It's a sight only a biologist — or a fisherman — might love: a very big, and very ripe, spawned-out king salmon, tucked under...

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It’s a sight only a biologist — or a fisherman — might love: a very big, and very ripe, spawned-out king salmon, tucked under a log in a side channel of the Green River.

This fish, this log and this meander wouldn’t have been here even a year ago. A levee used to fence the river and the fish out. Then King County did what until recently was unthinkable: It ripped the levee out and gave this bit of land north of Highway 18 in Auburn back to the river.

The project, completed last year, reconnected the Green River with a side channel to provide refuge for salmon and water to recharge the aquifer. The county planted native vegetation on the banks, and hauled in woody debris to slow the current and create hidey holes for fish.

It’s just one of many projects planned to eventually return stretches of land totaling more than 33 miles along the county’s major rivers back to nature — rather than fighting it.

The difficulty of trying to hold a swollen river to its banks was on frightening display earlier this month in southwestern Washington, where flooding damaged or destroyed hundreds of homes and dozens of businesses in the Chehalis River watershed, and closed Interstate 5 for four days.

Levees expected to protect roads and buildings were overtopped by the floodwaters.

The work along the Green River is a new approach for King County, which already has allowed more than $7 billion in development in its floodplains, and has suffered eight federal flood-disaster declarations since 1990, most recently in November 2006.

“Rivers don’t negotiate with you. So you have got to figure out what the river’s behavior is going to be, and accommodate that,” said Ron Sims, King County executive. “Nature has the last vote and the longest memory.”

Increasing flood risk

And flood risk is only going to get worse, scientists say. That’s because of two converging trends: climate change and development, in a place uniquely sensitive to both due to our topography and weather pattern.

The Cascades are low mountains. And our weather pattern sees most of its precipitation in winter. That means that if average temperatures rise just three degrees, about a third of the precipitation switches from snow to rain, said Philip Mote, research scientist with the Climate Impacts Group at the University of Washington.

“It’s very likely that flood risk will increase in the future,” Mote said.

Cliff Mass, an atmospheric scientist at the University of Washington, predicts a 30 to 50 percent loss of snowpack by 2050, and a 75 percent loss by 2100. If people pump less greenhouse gas into the air, the impact could be reduced, but some losses are already certain, Mass said.

And changes in land use will make flood risks — already heightened by more rain and runoff — worse. That’s because as more land in the Puget Sound area is developed, forests that used to soak up rain and hold it for slow release are replaced by roofs and pavement. That sends more water into rivers more quickly.

No place in the country has undergone a bigger change in land use between 1973 and 2000 than what’s called the Puget lowlands ecoregion. During that time, 29 percent of the land, or 1,943 square miles, converted from one type of use to another, according to preliminary findings of a study by the U.S. Geological Survey.

The study, which is still under review, analyzes changes in the landscape in 11 categories — such as forestland, water and development — in sample grids all over the country.

The Puget lowlands ecoregion stretches from the Canadian border to Longview in the south, the Cascade Crest in the East to the Olympics in the West.

The high rate of change is driven primarily by the turnover of forestlands. In all, about 337 square miles of forest have been converted to development between 1973 and 2000, according to the study. That does not include land that’s been clear-cut but is still in forestry.

“Adaptation strategy”

In King County, there is already so much development built in the floodplain — home to some 65,000 jobs, including one-third of the county’s aerospace employment — the county has to deploy a combination of tactics to reduce flood risk.

Where it has billions of dollars of development to protect, the county is spending millions on a containment strategy, repairing levees that in some cases were cracked and vulnerable to failure.

The County Council last month increased property taxes by 10 cents per $1,000 of assessed value to raise money for levee repairs and other flood-control projects.

Where it’s still possible, the county is giving the river some space to flood. In those places, instead of disaster, floodplains can actually deliver benefits: aquifer recharge, open space, recreation, wildlife habitat and farmland preservation.

The county also has severely restricted new development in the floodplain. Some new construction is banned, and new fill in the floodplain must be offset by excavation at the same elevation. The policy is intended to help prevent an increase in flooding due to new development.

It’s a mixed approach to living with Mother Nature, in a place where a lot of choices have already been made. “This is an adaptation strategy,” Sims said.

“There comes a time when nature speaks to remind us of our insignificance. I accept that,” he said. “You are never going to constrain a river system in a way that says there is not going to be flooding. Sometimes you can let nature do what nature does best.”

Lynda V. Mapes: 206-464-2736 or lmapes@seattletimes.com