SEATTLE (AP) -- The rows of giant sandbags now lining the Green River may calm residents who feared water could swamp its banks.

But the new barrier only masks risks still hidden below.

Beneath the stacks of plastic designed to prevent flooding from a damaged dam rest aging levees in serious decay. The right mix of storms could wipe them out and flood the valley before the river even hits its crest.

About a half-dozen stretches between Auburn and Tukwila are so porous or unstable that they're considered a failure risk -- even if the Howard Hanson Dam is operating normally. If problems with the dam force operators to release enough water to reach the sandbags, odds that the levees won't hold increase.

"In 50 years, the water's never been that high," said Tom Bean, a King County flood plain engineer. "Nobody can say what will happen."

Storms last winter damaged an abutment to the dam, and the Army Corps of Engineers concluded this year it might have to release more water than usual and cause flooding to prevent a catastrophic collapse.

After months of panic and millions of dollars in preparations, the corps said its temporary repairs this fall dropped the possibility of a significant flood from 1 in 3 to 1 in 33.

But permanent repairs that would lessen the threat further may be five years away. And concern for the levees is not new.

Some sections are so old and loose that high water could saturate and blow them out. Others have been undercut or carved into cornices or have deteriorated until muddy clumps slump off and wash away.

Rushing water already tunnels beneath some banks, forming muddy lakes along adjacent roadways even in mild rains. In one spot, levee instability caused an asphalt path above to crumble into a 4-foot sinkhole.

Shoring up the levees is slow and expensive, even as the damaged dam elevates risks. And the corps' best shot at preventing flooding -- quickly draining the reservoir between storms -- may, in some cases, cause even more levee damage.

"That's the thing that's causing me the greatest concern," said Hillman Mitchell, public-works director for the city of Tukwila. "Rapid drawdown of the river creates suction on the earthen structure and actually pulls pieces of it into the water."

An easy place to track the depth of levee problems is a half-mile stretch in northeast Auburn. On a recent winter morning, Bean, the county engineer, wandered a path above the river pointing out disrepair.

During high water, flood patrollers used to drive these banks 24 hours a day, looking for signs of damage. But the rows of thick sandbags means vehicles don't fit, so inspections can take four times longer.

Bean pointed to a tree trunk curved like a pistol rising from the bank -- evidence that the levee had shifted over time. Cracked pavement farther downstream confirmed his analysis. A few hundred feet beyond, he saw erosion-steepened banks, which further increases the risk of collapse.

And there are other issues. When the river is high, flap gates that cover storm culverts draining into the river can get propped open by debris, causing water to run backward through the pipes. On this day, water was seeping through the levee and creating ponds in nearby woods. The cloudy brown water told Bean the moisture was thick with sand, which suggested the levee actually was liquefying. County engineers warn that can lead to a blowout.

All this is taking place a few dozen yards from a mobile-home park that already fills with river water during modest flows within the normal range. A levee failure could flood these homes -- and cause millions of dollars in damage downstream.

"I have relatively high concerns about the levee system as a whole," said Steve Bleifuhs, who runs the county's river plain section. "That's one of the places we worry about most."

There are several others like it downstream.

It's no secret why the levees are in bad shape: Poor construction, decades of neglect and the power of moving water over time have weakened these mounds of sand and earth severely. Most were constructed at least 50 years ago by farmers who pushed mud and rock up with plows or bulldozers. Some levees are mixed with old tree stumps and rusting car frames.

They never were designed to last this long, but the Howard Hanson Dam upstream provided false security. Between Highway 18 and Interstate 405, the levees are high enough generally to hold flows of up to 13,900 cubic feet per second (cfs.) of water. But the corps never has let the flows exceed 12,400 cfs. since the dam was built in 1961. There hasn't been a major flood here in a half-century.

But decades of wear have taken a toll. So much water leaks through levees on a heavily industrialized stretch between Kent and Renton that engineers have reported seeing "sand boils," volcanolike burbles of water and earth that appear on what should be the dry side -- another sign that the levees are eroding from within.

A stretch near a massive housing development regularly sloughs into the water. Condominiums in one short section near a horseshoe bend in Kent are stacked within 10 feet of a dangerously steep river bank.

Recognizing that a changing climate might only make winter rain more frequent, the county in 2007 created a flood-control district and started collecting taxes to fix crumbling levees. It made repairs on the Green River a priority. Some parts, including a stretch in downtown Kent, are finished.

But the district's budget for repairing levees on rivers across the county is $39 million a year, while rebuilding levees only on the Green is expected to cost nearly $300 million.

Combined with emergency money from Kent and the state, about $65 million has been allocated for Green River bank reconstruction.

Many fixes are years away. Design of repairs to the section near the Auburn mobile-home park will come next year, but construction won't start until the year after, if not later.

"In some cases the cost is tens of millions of dollars," said Mark Isaacson, who runs the county's water and land division. "In some cases we don't even own the land."

Private developers often are reluctant to sell riverfront property. In one stretch near Kent, a developer plans to fix the levees across his land, but only in the course of developing his property, a multiyear process.

The Legislature set aside $10 million to begin acquiring land along another section next year, but improvements will take several years.

Meanwhile, Tukwila has built temporary retaining walls, similar to more permanent structures erected in New Orleans, some of which failed during Hurricane Katrina. Meanwhile, the levees continue to decay.

Even with temporary fixes to the dam, the reservoir can't hold nearly as much water as normal.

The corps is counting on being able to release up to 13,900 cfs. of water -- the capacity of most of the levees -- in heavy storms.

But some of the most troubling stretches have a 2 to 6 percent chance of failing every time they near their capacity. And a 100-year flood -- always a possibility -- could leave the river running at full bore for more than a week.

"When we have a higher likelihood of high flows, we'll have a higher likelihood of failure," said Larry Karpack, a consulting engineer for the county.

For now, the county and the corps seem to be doing what they can, said Derek Booth, a University of Washington geology professor who has studied Green River flooding scenarios. But Booth is convinced the true odds of serious flooding probably are higher than 1 in 33.

"We're looking at having to go five years without any problems," Booth said. "But it only has to come up bad once."

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