Stream rehabilitation gets back to nature

Wolf Creek in Northern California does not look like an engineering classroom.

In fact, five years ago, Wolf Creek did not look like much of anything. More than a century of logging and construction had left it a dumping ground for old concrete and cars — a nearly straight, wide, deeply incised channel used only for refuse and gravel extraction.

But the creek was the site of a project that resulted in a spectacular semi-failure and has focused attention on an innovative method of stream rehabilitation that may change the way hydrologists look at reviving stream systems. Flood-prone Wolf Creek, which has long been considered a bad-tempered stream, runs through downtown Greenville, Calif.

The first phase of its rehabilitation was completed in late 1990. Twenty-eight-hundred-feet long, the rehab project involved the moving of 15,000 yards of dirt and gravel and placement of 1,200 logs and 400 root wads with logs attached (donated by the U.S. Forest Service), 800 yards of small riprap, 450 three- to eight-foot boulders and a few hundred smaller boulders The work was done by Wilburn Construction, a local company, using men who have worked most of their lives in the woods as “gyppo” or small-time loggers. The result was the first major Rosgen stream rehabilitation project in California.

Named for its inventor, Dave Rosgen, the method of using heavy equipment and careful design in stream rehabilitation projects has achieved prominence in Colorado and other western states.

Still, it was a major leap of faith when the non-profit Plumas Corp. and 17 other local, state and federal agencies pinned their environmental and flood control hopes on a mass of salvaged trees and roots anchored in sweeping curves by the remains of former concrete flood control structures that had previously failed.

The work was innovative in other ways, too. For instance, members of the local Native American community received job training and retraining to help with the restoration work.

And 10-year monitoring mandated by the U.S. Environmental Protection Agency, which would have cost more than the construction itself, was instead being done by the conservation classes at Greenville High School, a national first.

Because of the complexity of the engineering and its application, Rosgen usually builds his own projects, bringing in his own operator to run locally leased equipment. But at Wolf Creek, local crews made up of loggers and operators — who are well aware that theirs may be the last generation in their families to make a living in the traditional logging industry — were trained to do the work.

Rosgen is a natural teacher, constantly asking questions and demanding the calculations and observations of his listeners, whom he urges to “think like a stream.”

His method, complicated as it is, involves no metal or concrete (except as scrap sometimes) and may cost as little as one-fourth what standard flood control structures go for. It has its own requirements though, among them great precision and that ability to learn to think like a river. “There are no cookbooks for this,” says Leah Wills, erosion coordinator for the Plumas Corp., a non-profit organization that deals with erosion problems and watershed degradation in Plumas County, which spearheaded the rehabilitation project.

Early monitoring results were encouraging. “For the first time,” Wills noted then, “black flies aren’t the number one invertebrate species in Greenville.” Swimmers and picnickers began returning to the creek, which Wills says was “clean and shady where it was just scummy and hot before.” Additionally, fishing picked up when the trout began reappearing.

Rosgen and the townspeople were cautiously optimistic that the stream rehab would hold under flood conditions, especially since other Rosgen projects had withstood massive floods within days of their construction without sustaining major damage.

But 1994 produced one of the wettest winters on record in Northern California, with warm rain on deep snow producing a “bank-full event” that lasted several months.

The narrow stretches of the creek above and below Greenville held; but when a 70-acre swamp area upstream developed a headcut, the part that ran through town was too confined to handle the 10 feet of water and 130,000 tons of sediment that came pouring downstream.

But the spectacular failure produced positive results. Land, bridges and houses that had suffered severe damage in past floods were largely unaffected, despite a worst-case-scenario flood. Additionally, the rehab gave local workers skills and training that will serve them well in the future.

And it sent Dave Rosgen back to the drawing board.

THE BIG PICTURE

Rosgen’s method of stream rehabilitation has four objectives:

* balancing sediment input and outflow;

* stopping downcutting — the process of stream channels becoming more deeply incised below the flood plain level;

* lowering the width-to-depth ratio so that the normal flow channel is narrower and deeper; and

* raising the groundwater level.

Appropriate width-to-depth ratio is established by creating a sinuous meandering channel that mimics the sequences of deeps and shallows (called pools and riffles) that are characteristic of almost all natural channels. Rosgen generally begins by mapping a channel via aerial photographs, so that vegetation and residual remnants of the original floodplain are preserved to every extent possible.

Traditionally, engineers have used concrete in stream rehabilitation projects. That, while appropriate in some circumstances, is a short-lived solution at best, according to a number of hydrologists and land-use planners. Concrete, even when functioning according to design, simply moves the costs — sediment transport and water velocity — downstream to the next unprotected section of the stream. In fact, traditional concrete and riprap channeling actually increase water velocity at the lower end of a project and leave sediment accumulating in downstream reservoirs.

Ideally, Rosgen’s method slows the velocity and raises the channel; thus, when high flows occur, the surface roughness and area of the floodplain allows a higher volume of water to flow at a lower velocity. When the water is slowed, it drops its carried sediment onto the floodplain.

Natural vegetation, a key part of Rosgen’s rehab method, and one that is frequently conspicuously absent in concrete projects, also has a slowing effect on floodwaters.

Perhaps the main lesson of the Wolf Creek failure is, as Wills says, “You can’t just take a little segment of a stream in a big watershed without considering the parts at either end.” That means, she says, the question