With four to 10 inches of rain in the forecast, operators at Northern California dams are opening the gates.
The federal Bureau of Reclamation says Lake Shasta and Folsom Lake are above the levels the agency wants with heavy rains on the way.
"Shasta Reservoir is approaching 3.5 million acre feet or about 77 percent full. Folsom is around 606,000 acre feet or about 62 percent full," said bureau spokesman Todd Plain.
Releases at Folsom Dam are accompanied by releases at Nimbus Dam downstream.
According to the bureau, releases at Folsom could double the current volume from 9,700 cubic feet per second to 20,000 by 8 a.m. Tuesday, and to 30,000 by 7 a.m. Wednesday.
"We want to maintain that designated safe space for the storms and the inflow to come in. There's enough room to hold that where we don't have to increase releases too much to put people at risk downstream," Plain said.
Sacramento County says park rangers have notified homeless people in Discovery Park to move to higher ground.
The bureau says it will increase outflows significantly when capacity becomes a concern, but takes a different approach when it comes to reducing the volume of water released.
Plain says the bureau will reduce flows over a number of days to allow fish to leave areas where they might otherwise be stranded with a sudden reduction.