When it comes to trees and levees, the U.S. Army Corps of Engineers needs to revise its one-chainsaw-fits-all policy.

A new study by the Corps reveals why.

The study, conducted by an Army Corps research unit in Mississippi, examined how trees affect flood-control levees in California, the Pacific Northwest, New Mexico and Mississippi.

It found that trees actually strengthen levees in some situations. It also urged that engineers conduct site-specific evaluations to determine if trees on levees are harmful or beneficial, according to a report Saturday by The Bee's Matt Weiser.

The Corps didn't need to commission a study to inject some common sense into this debate. But we are glad it did.

Ever since Hurricane Katrina, the Corps has been rigidly enforcing a policy of no trees on levees, regardless of circumstance. If that policy were to stand, local flood agencies would have to spend millions removing trees in California. And people who have grown accustomed to see gorgeous old cottonwoods and other trees along waterways would have to encounter riverbanks denuded of shade and wildlife habitat.

Already the new policy is forcing flood control agencies to spend scarce dollars that would be better devoted to actual levee repair. The city of West Sacramento is working to strengthen a section of levee along the Sacramento River near Bryte Park. To comply with Corps policy, it had to spend "a couple of hundred thousand dollars" on a contract to remove 37 trees in the vicinity and replace them with new plantings, according to Michael Bessette, the city's flood protection manager.

Compliance with Corps policy has also driven up the cost of the Sacramento Area Flood Control Agency's big levee project in Natomas.

There's no doubt that trees need to be removed when they inhibit inspection of a levee or are at threat of toppling, possibly gouging out part of the levee with their root balls.

But the Corps' new study shows the peril of continuing with an inflexible policy. If the Corps were to require removal of all trees near or on levees, it could actually undermine levee integrity, since some of these tree roots reinforce the soil.

Maj. Gen. Merdith W.B. "Bo" Temple became acting commander of the U.S. Army Corps of Engineers in June. He and Jo-Ellen Darcy, the nation's assistant secretary of the Army for public works, should heed the findings of the Corps study and develop a more flexible, case-by-case policy for levee maintenance nationwide.

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