UPDATE 5:47 p.m. Weather experts are warning residents of the greater Sacramento area of quickly-rising small creeks and streams.
The National Weather Service says if you see a flooded roadway, turn around. Do not try to cross it.
Forecasters say streams have moved above what's considered the monitoring stage.
The storm is expected to peak this evening.
The Sierra Nevada is under a winter storm warning through tomorrow night at six.
Heavy rainfall also means swollen and dangerous rivers.
The Sacramento Fire Department says three adults and a dog were rescued today on islands near the Howe Avenue boat ramp on the American River.
ORIGINAL: Rivers in the greater Sacramento area reached levels not seen in a decade Thursday night.
The California Department of Water Resources (DWR) said Friday the flow being released from Folsom Lake was at its highest rate since 2006, as 35,000 cubic feet per second (CFS) began being released at 12:30 p.m.
Spillways engineered along the Sacramento River were kicking in Thursday as well. They're called weirs, and a couple had already begun sending water into designated flood planes farther north, which is not that uncommon. But, DWR's Boone Lek said one closer to Sacramento was about to be used, and that's far more unusual.
"This one, the Fremont weir, is expected to start spilling on Saturday afternoon and through the weekend into the Yolo Bypass, and that's something that doesn't happen very often," said Lek.
Several rivers throughout the northern half of the state were expected to hit flood stage between Thursday afternoon and Saturday night.
Lek said the Truckee River at Truckee probably poses the biggest threat to a population center, with some minor flooding in town expected.