River of Return

The natural habitat along the Cosumnes has sprung back to life. Why? Ecologists are breaking down the levees that have tamed the river and allowing nature to take its course.

Wild thing: The Cosumnes River doing its thing—on a spring day…

Wild thing: The Cosumnes River doing its thing—on a spring day…

Photo by Larry Dalton

Dropping down into California’s Central Valley, say on I-5 coming south from Redding, the landscape you see is nothing like the one that existed even a century ago. European settlers, agriculture and then modern real estate development have transformed hundreds of thousands of acres of forest and native grassland, floodplain and vernal pools into the cities, towns and farms that exist today.

Much of the Valley forests, once thick with native oaks, was cut for fuel, then the plowing and paving changed the face of the Valley even more. But perhaps no human activity has had a more profound effect on the valley ecosystems than our control of its water—the capture and channelization of the rivers and streams that fall out of the Sierra Nevada. Up and down the state, every major river has been tamed, regulated and cut off from its natural floodplain by levees and dams built to keep floodwaters out of our farm fields and backyards, and to harness all that water for our own purposes—drinking water, irrigation and hydroelectricity.

In our efforts to use the rivers, and to protect ourselves from them, we have often overlooked the natural purpose of a river on a floodplain—that is to flood and dry up and change course, all the while constantly nurturing and reworking the landscape, and in so doing to create a complex and ever-changing mosaic of natural habitats for native species, from the massive valley oak to the tiny salmon fry.

Ecologists are seeing with increased clarity how the disconnect of our rivers from their floodplains by dams and levees has interrupted processes that are vital to these native species. But in a few places—such as on the lower Cosumnes River—they are using the new tools of “natural process restoration” to put riparian ecosystems back together again.

The Cosumnes is in some ways a typical Sierra river.

It begins in the Sierra Nevada almost 20 miles southwest of Lake Tahoe, and travels about 80 miles to where it ends in its confluence with the Mokelumne south of the city of Elk Grove. From there the two rivers continue on to the Sacramento-San Joaquin River Delta.

Once a meandering web of streams and densely forested islands, the river has been heavily channelized, cut off from its natural floodplain by agricultural levees and small dams.

Water wells also had a serious impact on the Cosumnes, particularly in the second half of the 20th century, as agricultural pumping drew down the groundwater that recharges the river in the dry months.

But the Cosumnes has one great distinction when compared to the Sacramento, American and even the nearby Mokelumne River: it has never been completely dammed.

Controlled and damaged, yes. But free of any major dams, the Cosumnes has been able to act as it has for thousands of years, earning it the title as the Central Valley’s last “wild river.”

“It’s a pretty amazing concept,” says ecologist Keith Whitener, somewhat tongue-in-cheek. “It rains and the river rises.”

The fact that the Cosumnes is unregulated, unlike other nearby rivers, has given the little river a leg up as ecologists attempt to restore it to something approaching its natural pristine state. And the Cosumnes’s relative wildness has made it an ideal laboratory for studying the Central Valley’s nearly vanished riparian ecosystem. What ecologists are learning on the Cosumnes will help restoration efforts on other Central Valley rivers. One day, at least pockets of the Valley could return to something approaching their once wild, natural state.

…and during winter floods.

Courtesy Of The Nature Conservancy

Perhaps the best way to get a sense of the river is to spend time in a canoe on the lower portion of it, in the protected Cosumnes River Preserve.

Here the river hardly looks like a laboratory. Paddling away from the visitor center on Willow Slough, a narrow, muddy side-channel of the river, it’s tricky going at first. The slough is densely overgrown, low hanging branches and submerged logs make it a dark and crooked path to the main channel.

Because this section of the Cosumnes is so close to the Delta, water levels can rise up to five feet in a single day. If you time it right you can let the incoming ocean tide do most of the work and push you upstream with a minimum of paddling.

