In February 2017, the main spillway for the Oroville Dam, located approximately 90 minutes north of Sacramento in Northern California, experienced a catastrophic failure when heavy rains and increased outflows caused a large crater to form in its long, concrete ramp.

           

      As Lake Oroville continued to fill with water, dam operators kept releasing water over the damaged spillway, which enlarged the size of the crater. But when power lines downstream were threatened by the growing hole, operators reduced the outflow until more water was flowing into the lake than they were letting out.   

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     The situation came to a head on February 11, when water started flowing over the dam’s uncontrolled earthen emergency spillway for the first time since its construction in 1968. But then a new danger emerged: the water started eroding the dirt hillside downstream.

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     As the erosion worked its way back up the hill toward the dam, it threatened to wash out the dirt underneath a concrete weir holding back the lake. If flows over the emergency spillway couldn’t be stopped, a full-scale breach of the nation’s tallest dam now seemed possible.

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     In the sprawling Sacramento Valley below the dam, more than 180,000 people were ordered to evacuate their homes, which were now directly downstream of a looming environmental disaster.

     

     To avoid that outcome, dam operators increased flows over the main spillway again, causing more damage on the lower part of the concrete ramp, but eventually dropping the lake level enough to prevent water from running over the emergency spillway. The dam held, and the potential catastrophe was averted.  

 

     But the damage had already been done. Estimates now run higher than $870 million to repair the destruction and fix the dam’s underlying problems.

     

     Subsequent press reports and government investigations revealed that the California Department of Water Resources, which owns and operates the dam, had been warned of the potential hazards of using the earthen emergency spillway at least a decade earlier, and that the main spillway itself was structurally defective.