For years, California flood control officials have pleaded with the U.S. Army Corps of Engineers to reconsider a policy that bans trees on levees, urging the agency to heed years of independent research that found little risk to flood safety.

Now, a new study by the Army Corps itself has found that trees actually strengthen levees in some situations. The conclusion could force the agency to reconsider its policy, which would otherwise eliminate millions of Central Valley trees.

The controversial levee maintenance policy, which emerged in the wake of Hurricane Katrina, allows only grass on levees, on the grounds that trees could damage levees if they fall down or that their roots may promote seepage that would undermine a levee.

The policy applies nationally. But its consequences may be greatest in California's Central Valley, where millions of trees line hundreds of miles of levees. Making up the Valley's last remaining riparian habitat, the trees are considered crucial to migrating salmon, hawks and a variety of other wildlife.

California has long operated under separate rules – once endorsed by the Corps – that permitted large vegetation as long as plants are maintained to allow inspections.