Nearly 200,000 Californians living just north of Sacramento were ordered to evacuate because of major flooding concerns.
Authorities issued the order because the emergency spillway at Oroville Dam is in danger of failing. The local sheriff emphasized this is "not a drill," and the governor declared a state of emergency.
Spillways are safety valves used to release water in a controlled manner. This prevents Lake Oroville from spilling over the dam.
The dam itself is separate from the spillway and is structurally sound. The California Department of Water Resources said water stopped flowing over the emergency spillway Sunday night. But it's not clear when residents might be able to return home.
The large amount of precipitation this winter comes after a five-year drought in California.