"This project involves the largest scale bioengineering of its kind ever attempted in the Yukon," says Jane Koepke about the new culvert installed at Mendenhall River. The culvert replaces an aging bridge as part of this summer's completion of the Champagne Revision, the Alaska Highway upgrade between Whitehorse and Haines Junction.

Koepke is the environmental coordinator for Yukon government's Transportation Engineering Branch, and she's describing the ambitious solution that YTG and Fisheries and Oceans Canada (DFO) came up with to address both engineering considerations and fish habitat compensation requirements.

A replacement bridge would have been significantly more expensive than a culvert. But as Koepke points out, the major problem with a culvert is its larger environmental footprint.

Raquel Roizman is a DFO habitat biologist working on the project. "Fish habitat is generally better protected by using a bridge," she says.

"But we looked at the energy of the river -- the Mendenhall is very snakelike and slow, with lower freshet (spring melt)," she says. "The Mendenhall was an ideal candidate for trying a culvert of this size (6.5 metres wide and 31 metres long). Also, while the river's fish values are important, we felt that they could be adequately compensated in this case."

Starting from Taye Lake north of Champagne, the Mendenhall River flows south and then east into the Takhini River. A relatively high number of fish species are found in the Mendenhall, including Arctic grayling, lake chub, northern pike, round whitefish, slimy sculpins, suckers and minnows. Juvenile Chinook have been found as high as the Mendenhall bridge, but their occurrence there is not well documented.

"The design actually creates more habitat than before, which is one of the reasons we liked it," says Roizman. "Maybe we'll see more Chinook in there. There will be a fairly lengthy monitoring program."

"It's quite a challenging project," explains Koepke, tracing the route on a map. "We needed to address a number of important issues, like diverting traffic in the busy summer season, managing erosion, and protecting fish."

"The best solution was to relocate the actual river crossing. This way we could keep the river flowing under the bridge and the construction work as dry as possible."

The new culvert connects to an old oxbow on the north side, and to a portion of the river that runs parallel to the highway on the south side. In essence, the project cuts out a small portion of the river that ran under the old bridge. Opening up the abandoned oxbow channel presented an opportunity to reclaim lost fish habitat.

"Air photos taken in 1964 actually show that this oxbow was the active stream channel, while a 1992 aerial photo shows the river in the more recent channel with the oxbow abandoned," says Koepke.

To make the new culvert work, this winter a crew will remove plugs of soil at either end of the oxbow, and block the existing channel so water flows through the oxbow and into the new culvert. As Koepke explains, work with heavy machinery to excavate the plugs must be done when the ground is frozen in order to minimize sedimentation.

But constructing the culvert and redirecting the channel are really just Phase One. Phase Two involves the protection of the new channel from erosion. Normally, crews would stabilize the banks with rock, called 'rip rap', to above the high water mark. But Koepke explains that this project is taking a different approach.

"Typically, rip rap is engineered for the one hundred year flood level. But DFO saw an opportunity for more creative habitat compensation," she says. "Given that the design requirements of this project were a little more forgiving, we opted for a more natural approach."

In addition to rip rap that has already been placed, crews will bioengineer a more natural outcome. Next spring, a major re-vegetation project will see literally the planting of thousands of willows.

Koepke explains how it's done. "First we lay a soil wrap-it's like a pillow made of biodegradable cloth and filled with earth. Next we lay willow stakes on top, and they'll take root and create shade and habitat along the river, and introduce leaf matter and support insect-loving fish."

She also explains how they have used 'root wads': inserting trunks of dead trees into the streambed so the roots protrude into the channel. "Root wads create natural cover for fish and increase the complexity and variation in the velocity profile of the river," she says. In other words, the roots make swirls and eddies, and create places for fish to rest and feed.

"The planting will maintain the aesthetics of the spot," says Roizman, "and it will allow this disturbed environment to function more closely to natural riparian areas."