This past Sunday, officials ordered more than 100,000 people to flee the area near the Oroville Dam, north of Sacramento, California. Authorities feared that one of the dam’s spillways—structures used to provide a controlled release of water—could collapse. If that happened, it would have unleashed a flood of water from Lake Oroville.
Oroville Dam’s troubles began several days ago when heavy rain and snow caused water levels in Lake Oroville to drastically rise. Dam operators attempted to release the excess water through the dam’s main spillway. But a section of the concrete structure caved in, leaving a gaping hole. It stretched 60 meters (200 feet) wide and 9 m (30 ft) deep.
Engineers were forced to switch to the dam’s emergency spillway. It was the first time it had been used since the dam—the tallest in the U.S.—opened in 1968. Water couldn’t be removed from the lake fast enough, though. It overflowed the banks of the emergency spillway, eroding (wearing away) the hillside beneath. Officials worried the damage would cause the spillway to collapse, sending a massive wall of water cascading into the valley below the dam.
SAFE, FOR NOW
Since the warning to evacuate (move people from a place of danger to a safer location), lake levels have been steadily falling and the threat of flooding has lessened. Dam operators decide to reopen the damaged main spillway to release as much water as possible from Lake Oroville. The current flow is about 2,830 cubic m (100,000 cubic ft) of water per second. That amount of water is slightly more than it would take to fill an Olympic-size pool.
But more storms are headed to the area, which could change the situation. People may not be able to return home until engineers are able to shore up the dam. Workers have begun using helicopters and dump trucks to patch the emergency spillway with rocks to prevent further erosion. In total, repairs to the dam are estimated to cost $100 million and could take months to complete.