The banks along this stretch of the river are mostly covered in spiky green willow trees. This low, tangled willow forest is home to all kinds of birds, judging from the constant squawk and whoop that emanates from either side as you paddle upstream.

Genelle Treaster lives in the nearby town of Wilton and is our tour guide this afternoon. She calls the 40,000-acre preserve the “last little slice of everything,” because the area is critical habitat for dozens of native species, including the sandhill crane and Swainson’s hawk, both listed as threatened, and the tiger salamander.

An avid birder, Treaster can discern even at great distances the calls of wood ducks and mallards that are floating all along the river this afternoon.

Clumps of tule grass, also called bulrush, grow in the marshy space in between the river channel and the shore. Their thickets of dark green stalks will soon provide perfect hiding places for dozens of little ducklings, judging by the numerous duck couples that are cavorting on the river today.

Treaster can name an impressive array of animals within a few moments of listening and glancing around. It’s partly a tribute to her knowledge of the area wildlife, and partly a testament to how much life there is out here. She picks out the tune of a meadowlark, a fast and complex arpeggio that is her favorite birdsong.

Listening intently, she nearly jumps out of the canoe when a loud “thwak!” explodes on the side of the canoe, and a small wave splashes into the boat.

“I think it was a beaver,” she laughs.

She steers the boat into Tihuechemne Slough, named for one of the Miwok groups that lived in this particular stretch. The Cosumnes was once one of the most densely populated areas in pre-European America, sustaining the mighty sum of 11 people per square mile at one time. It could support such a relatively dense population for two main reasons. The first is salmon. Today, the number of salmon running on the Cosumnes trickle along in the low hundreds. But historically the river has boasted runs as high as 5,000 fish. Indeed, the name Cosumnes comes from the Miwok word for salmon, kossum.

The other advantage for the Miwok was acorns. The once sprawling forest of valley oaks here provided plenty of acorns, rich in protein and carbohydrates, which were a staple of the Miwok diet.

In a meadow overlooking Tihuechemne Slough stands a single solid-looking old valley oak, with a trunk so thick that three people holding hands couldn’t embrace it, the branches already full with leaves. Cooling off in the oak’s massive shadow, it’s easy to get lost in quiet contemplation of the forest that used to be, and the people who once thrived beneath it.

Inspired by the “Accidental Forest,” ecologists learned that breaking levees was a good way to bring back natural habitat.

Courtesy Of The Nature Conservancy

So quiet and so contemplative is the afternoon, that no one notices the canoe has slid off the bank and has been blown by the wind some 30 yards to the opposite side, where it has wedged under some low-hanging branches. There is nobody on the river to offer any help, except for a swimming muskrat who occasionally pokes his reddish brown head above the water, curious about the humans on shore.

As Treaster hikes the mile or so to the closest house to ask for assistance, there’s time to inspect a field of new oak seedlings planted in the meadow near the slough. The couple of dozen or so oak seedlings here, encased in white plastic tubes for protection, are just a few of the hundreds of hand-planted oaks that dot the preserve, in hopes of recreating at least part of the oak forest that once existed here.

These few old stands of valley oak and connected riparian habitat, stuck in the middle of this intensively farmed area of the south county, attracted the interest of The Nature Conservancy (TNC) in the early 1980s. Quiet and non-confrontational as environmental groups go, TNC stumbled upon an 85-acre stand of valley oaks on the Cosumnes that was up for sale in a bankruptcy auction. Since only about 5 percent of the state’s riparian oak habitat remains, TNC jumped at the chance to buy the parcel. And for a while, that was that.

“At the time, we were satisfied with buying these small properties, putting a fence up and calling it a preserve,” said Mike Eaton, director of TNC’s Cosumnes River Project.

TNC continued to buy little parcels in the area for a few years. But for a time, the thinking continued to be that a preserve was nothing more than a fenced-off piece of land, protected from subdivision and development. Along the way, TNC acquired one of the preserve’s crown jewels, a mature 130-acre parcel of oaks that folks here call the Tall Forest.

