MODESTO, Calif. — For years, there has been a movement in California to restore floodplains, by moving levees back from rivers and planting trees, shrubs and grasses in the low-lying land between. The goal has been to go back in time, to bring back some of the habitat for birds, animals and fish that existed before the state was developed.
But in addition to recreating the past, floodplain restoration is increasingly seen as a way of coping with the future — one of human-induced climate change. The reclaimed lands will flood more readily, and that will help protect cities and towns from the more frequent and larger inundations that scientists say are likely as California continues to warm.
“We thought we were just going to plant some trees out here and get some birds to move in,” said Julie Rentner, executive vice president of River Partners, a conservation group that is restoring hundreds of acres of farmland on the outskirts of Modesto in the Central Valley, where agriculture has overwhelmed the natural environment. “Now we’ve got this whole much larger public benefit thing going on.”
Researchers say it is unclear whether climate change will make California drier or wetter on average. What is more certain is that the state will increasingly whipsaw between extremes, with drier dry years, wetter wet ones and a rising frequency of intense periods of precipitation.
Climate models agree that “this really big increase in wet events is quite likely,” said Daniel Swain, a climate scientist at the University of California at Los Angeles and an author of a recent paper on the expected changes. “There’s just so much more moisture in the atmosphere in a warming world.”
As more so-called atmospheric river storms blow in from the Pacific Ocean, and more precipitation falls as rain rather than snow in the Sierra Nevada, where most of the state’s main rivers begin, increased runoff may force reservoir operators to release more water from dams or may otherwise cause flooding downstream.
River Partners’ project, Dos Rios, covers more than three square miles of farmland here at the confluence of the Tuolumne and San Joaquin rivers. It will benefit endangered animals like the riparian brush rabbit and birds like the least Bell’s vireo, but it is also designed to absorb some of the floodwater, holding it or slowing its flow to reduce levels in the nearby town of Grayson and elsewhere along the rivers.
Dos Rios is only one of many such efforts. Kris Tjernell, deputy director for integrated watershed management at the California Department of Water Resources, said the state was actively working on “upward of 20 or 30” projects, some on its own and some in concert with groups like River Partners.
That number is expected to grow significantly since California voters last month approved Proposition 68, which includes $300 million for floodplain projects in the Central Valley.
California, and especially the Central Valley, is no stranger to floods. The biggest one in modern times occurred in 1861-62, when 40 days of rain turned the valley into a 250-mile-long lake and Leland Stanford, the state’s new governor, took a rowboat to his inauguration in Sacramento.
That flood occurred before most of the state’s dams, levees and other flood-control works were built (and when the population was one-hundredth of what it is today). While not as big, some more recent floods have been severe, including one in 1997 that killed nine people and caused nearly $2 billion in damage.
Most recently, last year, after five years of drought, a string of storms dumped heavy rains on Northern California that led to a near disaster at the Oroville Dam when spillways were damaged as runoff forced the dam’s operators to release large amounts of water. About 200,000 people along the Feather River were evacuated from their homes, and repairs to the dam cost $870 million.
A study published last month found that climate change contributed to the problems at Oroville, as human-caused warming in the Sierra Nevada led to more rain and less snow and thus greater winter runoff.
John Carlon, the president of River Partners, said floodplains that the group had restored on the Feather proved their worth in that event.
“They just absorbed that floodwater beautifully — they acted like a shock absorber,” said Mr. Carlon, who farms blueberries in the northern valley town of Chico. “It was a big test for this concept and we’re really pleased with how it worked.”
Dr. Swain said his research suggested that in California, as in many other parts of the world, severe floods are far more likely as warming continues. “For me the most surprising aspect is that the likelihood of seeing a repeat of this 1862 event over the next 40 years is greater than 50 percent,” he said.
The United States Geological Survey has estimated that a similar event today would force the evacuation of up to 1.5 million people in the Central Valley. Statewide, damages would exceed $300 billion.
No floodplain would help in such a disaster, but projects like Dos Rios can lessen some of the impacts of smaller floods. By allowing water to sweep over its acreage, a floodplain can reduce the river level in a nearby community.
Even lowering a flood stage by a fraction of a foot can in many cases buy time — for farmers to move their cattle, for instance, or for a town to shore up its flood protection or evacuate the area if necessary.
About three-quarters of the land at Dos Rios was owned by a dairy farmer who was tired of dealing with flooding and sold it in 2012 to River Partners, with various state and federal agencies financing the $21.8 million cost. An adjacent farmer then sold his land to the group in 2014 for $9.3 million.
The result is 2,100 acres, including three miles of frontage along the Tuolumne River and four and a half along the San Joaquin. River Partners is removing berms along the riverfront that the owners formed from earth and rubble to keep the smallest floods out.
A higher, more permanent levee, built by the United States Army Corps of Engineers in the 1950s, protects about 1,000 acres in the core of the project, back from the rivers. Modifications will allow water to enter, so the area will serve as a giant bathtub, holding up to 10,000 acre-feet of floodwater.
This will not only reduce river levels, but as the water percolates into the land it should provide another benefit, helping to recharge aquifers that have been depleted statewide because of pumping for agriculture.
About 600 acres inside the levee have been converted from farmland to riparian woodlands, and another 700 acres inside and outside the levee are in the process of being restored. The entire project should be finished by the middle of next decade, Mr. Carlon said.
River Partners plants many species — trees like cottonwood and black willow, shrubs like golden currant and valley elderberry and a variety of grasses — all of which can recover after being underwater for months.
The planting, much of which is done by young workers with the state-run California Conservation Corps, is done with all the precision of modern agriculture. A field about to be restored looks much like any cropland, with furrows and markings where the various young plants are to be placed, all determined ahead of time.
Eventually the rows disappear — as evidenced by the land directly across the San Joaquin, where beginning 17 years ago River Partners restored about 3,000 acres of farmland that now appears to be a natural woodland, part of a national wildlife refuge.
The exact mix of species to be planted in a given spot depends on what River Partners and state engineers want the water to do as it spreads across the land.
Sometimes the goal is for the floodwater to slow down, as this can reduce downstream levels even more than floodplains alone. So in those areas, Ms. Rentner said, the mix will include species with relatively stiff stems or trunks that help impede the flow.
“There’s always an opportunity to use vegetation and this green infrastructure to lower flood risk — move the water where it will do the most good and redirect it from places that will do the most harm,” she said. “And you can do that in a way that is durable and sustainable over time.”
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