During a heavy rainstorm this winter, the waters of the Green and White rivers could rise earlier and remain high longer than in the past, but river watchers shouldn’t panic.

The high water will be intentional.

Federal flood control managers on the weakened Howard Hanson Dam said Tuesday that they will try to take the edge off any storm in the Green River basin by releasing water early to prevent a dangerous buildup in the reservoir behind the dam.

The annual task of weather forecasting and balancing flood control and dam safety for the Green River is always difficult, said Larry Schick, a U.S. Army Corps of Engineers meteorologist and a manager in the Corps Reservoir Control Center in Seattle.

With the dam able to hold back safely only one-third of the water it used to, he said that balancing act is going to be even more delicate.

“We have to start working before (a flood event) actually starts,” he said Tuesday during a media tour of the center.

Mud Mountain Dam has managed flood risk on the White and Puyallup rivers since 1947. It is working fine and can hold back plenty of water if it has to.

The White River, however, can no longer take as much floodwater from the dam as it has in the past. Last January, the Corps released water and ended up flooding parts of Pacific and the Sumner industrial park.

With that in mind, the National Weather Service has lowered the flood stage on the White River below the dam from 10,000 cubic feet per second to 8,000 cubic feet per second.

The practical result is that the Corps won’t be able to get rid of as much water from the dam reservoir as fast as it has in the past.

Schick said regulators will release a bit more water during a storm event than in the past in order to keep the reservoir as empty as it can in case there is another rainstorm on the way.

The Green River is the real worry this flood season.

If a big enough storm occurs, there is the possibility of catastrophic flooding in the Green River Valley. The valley floor and the cities of Auburn, Kent, Renton and Tukwila have escaped flooding since Howard Hanson Dam was built in 1962.

Schick said decisions this winter about holding water at the dam or releasing it into the Green River will be up to Col. Anthony Wright, the district Corps commander.

There will be people walking the levees to check on the river level and the condition of the levees. The Corps also will have a liaison in the King County Flood Warning Center.

Work is almost done on the grout wall installed in the right abutment at Howard Hanson Dam. The wall is a temporary repair to help prevent water leaking through it from the dam reservoir.

Efforts to prepare for a major flood in the Green River Valley also continue. Sandbags are protecting homes, levees and businesses.

Mike Archbold: 253-597-8692

mike.archbold@thenewstribune.com