If the bird chatter on the river gets a little spirited at times, walking into the Tall Forest is like walking into a party at full swing, there’s that much birdsong and flight all up and down the canopy. But it is like a party in a cathedral, because many of the trees in the Tall Forest are easily a hundred feet tall, and probably as many years old.

It was the Tall Forest that Rich Reiner, project ecologist for the preserve through most of the 1990s, wanted to recreate on other parts of the preserve. The idea early on was to do it with the labor of hundreds of volunteers, hand-planting valley oaks in little plastic tubes. It’s a great educational tool, and a lot of trees get planted that way.

“At one point we had 500 people out there planting acorns, all on the same day,” recalled Reiner. “It was like some sort of rock concert.”

But Reiner and his colleagues quickly realized that it would take decades of methodical planting to get anything approaching the size of the Tall Forest. And the truth was that many of the hand-planted trees didn’t grow very well.

Then there was a breakthrough.

Not far from the Tall Forest was a smaller forest of willows and cottonwoods, covering maybe 15 acres. Underneath the willow forest, some tiny oaks were beginning to sprout up. One day Reiner was looking at old aerial photographs of the preserve and noticed something odd. On the photo, taken 10 years earlier, the smaller willow forest wasn’t there. What had changed to produce the willows?

Reiner was curious and contacted the farmer who owned the land before the preserve. It turned out that the new trees stood in a field that had, until just a few years earlier, been regularly planted with beans and tomatoes and other crops. But one spring, when it was time to plow the field, the farmer discovered that a nearby levee had broken, and the ground was covered with a great splay of sand, spread by the floodwaters across his field.

The farmer patched the levee, but didn’t have time to take the sand out of the field before planting. When he returned the next year he found that tiny trees had begun to sprout in the sand splay. This time he let the sandy patch of ground remain for good, and cut his farm field around it.

When the levee breaks: An intentional levee breach on the lower Cosumnes.

Courtesy Of The Nature Conservancy

And so the “Accidental Forest” had begun.

Reiner began to piece it together. The sand deposited by the flooding river was ideal for the root systems of willows and cottonwoods. And willow forest makes a great home for nesting scrub jays, which bring home oak acorns and bury them on the forest floor. Oaks begin to grow in the shade of the colonizing willow forest, barely noticed for years. But since willows and cottonwoods are comparatively short-lived, what you are left with after many years is a nice oak forest. In fact, the Accidental Forest is today transitioning to oak forest, helped along by local beavers that find willows delicious. Once the oaks come in, the new forest provides a favorite perch for wintering birds of prey and habitat for dozens of other native species.

If Reiner could replicate the process that created the Accidental Forest, he reasoned, it would be far more effective than merely planting oaks. That meant getting permits to break through part of a levee and letting nature take its course. First, he had to find a spot where a levee breach wouldn’t flood the preserve’s neighbors, which meant hiring consultants to do some careful study and modeling of where floodwater would go if allowed outside the levee. That done, he had to negotiate a maze of government bureaucracies to get a permit for the levee breach. It took a year and a half. “It turns out there’s no desk you go to if you want to take down a levee,” Reiner laughed.

But eventually he prevailed and, late in 1995, work crews used a bulldozer to remove a 100-foot section of a levee just to the south of the Accidental Forest.

Predictably, when the winter storms arrived water rushed through the hole in the levee and flooded several hundred acres of bare ground. Lo and behold, as the floodwater receded over the following months, Reiner found that thousands of young cottonwoods, smaller than your finger, had begun to sprout. Today those trees are close to 10 feet tall, and cover approximately 25 acres.

