Water will rush down the rebuilt spillway at the nation’s tallest dam for the first time since it crumbled in heavy rain two years ago and drove hundreds of thousands of California residents from their homes over fears of catastrophic flooding.
The state Department of Water Resources said it anticipated releasing water down the spillway as early as Tuesday due to storms feeding the enormous reservoir behind Oroville Dam in the foothills of the Sierra Nevada.
The spring storms follow a very wet winter that coated the mountains with thick snowpack, which state experts will coincidentally measure Tuesday to determine the outlook for California’s water supplies. Heavy winter rain and snow has left the state drought-free for the first time since December 2011, experts say.
The dam’s main spillway is fully functional and “was designed and constructed using 21st century engineering practices and under the oversight and guidance from state and federal regulators and independent experts,” Joel Ledesma, deputy director of the department’s State Water Project, said in a statement.
The original spillway on the 770-foot-high (235-meter) dam, which is 150 miles (241 kilometers) northeast of San Francisco, was built in the 1960s.
In early 2017, storms drenched the state and the massive spillway broke apart as it carried heavy flows.
Dam operators reduced the flow and allowed water to run down an emergency spillway — essentially a low area on the reservoir’s rim — but the flow began eroding the earthen embankment that had never been used. Authorities suddenly had to order an evacuation of nearly 200,000 people living in communities downstream.
The threat of a dam collapse that would unleash a torrent of water did not happen, however, and people were allowed to go home days later.
In January 2018, an independent panel of dam safety experts released a nearly 600-page report that blamed the crisis on “long-term and systemic failures” by California dam managers and regulators to recognize inherent construction and design flaws in the dam.
Repairs have cost $1.1 billion. California requested about $639 million from the Federal Emergency Management Agency for the fixes, but the federal government has rejected $306 million of those reimbursements. U.S. officials say the dam’s upper gated spillway was damaged prior to the heavy rain two years ago.
Local water agencies are already paying some of the repair costs, and they would cover anything not paid by the federal government.