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Tree Drop Restores Au Sable River Habitat
November 6, 2008
Department of Natural Resources fisheries biologists have learned that, given time, Mother Nature will take care of itself quite well. But that doesn't mean she can't use a little helping hand to get through some rough patches. And that's exactly what's going on as a 10-year, $2 million habitat improvement program reaches its midpoint.
The Au Sable Headwaters Restoration Project is working on improving Michigan's most famous trout fishery from Mio Pond upstream. The project -- funded by the Sport Fish Restoration Act, which is the money collected in excise taxes on sales of fishing equipment -- includes the mainstream of the Au Sable as well as the North Branch, the East Branch and the South Branch. Money for the work is available on a 75/25 split with the federal portion paying the lion's share.The purpose of the project, said DNR Fisheries Biologist Steve Sendek, is to make up for the environmental damage done more than a century ago.
In the natural scheme of things, trees along a river get old, die and fall in. But that hasn't happened on the headwaters of the Au Sable since the late 1800s, when loggers denuded the banks; there simply haven't been any trees to fall into the river. So, as part of the restoration project, fisheries workers are dropping whole trees into the waterway.
"When the lumberjacks clear cut along these rivers, they interrupted the natural processes," said Sendek, who oversaw the placement of 300 trees in the section of the mainstream from Wakeley Bridge to just below Connor Flats this past summer. "Most of those tree species have about a 150-year life span. We're allowing the river corridors to revegetate and grow back, but we're not there yet. So this process is designed to get us another 20 years or so when the natural process can take over."
As one might suspect, putting whole trees into the river involves plenty of heavy equipment, including a helicopter that transports the trees from the "pick site" to the river and drops them in place.
The pick site for this summer's project, for instance, was state forest land not far from the river. The site was a part of the forest included in a recent forest management unit compartment review that had approved timber harvest for the area and was in a place where a wildlife opening was recommended.
The trees were knocked down with an excavator before being airlifted to the river.
"That way we get the root wad and everything," Sendek said. "We're counting on the root wad to anchor the tree in the stream. But if it doesn't anchor where we put it, it just sweeps downstream and hangs up somewhere else. We're trying to mimic nature in creating diverse in-stream habitat. Woody debris has always been a key component for in-stream habitat for a variety of reasons."
For one thing, woody debris provides cover for fish to help protect them from predation.
"Our population trends that we've collected over the last 50 years show that if we can carry a year-class through, year after year after year, we can provide a very nice fish community for anglers to catch," Sendek said. "We need them to survive for three or four years.
"We put these trees both in the shallow, muddy water for protection for nursery areas for larval fish and young-of-the-year fish as well as in deep water areas so the larger trout have refuges. We also try to put them near spawning areas because we've found that predation is very high on spawning fish and overhead cover increases survivability and allows them to complete their spawning cycle."
But fish cover is just one of the positives associated with woody debris.
"The trees also help funnel current, diversifying the stream channel, creating pockets and pools and removing excess sand sediment toward the bank where it can revegetate and restabilize," Sendek said. "By doing that, we uncover buried woody debris and various gravels and cobbles that are important to benthic (aquatic insect) life."
Large woody debris also helps increase the productivity of the river by trapping vegetation, an important factor in the food web.
"The Au Sable is very limited in nutrients," Sendek explained. "The sandy soils are very infertile soils so very few nutrients are transferred into the water. Every year organic matter -- grasses and leaves -- falls into the stream. But in order for it to benefit the food web, it has to stay in the river for awhile so the biological processes -- bacteria, insects, etc. -- can break it down and make it available.
"In a clean channel, all the organic material basically blows out downstream and isn't transferred and utilized," he continued. "The large woody debris catches this material and allows the time necessary for those biological processes to take place."
The Au Sable River maintained its fishing quality for many years despite a lack of woody debris because it was receiving nutrients from another source. Prior to the passage of the Clean Water Act, the city of Grayling dumped its sewage into the river. The Au Sable held a biomass of trout of up to 175 pounds per acre in the early 1970s.
"In the late '80s, it dropped to about 60 pounds per acre," Sendek said. "This fall, we sampled it again and we're up to 145 pounds per acre. We're almost back to the good old days and the only thing we've changed is the amount of debris in the river."
The Au Sable Headwaters Restoration Project's work on the North Branch took three years to complete and the work on the mainstream is two-thirds complete. There's still a summer of work to do on the mainstream, a year on the East Branch and then three years on the South Branch.
And after that? Sendek, for one, is hopeful that Mother Nature will take over again.
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