Lake Berryessa has a capacity of approximately 521 billion gallons of water, above which it will begin to overflow. At maximum capacity, the surface of the reservoir rises more than 440 feet above sea level. But if the dam reaches its limit, then the excess water has to go somewhere. If it didn’t, of course, the structure itself could be damaged and might even collapse.
Ordinarily, the water level wouldn’t be a problem. After all, California is famous for its dry weather and frequent droughts, and the state only lifted its most recent set of water restrictions in April 2017. As a result, it had been more than a decade since Lake Berryessa was last in any danger of overflowing. Even in October 2016 the lake was barely half-full, with the water level sitting below 400 feet.