Host: California is getting ready for war with the U.S. Army. A legal war, anyway, between California water and wildlife agencies and the U.S. Army Corps of Engineers. The battleground? 1,600 of California levees that protect the Delta and Central Valley from flooding. The Corps wants the state to cut down most of the trees on those levees for safety's sake. But state officials want the trees to stay. Not only because they're valuable for wildlife and as scenery, but because they may actually make the levees stronger. Thibault Worth reports. 

Thibault Worth: Twitchell Island … in the Delta … south of Sacramento. Scrubby bushes and trees cover the levees here - their branches droop lazily into the slow-moving water. On an abandoned levee, UC Berkeley researcher Michelle Shriro is getting ready to dig a trench and fill it with water.

Michelle Shriro: So we'll be simulating something very similar to a flood event, but the water will be coming from the center of the levee rather than the side.

Worth: Shriro wants to find out whether tree roots might help a levee -- or hurt it during a real flood.

Shriro: Do we see that the seepage is increased by the presence of the tree? Or is the permeability of the soil around the tree root reduced? These are questions that we'd like to have answers to.

Worth: And those answers could shape the future of California's levees. The U.S. Army Corps of Engineers wants the state to meet a national standard that keeps levees clear of trees and shrubs. That standard is based on the Corps' belief that trees and their roots weaken a levee's structure. To comply with the rule California would need to cut down what amounts to a sprawling forest that winds through the Delta and up and down the valley.

Rodney Mayer: It's a kind of a one-size fits all policy that's really hard to live with and would be very expensive to comply with.

Worth: That's Rodney Mayer, a flood safety expert with the state Department of Water Resources. He says that trees have been growing on Delta levees for decades with Corps oversight. Today, they comprise the Central Valley's last riparian forest.

Mayer: We think they are important for the habitat they provide to endangered species. And also, there's value in just the trees themselves with respect to aesthetics.

Worth: That's an abstract value. But here's one that's not.

The Department of Water Resources says it would cost more than $7 billion to clear the trees from all 1,600 miles of the levees mostly in the Central Valley. And it says all that money wouldn't buy much in the way of public safety.

Mayer says a study looking at 300 historic levee failures over the past century shows that not a single one was caused by trees.

The Army Corps' proposed solution to a problem that may not exist angers East Bay Congressman John Garamendi -- a Delta native. He says in the past, the Corps not only tolerated, but encouraged the planting of trees on levees.

John Garamendi: Now the Corps is coming back and saying, 'Take it all out, we didn't mean it in the first place.'  Now wait a minute, first of all you've got habitat issue. You got endangered species issues, both in the water and also on the land.  Besides. What do you want?  A concrete floodway here? Is that what you want Corps of Engineers?  Well, we're not going to let that happen.

Worth: Garamendi says he's thinking of proposing a bill that would exempt California from the Army Corps policy. The Corps of Engineers says the "optimal" vegetation for levees is "well-maintained grass cover." The agency, which has nationwide responsibility for maintaining levee safety, is in the midst of standardizing its vegetation policy in the wake of Hurricane Katrina.

Trees aren't bad says Meegan Nagy, the Levee Safety Program Manager in the Corps'  Sacramento office. They just create more uncertainty about a levee's integrity.

Meegan Nagy: What you saw after Katrina was, a realization that we have to do a better job. And the "we" is the shared responsibility "we" -- local, state, federal agencies -- at setting our standards and living up to those standards.

Worth: But state officials say those standards are inconsistent. The Army Corps' own research shows that in some circumstances trees actually may strengthen or protect levees. And the Corps has granted itself a "variance" from its no-tree rule and will plant 30,000 trees on Valley and Delta levees later this year in order to enhance river-side habitat that's important for threatened species like chinook salmon.

That habitat issue is at the heart of a threatened lawsuit by the state Department of Fish and Game against the Corps policy. The department contends the Corps violated federal environmental laws in developing and imposing its policy.
State officials are also working on alternatives that don't involve lawyers. One would remove levee trees gradually -- as they die out. Trees and bushes on the water's edge would be allowed to remain.

The state hopes that will provide a way to help wildlife, save some of the Valley's last trees and avoid a long and costly legal war with the Army Corps.

For The California Report, I'm Thibault Worth.