Now ecologist Whitener, with TNC, is giving a reporter a tour of a second break, breached in 1997 somewhat to the north of the Accidental Forest. Twenty feet high, and covered in grass, this levee drops abruptly to a sand-covered beach where bulldozers broke through and then rises again 100 feet downstream. Just as at the other breach, the three-foot-deep sand splays gave rise to trees. And interestingly, there is a whole array of different aquatic plants here now, species that weren’t present at the earlier levee breach.

No one is sure exactly why that is, says Whitener. But underlying soils could be slightly different, more or less accommodating to certain species of plants. The shape and elevation of the ground surrounding the levee break, the velocity of the water that broke through, how long it took to recede and the plant matter that was present in the stream at the time of flooding are all variables that led to a slightly different group of species colonizing the newly formed beach. That is one of the great benefits of this kind of restoration, says Whitener, it reintroduces complexity into the environment. And for a healthy ecosystem, complexity is a beautiful thing.

It should be noted that Whitener and The Nature Conservancy are not trying to rid the south county of agriculture and return vast tracts of farm fields to the wild. In fact, over 85 percent of the Cosumnes River Preserve is actively farmed.

Just to the west of this levee breach, behind another much smaller levee that TNC actually built, is an organic rice farm being run on land owned by Sacramento County (the county is a partner in the preserve, along with a host of other government agencies).

The rice field is actually great habitat as well, a favorite spot for wintering cranes. Again, complexity is key. Farm field, willow forest, oak forest, all different habitats interacting with each other to form a rich, complex web of habitats.

While the levee breaches and other restoration efforts were intended to recreate a specific kind of habitat—oak forest—ecologists found that setting the river free had all kinds of unexpected benefits as well. “At first, it was all about the trees,” says Whitener, whose own background is fish biology. He is particularly interested in how reconnecting the river to its floodplain has set in play all sorts of other natural processes. For example, he said “we found out we had accidentally created really great fish habitat as well.” When the river floods, he explained, hundreds of acres of ground become a vast shallow pool for days, weeks, even months at a time.

“And these floodplains are incredibly productive,” says Whitener. The shallower, warmer water simmering in the floodplain is a cauldron of living things. It produces 100 times as much zooplankton as the river does. Fish such as the threatened splittail, a native minnow, were spawning and thriving in the restored floodplain. And salmon were growing fat and happy before making their journey upstream to spawn. And the floodplain was giving rise to a whole suite of little-noticed insects and other invertebrates, in turn making great habitat for waterfowl and other birds.

And it isn’t just the presence of water on the floodplain that is good for native species, said Whitener. He said fish like the salmon and splittail are genetically hardwired to take advantage of the boom and bust cycles of flooding. The splittail, for example, spawns almost exclusively in floodplains. And they know instinctively when it is time to get off the floodplain and head upstream. Restoring the floodplain also helps in the fight against non-native species that are competing—often out-competing—with the natives. Bass, for example, which have been devastating to native species throughout the Delta, will also wander out onto the floodplain, but the conditions aren’t right for them to thrive here. And the bass don’t “know” what to do while they are out there as the splittail does. When the floodwaters subside, the bass tend to get stranded and die, and so the floodplain acts as a sort of filter against invasive species that shouldn’t be there.

Ecologist Keith Whitener says the Cosumnes is a model for restoring riparian ecosystems.

Photo by Larry Dalton

More than merely growing a pretty forest, flooding induces a whole world of habitat that can’t exist when the river is reduced to, in Whitener’s words, a “rip rap lined flood conveyance channel.” Take away the floodplain and you seriously compromise the whole system. Put it back, and you make the ecosystem whole again, or more whole at least.

But we humans are far more comfortable with half an ecosystem. We like the river, but we hate the floodplain. Floods are awfully inconvenient, and they can destroy crops and real estate alike.

While The Nature Conservancy has latitude now in breaching levees on some of the property it owns along the river (some 40,000 acres now), the increasing urbanization in south Sacramento County, particularly from Elk Grove and Galt, may inevitably demand more, not fewer, levees on other upstream parts of the river.

