Thousands remain in shelters after California dam evacuation

"It is important to recognize that during a rare event with the emergency spillway flowing at its design capacity, spillway operations would not affect reservoir control or endanger the dam", wrote John Onderdonk, a FERC senior civil engineer in a July 27, 2006 memo to his managers.

Trouble began last week when a 250-foot-long hole gave way in the dam's primary, concrete-lined spillway. Normal flows down the main spillway are about 55,000 cubic feet per second. Spillage from the dam has stopped as a result of the lower water levels, The Associated Press said.

This spillway was used for the first time since it had been built 50 years ago, but when it was put into use, something went wrong, and officials began to fear a sudden collapse that would send a wall of water into the town. Emergency spillways are usually reserved for last-resort situations.

KQED describes the emergency spillway as "essentially an ungated 1,700-foot-wide notch in the rim of the reservoir".

Were officials warned about the potential for erosion? Authorities believed the emergency spillway, which is earthen, was on the verge of collapse and issued swift, stern evacuation orders.

The groups said rocks and other debris could be swept into the river, damaging highway bridges and power plants downstream.

California Department of Water Resources Director William Croyle said yesterday that he was unfamiliar with the environmental groups' 2005 filings. "This was a new, never-having-happened-before event".

Construction and helicopter units worked today to shore up an overflow channel and drain some water from the reservoir at the dam, in the hopes they can prevent spillage before more rain hits the already saturated region of California. The cost of reinforcing the hillside was not immediately clear. A second spillway mainly made of earth serves as an emergency backup.

On Friday, Brown requested a Presidential Major Disaster Declaration for recovery efforts following January storms that tore through California, causing flooding, mudslides, power outages and damage to critical infrastructure.

Still, officials are planning for the worst as the situation could yet become risky.

As the Sacramento Bee reports, officials were cautiously optimistic Monday following two days of more high-pressure releases down the main spillway, which appeared not to have caused any further significant erosion.

A damaged spillway with eroded hillside is seen in an aerial photo taken over the Oroville Dam in California on February 11. The level was 884 feet on Tuesday morning. He also said DWR engineers will spend the coming weeks calculating whether it makes sense to fix the existing concrete structure or build an entirely new spillway nearby.

State and federal regulators dismissed those fears at the time, saying they were confident the hillside that helps hold back hundreds of billions of gallons of water was stable and did not need to be reinforced with concrete.