Newsom said Thursday the emergency proclamation is for the counties of Amador, Glenn, Lake, Mendocino and Sonoma, where hundreds were marooned.
The governor says the emergency declaration directs state officials to immediately request federal assistance to help communities recover from flooding, mudslides and damage to critical infrastructure.
The Russian River in Sonoma County crested at more than 46 feet (14 meters) Wednesday night, flooding about 2,000 buildings. More than 200 miles (322 kilometers) to the north, a man drowned in floodwaters while trying to reach his home, where three children were trapped.