University of California



Knights Valley, CA -- To a fish, such as a coho salmon or steelhead trout, the perfect home is a deep pool of water framed and shaded by a jumble of big branches, root wads, tree trunks or parts of living trees leaning over the stream.

To Millie Bisset and Craig Enyart, that perfect fish home used to look like a junked-up stream in desperate need of spring-cleaning. That is, until Jeff Opperman, a stream ecologist, explained that large woody debris in a stream means it’s healthy.

Decades ago, fish and game agencies found that timber companies’ logging debris, called “slash”, was blocking fish migrations. When they removed the “slash”, they began removing natural debris jams, too. “That legacy persisted,” says Opperman. “People still often think that large woody debris is ugly, bad for fish, or they use it as firewood.”

But research shows that natural debris jams are almost never barriers to migration, says Opperman. After all, fish managed quite well swimming through them in the centuries before humans settled around Northern California streams. In fact, without the large, messy-looking woody debris, fish don’t stand a chance of survival, says Opperman. The disappearance of woody debris from the state’s streams is another reason the fish disappeared, too.

Bisset and Enyart have 32 acres of land in Knights Valley, where more oak trees than vineyards still populate the rolling hills. A half-mile stream, most of which is lined with large trees, winds through their property. The previous owner allowed cattle access to the water. The cattle eroded the stream banks and ate tree seedlings.

“The idea was to save the land and preserve it as a California oak woodland,” says Bisset. The first thing she and Enyart did was fence off the cattle from the stream. They also planted over 300 native oaks on the property.

Then they began to think about clearing all the dead wood out of the stream. And when one enormous tree, more than 10 feet in diameter, began a slow tilt over the stream, they didn’t know what to do.

They heard about a seminar on stream management organized by Adina Merenlender, a cooperative extension specialist in University of California’s Integrated Hardwood Management Program, David Lewis of University of California Cooperative Extension (UCCE), and Jeff Opperman, then in UC Berkeley’s Department of Environmental Science, Policy and Management. The seminar, which was attended by about 30 other people who own small parcels of land with streams, was funded by a $10,000 grant from the Renewable Resources Extension Act, administered by UCCE. What Bisset and Enyart learned changed their whole approach to managing their stream.

These are the basics, Opperman told them:

More wood means more invertebrates, otherwise known as fish food, because the debris traps organic matter that drives aquatic food webs.

More fish food means more fish – juvenile salmon, resident trout, sculpin and other native fish.

More wood means more pools, as wood directs water to scour out deep parts in the stream channel.

The deeper the pools, the longer they last, and fish and their food have a better chance surviving the dry summer bottleneck. Juvenile coho salmon need to spend one summer and a winter in a stream before returning to the ocean. Steelhead trout need two summers and two winters before they migrate to the sea.

The other benefits of woody debris include:

Fish can hide under tangled branches from predators such as heron, raccoons and otters.

Partial dams created by woody debris can store the gravel that salmon need for spawning.

The sediment accumulating upstream of a debris jam can provide a place for riparian tree seedlings to root.

Natural debris dams control the shape of the stream channel and slow the water so that young fish don’t have to spend energy fighting the current or be flushed downstream before they’re ready to migrate.

Woody debris piles are actually small, diverse communities, where fish occupy pools, and various species of invertebrates colonize the wood – some burrowing into rotting wood, others living on the wood above the water, and some using the wood below the water. Fungi and algae grow on the wood’s surface to form a film that’s a primary food source for other species of invertebrates called scrapers. All these invertebrates are food for steelhead trout, coho salmon and sculpin.

Wood finds its way into streams through erosion of stream banks, landslides, wind-throw, or tree death. It may take several human generations for a large tree to die and make its way downstream. Some trees fall over streams, but remain living and rooted to the banks, and begin sending branches vertically. Streams change from year to year and so does the amount of wood in the stream. For example, in 1987, 25 debris jams occurred along a four-mile stretch of Wildcat Creek in Alameda County; in 2001, it had well over 100.

One of the reasons Opperman wanted to offer the seminar is that most of California’s hardwood ecosystem is privately owned by people who have small parcels of land. For years, foresters have been given information about the importance of leaving wood in streams. But on land where trees aren’t harvested, “there’s very little information available to owners,” he says.

In looking at a sample of hardwood streams, Opperman found that oak woodland streams on private lands generally had very low amounts of large woody debris. In certain areas, such as the Russian River where 95 percent of the watershed is privately owned, he considered it critical to educate landowners if streams were to see healthy runs of migrating fish.

During the seminar, the landowners learned that wood shouldn’t be removed from streams for firewood or to improve aesthetics and, in general, debris jams were beneficial – not harmful – to fish. They were also directed to resources, such as fisheries biologists, if they thought a debris jam looked like a barrier to fish migration. A biologist can remove two or three pieces of wood to make the jam passable, rather than a landowner going through the expense of removing the entire debris pile.

If landowners weren’t able to assess whether woody debris would divert water toward structures, such as houses or barns, or threaten the safety of a bridge, they were advised to contact their local water district or Natural Resources Conservation Service for advice.

After the seminar, a trunk and root wad of a large oak that once seemed like an unsightly mess looked different to the couple. And Bisset stopped worrying about the large valley oak that was leaning ever further across the stream. “If the tree continues to fall, it may still live and be a beautiful bridge and have a habitat underneath,” she says. “So I’m not losing sleep over that tree anymore.”

-- by Jane Ellen Stevens
November 2004
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