Feds: Dams helped prevent more severe Skagit River flooding

By Kimberly Cauvel / Skagit Valley Herald

Skagit County’s first major flood of the season — and the first in at least 15 years — could have been more catastrophic if not for the dams that regulate the flow of the Skagit and Baker rivers, according to the federal agency that oversees operations of those dams during flood events.

The Army Corps of Engineers estimates that the height of the Skagit River during flooding two weeks ago could have been 8 feet higher — a level that would have had the river going over the top of Mount Vernon’s downtown floodwall, pouring over levees and bringing catastrophe throughout Skagit County — if not for the Ross and Upper Baker dams holding back the rush of rainwater.

The corps’ Seattle District Reservoir Control Center began its round-the-clock emergency oversight of those dams on Nov. 14.

Corps staff directed Seattle City Light, which operates Ross Dam, and Puget Sound Energy, which operates Upper Baker Dam, on how to capture the rain-driven inflow behind those dams that set record highs.

Sonja Michelsen, the corps’ Western Washington senior water manager, said while Upper Baker Dam saw a slightly higher record flow of 41,000 cubic feet per second, Ross Dam saw a significant new record of 60,000 cubic feet per second over its previous record of 50,000 cubic feet per second.

The corps reported that its ability to capture most of that water behind the dams decreased the peak flow of the Skagit River downstream by 40%.

“The reservoirs offer quite a bit of flood risk reduction,” Michelsen said.

While the record-setting flows poured into the reservoirs — into Ross Lake above Ross Dam and into Baker Lake above Upper Baker Dam — those reservoirs saw about 12-foot and 14.5-foot rises, respectively. Michelsen said Ross Lake filled to 99% of its storage space, and Baker Lake filled to 98%.

“They were within inches of full,” she said.

Ross Lake peaked within 3 inches of its normal full level and Baker Lake peaked within 9 inches of its normal full level.

During the flooding, the corps directed Seattle City Light and Puget Sound Energy to continue releasing 5,000 cubic feet per second of water from the dams. That’s the minimum flow required under the dams’ federal licenses.

That meant downstream Skagit River flows were shielded from the other 55,000 cubic feet per second flowing into Ross Lake and the other 35,000 cubic feet per second flowing into Baker Lake.

Those high flows were prompted by what Michelsen calls a “really big, monster atmospheric river” that dumped rain on Western Washington. While the weather pattern that spurred this flood was different from conditions that led to the last major floods in 2003 and 2006, there were also similarities between the three.

Michelsen said all three major floods came early in flood season — October and November — and the corps relied on similar interventions at the dams to limit downstream impacts.

“It was really similar in how we had to be intentional about how that space was managed,” she said.

Both Seattle City Light and Puget Sound Energy are required under their federal dam licenses to have space available behind their dams to hold back water. The amount of space required changes throughout the flood season.

At the start of the major flooding in mid-November, Upper Baker Dam was required to have 69,000 acre feet of storage space available. Ross Dam was required to have 58,000 acre feet.

Ross Dam had about two and a half times the required space, at 145,000 ace feet. Michelsen said that extra space made a big difference.

“If we didn’t have that extra storage space we wouldn’t have been able to hold back as much water and the peak flow at Concrete and Mount Vernon would have been higher,” she said.

The corps handed operations back to Seattle City Light and Puget Sound Energy a week ago, after emptying the reservoirs back to flood-ready levels.

“The corps prioritizes emptying the reservoirs quickly to prepare for the next flood,” Michelsen said.

While that means the Skagit River receded more slowly than it would naturally, it also means the dams are ready to spring into action should another flood come — like is forecast now to begin this weekend.

“We’re ready and prepared … and we are watching the forecast carefully right now,” Michelsen said.

The corps plans to again oversee operations of the dams beginning this weekend.