“To my mind, the biggest threat is urbanization,” said TNC’s Eaton. The current city limits are less than a mile from the river in many places. And ultimately there will be pressure to move the city limits closer.

The city of Elk Grove, seeking to make room for its own burgeoning population, has already attempted to expand its sphere of influence, where it might ultimately allow development, right up to the river itself. The city had to back off this aggressive plan for the time being, given the exhaustive environmental review that would be required. But Elk Grove planning director Eric Norris said there’s still a desire among City Council members to expand the city limits. And some local landowners, whose property straddles the river, are eager to sell their farms for development. Many of the area farms make use of agricultural levees. Norris said if the city did ultimately develop the area that it would likely put new houses and retail outside the current floodplain. Critics of Elk Grove expansion, however, say development so close to the river would ultimately lead to more aggressive flood control measures, bigger levees and further straightjacketing of the river.

The Nature Conservancy is trying to combat development pressure not only by purchasing land outright, but also by buying conservation “easements” from area farmers. An easement is basically an agreement by which TNC buys the development rights to the property but not the property itself. A farmer often could make far more money selling the property outright for development, but an easement may help struggling farmers that want to see their land stay in agriculture. Much of the property that makes up the preserve today is the assemblage of such easements, which serve as a buffer to urbanization and valuable habitat as well.

Eric Norris said the city is also interested in developing a Cosumnes River Parkway along the river. Few details have been worked out, but Norris said the city is interested in some “low impact” trails, and some trailhead areas with parking lots.

Much of the current Cosumnes River Preserve is off-limits to the public. While the trails would probably be quite enjoyable to Elk Grove residents and others who wanted to enjoy this wild area, it might also work against the efforts of The Nature Conservancy, said Eaton.

“There is a real danger of loving some of these habitats to death,” said Eaton. Still, he noted that Sacramento County has a great need for more trails and natural recreation areas, and that ways would have to be found for the Cosumnes to “bear its fair share.”

In the early days of spring, as the winter rains subside and Sierra snowpack begins to melt, the Cosumnes is in peak form. The heavy flow allows fish enough purchase to literally leap above the water. Sit on the bank long enough and you’re sure to see a few fish get airborne as they work their way upstream. In the meadows along the river’s edge, native wildflowers, such as the vivid goldfields, are coming up in eye-catching splashes of color here and there on the stretches of green grassland.

But by the end of summer, great stretches of the Cosumnes will be nothing more than dry, dusty ditches beneath the oaks, and the grasslands around will have gone brown. Where there is water near the confluence with the Mokelumne, it will be mostly the result of rising tide coming up from the river delta. The Cosumnes has always been ephemeral, tending to be dry in spots in the summer months.

But the trend appears to be that the river has been drier for longer periods of time. Aside from urbanization, the other big threat to the Cosumnes is the drying up of regional groundwater. Historically, even when the river wasn’t flowing, there was still groundwater stored right beneath the surface of the riverbed. But now, as a result of years of heavy groundwater pumping, the groundwater in the middle stretch of the river has dropped dramatically away from the river, in some places 50 feet or more. Where the river would have “reconnected” in certain spots after one good winter storm, it may now take two, three or more periods of heavy rain to get the river flowing again.

Most of the damage was done over the last half-century, when agricultural pumping really ratcheted up. There is some debate now over how much development in the south county is exacerbating the groundwater problem.

UC Davis geologist Jeff Mount says the Cosumnes is threatened by development and groundwater depletion.

Photo by Larry Dalton

UC Davis scientists are monitoring the groundwater situation, and what they discover will help provide information for the ongoing Water Forum process, a regional negotiation involving developers, water districts, environmental groups and government officials that began with the goal of ensuring regional water supplies for the next 30 years and restoring the American River. The first phase of the Water Forum all but overlooked the south county. Water Forum Phase II, as some are calling it, should take a closer look at the south county, and the Cosumnes, over the next year.

