North Delta island levee lowered in hopes of avoiding downstream flood damage

Levees threatened in the Delta.
Levees threatened in the Delta.

Work crews took emergency action Saturday to lower a portion of the ring levee surrounding the McCormick-Williamson Tract in an attempt to avoid a catastrophic “flood pulse” that last occurred in the floods of 1997.

The tract is a North Delta island just downstream of the confluence of the Cosumnes and Mokelumne rivers, just northeast of the Delta Cross Channel. The 1,600-acre island is owned by The Nature Conservancy in California. It’s adjacent to Staten Island.

Ryan Luster, Delta project director for the TNC, said Saturday that the TNC learned from experts on Friday that water flows along the tract would be strong enough to cause a levee breach similar to the one that occurred in the rainstorms of 1997.

Twenty years ago, a levee breach flooded the island, subsequently building up pressure and creating another breach and resulting flood pulse that swept up boats and smashed them into the Highway 12 bridge downstream, and created other property and structural damage.

Based on the current water flow projections and the 1997 history, Luster said the Reclamation District 2110 – essentially the TNC as the sole owner of the island – declared an emergency and opted to degrade and lower the levee at the opposite end of a natural levee breach.

The goal was to create a more controlled flooding of the island, without the building up of a damaging flood pulse.

TNC said the Sacramento County Office of Emergency Services, the Department of Water Resources and tract “neighbors” were among those notified of the planned emergency action. Luster said neighbors were highly in favor of the action.

TNC said a portion of the tract levee was breached on the eastern end about 8 a.m. Saturday, setting the emergency plan at the opposite end of the island in motion. That effort included use of a helicopter, lowering a backhoe and other equipment into the area. Work started around 11 a.m. and concluded about 4 p.m.

“So now we’re just waiting to see what happens,” Luster said. “We’ll be watching it tonight, tomorrow and Monday. The Mokelumne is forecast to keep going down.”

However, surrounding areas still could be at risk. The National Weather Service said Saturday night that flooding in the McCormack-Williamson Tract may threaten levees on nearby Staten and Tyler islands.

Eventually, Luster said, water will have to be pumped out of the flooded area. Over the long term, Luster said work is being done to preserve the island as a viable floodplain habitat.

Mark Glover: 916-321-1184, @markhglover