WALNUT GROVE, Calif. — Until a few weeks ago, the McCormack-Williamson Tract in the California Delta was an island of low-lying farmland, more than two square miles protected from the surrounding rivers and sloughs by earthen levees.
Today the tract is an immense lake, up to 15 feet deep, with fish prowling the water and ducks skimming the surface. The adjacent Mokelumne River, swollen by the intense storms that have drenched the state this winter, caused a levee to break, allowing the water to rush in.
Those same storms led to the recent near disaster at the Oroville Dam 100 miles north of here, which cast an uncomfortable light on the elaborate and aging network of reservoirs, aqueducts, levees and pumps that funnel water to the state’s 39 million people and its $50 billion agricultural industry.
The flooding at McCormack-Williamson was unintentional, but scientists and environmental groups say deliberately creating similar areas — floodplains to allow the state’s rivers to overflow more naturally and benignly — is a way to help ease the strain on this water infrastructure, especially as climate change poses new challenges.
“Nature has been dealing with the vicissitudes of water changes in California for millennia,” said Brian Stranko, director of the state water program for the Nature Conservancy, which bought the tract here, south of Sacramento, in 1999, and has long had plans to restore it. “There are certain things that nature can do that we can’t do as well.”
Moving some of the state’s 13,000 miles of levees back from rivers to make floodplains would allow dam operators to release more water without endangering population centers. Water percolating down through the flooded land would also help recharge aquifers, which, having been severely depleted by pumping for agriculture, are subject to a new state groundwater law requiring that they eventually be made sustainable. And the flooding could restore some of the fish and wildlife habitat that existed in California’s interior valleys before intensive farming began a century ago.
But as with everything else involving water in California, the subject of augmenting the state’s so-called gray infrastructure of concrete dams, aqueducts and other structures with floodplains and other “green” infrastructure, including better-managed watershed forests, is one of intense debate. There are concerns that new floodplains would take farmland out of production and that allowing benign flooding would reduce the amount of reservoir water available for agriculture and other uses.
“California water is complicated,” said Joshua Viers, a professor at the University of California, Merced, who was at the flooded tract last week with a team of researchers using water-sampling equipment and other instruments to monitor the changes taking place. “But I think we’re finding that there are softer paths.”
At Oroville, which was completed half a century ago during the heyday of the push to build gray infrastructure, the immediate danger is past. Trucks last week rumbled up the winding roads to the 770-foot-high dam, carrying rocks and other materials as crews continued to shore up an eroded spillway.
The reservoir level was well below maximum, operators having drawn it down using another spillway that is also damaged. The more than 180,000 people downstream who were evacuated in response to fears of a flash flood are back home.
But the episode was a reminder of the pressures on the major dams among the 1,400 dams of various sizes in the state, where water levels are actively managed to control floods, make electricity, provide water for drinking, irrigation and recreation, and keep the dams and the people below them safe.
After five years of severe drought in the state, in which water all but vanished from some reservoirs, this year is on pace to be one of the wettest on record. Dam operators follow specific rules on releasing water, set by state and federal regulators, which require leaving room for anticipated storms. This year that balancing act has been more delicate than usual.
The rush of runoff that affected Oroville when a warm storm hit the Sierra Nevada, bringing more rain and less snow, exposed the need for proper maintenance throughout the water system, and the potentially high cost when things go wrong. Repairs at Oroville alone could run as high as $200 million.
The pressure on dams and other parts of the system is expected to rise as global warming continues. Computer models suggest that climate change will exacerbate a trend toward extremes — very dry or very wet years. By producing more rain and less snow in the mountains, atmospheric warming will also alter the amounts and timing of the runoff that fills the reservoirs behind dams like Oroville. Operators may have to release more water earlier in the year.
But critics say the system is not flexible enough. At dams, like Oroville, that generate electricity, the rules on water releases are set for several decades as part of licensing requirements of the Federal Energy Regulatory Commission, known as FERC. Even nonhydropower dams have rules that are based on more static conditions.
“California’s dams must be adapted to address new risks from a changing climate,” Jeffrey Mount, a senior fellow at the Public Policy Institute of California, wrote in a blog post after the Oroville crisis. But, he added, “when it comes to changing course on dams, institutional inertia is a powerful countervailing force.”
A spokeswoman for FERC pointed out that the commission had the authority to reopen a hydropower license, and that the agency’s dam safety program reviewed rainfall and storm data every five years.
But Dr. Viers and others say climate change has not been factored in. In 2009, for example, FERC rejected climate change studies in relicensing several hydropower projects in Northern California.
John Andrew, the deputy director of the California Department of Water Resources responsible for climate change issues, said the state had been studying the impact of warming for years.
“We’re already looking at a changed hydrology,” he said. “And that’s going to continue to change. So how we operate dams as well as other water infrastructure is going to have to change.”
Mr. Andrew said a mix of conventional infrastructure like dams and alternative approaches — including floodplains but also technologies like desalination and wastewater recycling — should be used to manage the state’s water.
“The bottom line is we would like to see a diverse set of strategies, implemented at a local and regional level,” he said.
At the McCormack-Williamson Tract, after the levee was breached two weeks ago the Nature Conservancy made a cut at the opposite end to allow the river waters to flow through. When the wet season ends, the tract should drain and dry out, although there still may be occasional flooding in parts of it.
A more deliberate experiment is taking place about a half-dozen miles to the east, at another conservancy plot on the Cosumnes River.
Dr. Viers refers to the Cosumnes as the state’s “ugly duckling” river — it has never fit into water-management plans. As a result, unlike all the other significant rivers on the western slope of the Sierra, the Cosumnes is essentially undammed and flooding is largely uncontrolled. Farmers in the area have adapted, growing crops or raising orchards that can tolerate winter inundations.
The flooding can be a nuisance nonetheless. For days at a time over the last month, Michael Eaton, a former official with the Nature Conservancy who has a small farm at his home in Galt, has had to leave his car on one side of a flooded driveway and paddle a canoe a couple of hundred feet to reach the property.
“It gets a little tiresome,” said Mr. Eaton, who is thinking of having the road raised.
At times, Mr. Eaton said, he can see and hear air bubbles popping up through the water as it infiltrates the soil. “It’s groundwater recharge in real time,” he said.
The undammed nature of the Cosumnes also makes it a useful laboratory, both for studying how stream flows and flooding are changing under more natural conditions and for assessing the impact of green infrastructure. Not far from Mr. Eaton’s home, the conservancy bought a 500-acre plot along the river, called Oneto-Denier; punched a hole in a levee to turn it into a floodplain; and planted native vegetation as part of the restoration effort.
Dr. Viers and others have been studying the plot, and so far the results are encouraging. Among other things, they have measured a significant amount of water returning to aquifers.
“This little ugly duckling is now showing us the way in which we can better manage rivers downstream,” Dr. Viers said.