Lake Oroville, the largest reservoir in a state system that provides water to 27 million Californians, has already reached its peak level for the year, barely surpassing half of its capacity, according to the Department of Water Resources.

Officials had warned the lake — key to the roughly 700-mile State Water Project, which pumps and ferries water across the state for agricultural, business and residential use — was at “critically low” levels on May 8.