Some, such as UC Davis geology professor Jeff Mount, said government officials need to take special care not to allow new development to damage flows on the Cosumnes. As new subdivisions sprout up in the south county, and upstream in El Dorado and Amador counties, there will be efforts to pump tens of thousands more acre-feet of water out of the ground every year, possibly drawing groundwater levels down even further.

“This basin is already stressed. Any additional diversions will only exacerbate that. I’m not certain this river can withstand any more straws being stuck into it.”

There is another potential threat to the Cosumnes, one that is only now beginning to be understood. That’s global climate change. There is evidence that the Cosumnes, like all rivers in the Sierra, will get more rainfall in the winter, and less snowmelt in the spring, as a result of warming climate change. Scientists are only just now beginning to talk about climate change and the regional watershed.

But Mount offers an illustration of how climate change might affect the Cosumnes. In his office in the UC Davis Physics and Geology building, Mount calls up a Web site that gives flow information for all area rivers. First he pulls up a graph on the American River. There is very little fluctuation in the river flow levels over the past 10 days. “That’s flat line. That’s a dead river,” he explains. An undammed river would show more life, more up and down over a period of time.

Then he pulls up the graph on the Cosumnes. The graph shows that about 10 days earlier flows dropped off sharply, as the winter rains subsided and spring began. Then after about three days “as the air temperature started to rise,” little rises and dips begin to appear in the line. These are diel fluctuations, he explains, the signature of spring snowmelt. You don’t get these fluctuations on dammed rivers obviously, because snowmelt is filling reservoirs upstream.

But the diel fluctuations are a critical part of the Cosumnes life cycle. As he peruses the Cosumnes flow graph, Mount makes a happy mini-discovery. The melt-driven flows have been increasing over the last couple of days. “Today, right now in fact, it is flooding on the field by the Accidental Forest. And that water is getting warm. The fish are going crazy. Just loving it. It’s a very productive time right now.”

But climate change will probably mean that less snow will fall within the reach of the Cosumnes, and over many, many years, there will be fewer of these spring pulses. Even as scientists work to restore natural processes on the Cosumnes, even larger processes, global climate change in this case, may be working against the river.

Here, it is worth putting the Cosumnes in perspective. It is after all a relatively minor river, often overlooked—for better and for worse. And the Cosumnes River Preserve is a mere postage stamp compared to the great breadth of the Central Valley.

But what has been learned here has been tremendously valuable in scientific terms, says Mount. The Cosumnes is one of only a handful of rivers where scientists have been able to poke holes in the levees and watch what happens. Even where it is not possible to mount big restoration efforts, just learning more about how species rely on natural processes can help plan flow releases and restoration projects on other rivers. The Cosumnes has also provided a model for how other Central Valley rivers, the Feather, the Yuba, the lower San Joaquin to name just a few, can be similarly restored in at least some places.

It also gives weight to the argument for a comprehensive floodplain management policy that would discourage developing in floodplains up and down the Valley. Indeed, the Bureau of Reclamation and the U.S. Army Corps of Engineers are working on a comprehensive study looking at less intrusive ways to control floodwaters.

But will the next generations of Californians look out on to the Valley and see it again transformed, reforested and put back together again?


“I think we have to be realistic,” said Eaton. “We did some things that were pretty easy,” he noted of the levee breaches that TNC has done on the Cosumnes. These were levees that weren’t terribly effective in the first place, that houses weren’t built behind.

“Still, there are some very encouraging things going on,” he said of efforts on the San Joaquin and another Nature Conservancy efforts just getting underway to reforest a stretch of the Sacramento River between Red Bluff and Colusa.

“People recognize that the biodiversity that exists in the Central Valley is very important. I think that will energize efforts to recreate Valley habitats wherever it is appropriate.”

If the restoration of habitat catches on, then the landscape you see when you drop into the Valley from Redding might look at least a little more like it did hundreds of years ago.