Auburn, Kent, Renton and Tukwila face their most serious flooding risk in 40 years this fall and winter because of January damage to a flood-control dam on the Green River, authorities have warned.

Four South King County cities face their most serious flooding risk in 40 years next fall and winter because of January damage to a flood-control dam on the Green River, authorities have warned.

The Army Corps of Engineers, which built and maintains the Howard Hanson Dam, says it doesn’t know what caused a 10-foot-wide, 6-foot-deep depression in an abutment to the rock and earthen dam.

As a safety precaution, the Corps will store less water behind the dam until engineers can figure out what caused the problem in the reservoir wall adjoining the spillway — and how to fix it.

In the meantime, the Corps will be forced to release into the lower Green River essentially all rainwater from storms, and risk overwhelming the levees that protect low-lying parts of Auburn, Kent, Renton and Tukwila, the federal agency has warned.

“We need to prepare for a long-term possibility that over the next few flood seasons we may experience anywhere from significant to catastrophic flooding, depending on the event,” said Dana Hinman, a Auburn city spokeswoman.

Officials from the four cities, the Corps and King County have been telling businesses and residents about the danger and are urging them to buy flood insurance and be prepared to evacuate in the event of a disaster.

“We can protect life. We don’t know that we can protect property,” Hinman said. “We’ve estimated maybe in the neighborhood of 3,000 people [in Auburn] could be affected in a large-scale event.”

A larger number of homes and businesses could be hurt in flood-prone parts of Kent, where about 50,000 people work and 22,000 people live, said Mayor Suzette Cooke. She said the damaged abutment “clearly raises our level of concern” about levees downstream that haven’t been certified as meeting federal standards.

If the substandard Horseshoe Bend Levee were to fail, Cooke said, the Green River Valley could be flooded all the way to Interstate 405 in Renton, possibly severing Highway 167 and two main rail lines.

“For the first time in my life I will actually be hoping we have a drought for a few years,” Cooke said.

Renton’s floodplain has industrial parks, big-box stores and other commercial ventures but fewer homes than Kent and Auburn, said Renton spokeswoman Preeti Shridhar.

The Green River Valley, formerly a farming area, sprouted Boeing plants and warehouses after the Howard Hanson Dam put an end to once-regular flooding when it was completed in 1961. A deluge in December 1959 flooded some houses up to their second floors.

By 2007, the valley cities had attracted so much business that a study prepared for King County by ECONorthwest found that a one-day shutdown of commercial activity in the county’s flood plains would mean an economic loss of $46 million. The overwhelming majority of that commercial activity is in Kent, Renton, Auburn and Tukwila.

King County launched a new flood-control district last year to maintain and repair levees, but the Green River levees aren’t designed to handle the amount of water that may be released from the Howard Hanson Dam next winter if the damage isn’t fixed.

Col. Anthony Wright, commander of the Corps’ Seattle district, will brief the King County Flood Control Board on the issue Monday. Crews are preparing to dig into the dam abutment in an effort to find out what caused the depression.

Some participants in a public meeting with Corps and city officials in Auburn last month said they were told repairs might not be completed for two to four years and the cost could be $100 million.

But Corps of Engineers spokeswoman Casondra Brewster said officials won’t know how long or how costly repairs will be until they determine the cause of the depression and develop a repair plan. She said there is “a good chance” that some repairs could be made by fall.

The depression was discovered as water receded after storms dumped an unprecedented volume of storm water into the reservoir, filling it to its highest level ever.

“This is a complete anomaly. It has not occurred before. So it caused us concern,” Brewster said of the damage.

The dam itself has not been harmed, Brewster said, but the Corps has restricted the amount of water it will hold in the reservoir in order to make sure it isn’t damaged while the problem is being diagnosed.

She said the damaged abutment was built on the side of “an ancient landslide” whose stability needs to be studied.

Keith Ervin: 206-464-2105 or kervin@seattletimes.com