Two winters’ worth of snow has already fallen in the Sierra Nevada since Christmas, pulling California from the depths of extreme drought into one of its wettest winters in memory.

But as a series of tropical storms slams the state, that bounty has become a flood risk as warm rains fall on the state’s record snowpack, causing rapid melting and jeopardizing Central Valley towns still soggy from January’s deluges.

The expected surge of mountain runoff forced state officials on Wednesday to open the “floodgates” of Lake Oroville and other large reservoirs that store water for millions of Southern Californians and Central Valley farms. Releasing the water will make room for the storm’s water and melted snow, prevent the reservoirs from flooding local communities — and send more water downstream, into San Francisco Bay. The increased flows in the Sacramento-San Joaquin Delta could help endangered salmon migrate to the ocean.

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