, and we earnestly recommend that it should be done.”
State engineers from 1896 estimated the fix would cost $500. But the recommendation was not followed, and now the solution will likely cost millions and take years to implement.
Thompson said that the presence in Copeland Creek of steelhead trout, a threatened species, complicates efforts at finding a long term solution. State and federal agencies that regulate waterways and wildlife would likely need to issue permits for any work to maintain the creek.