People watch water being released through the Oahe Dam's Stilling Basin near Fort Pierre, S.D. on Saturday May 28, 2011. U.S. Army Corps of Engineers officials say they have to release more water than previously expected through the Missouri River dams because of expected heavy rains upstream. The corps now plans to maintain releases from Oahe Dam at the current level of 85,000 cubic feet a second until Thursday, June 2, 2011, and then step it up to 150,000 by mid-June - much higher than previously expected. (AP Photo/Toby Brusseau)

Hundreds of Thousands Evacuated in California Near Nation’s Tallest Dam

Over 180,000 Northern California residents have been evacuated from their homes in the vicinity of Lake Oroville.  The evacuation order went out Sunday evening as a “hazardous situation” developed on the auxiliary spillway section of the nation’s tallest dam.

From the Butte County Sheriff Sunday night:

This is an evacuation order.

Immediate evacuation from the low levels of Oroville and areas downstream is ordered.

A hazardous situation is developing with the Oroville Dam auxiliary spillway. Operation of the auxiliary spillway has lead to severe erosion that could lead to a failure of the structure. Failure of the auxiliary spillway structure will result in an uncontrolled release of flood waters from Lake Oroville.

In response to this developing situation, DWR is increasing water releases to 100,000 cubic feet per second.

Immediate evacuation from the low levels of Oroville and areas downstream is ordered.

This in NOT A Drill. This in NOT A Drill. This in NOT A Drill.

The Oroville dam holds back California’s second-largest man-made reservoir which has been filling rapidly as a result of repeated heavy rainstorms throughout Northern California this winter.  Damage was first spotted on Oroville’s primary spillway nearly a week ago, but in light of the ongoing rain, Department of Water Resources (DWR) officials had no choice but to continue to use it–albeit cautiously–in an attempt to keep the level of the lake manageable.  Despite their efforts, the lake reached 100% capacity Saturday morning and water began to crest the adjacent auxiliary spillway.

By Sunday evening, large sections of the auxiliary spillway had eroded prompting local officials to issue evacuations of the downstream areas of the Feather River Basin in Butte, Yuba, and Sutter counties including the towns of Oroville, Marysville, Plumas Lake, Live Oak, and Nicolaus.  Every road and freeway out of the area was clogged with evacuating vehicles last night fleeing to shelter areas in neighboring counties, the Sacramento metro area, and nearby Beale Air Force Base.

Fortunately as of this morning, the water level in Lake Oroville has descended below 100% and water has stopped flowing over the auxiliary spillway.  Inflow to the lake has dropped to 45,000 cubic feet per second (cfs) and outflow down the primary spillway remains at 100,000 cfs which should keep the lake below maximum for the time being.  Though the primary spillway is severely damaged, and there remains a high risk of exacerbated damage due to its heavy use, it is currently the only means the DWR has of controlling the level of the lake.  Crews have been working throughout the night to shore up damaged sections of the auxiliary spillway.

As the sun rises over the Pacific coast this morning, engineers and specialists will get their first good look at the dam and its spillways since the lake stopped overflowing late last night.  The evacuation order will remain in place as officials inspect the dam and determine how best to proceed.

They’ll have to work fast.  More rain is expected later this week.

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Russell Patten

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