It has fallen about a foot since then. Caltrans believes recent storms are to blame.
RELATED: Sonoma-Napa County Junction closed due to flooding
People who live nearby say the rain caused quite a few problems. "There's been a couple of rock slides up the hill and a tree fell down the other day, down Wooden Valley Road, so I mean the rains have their effect," resident Andrew Skolnick said.
SR 121, north of Napa, is closed btwn Wooden Valley and Circle Oaks due to section of road collapsing. pic.twitter.com/h0C4vdeg6i— Lilian Kim (@liliankim7) March 15, 2016
State Route 121 is in a rural section of Napa County but people traveling from Napa to Lake Berryessa rely on it.
PHOTOS: Rain, storms hit Bay Area during 2016 El Nino season
There's no word yet on how long 121 will remain closed. Caltrans must first determine if the road can even be fixed.
The agency hopes to know more by the end of the week.
In the meantime, drivers are being asked to use State Routes 128 and 29 as alternatives.
WATCH VIDEO: Watch Accuweather Forecast
Click here to download the ABC7 News App to get your weather forecast and severe weather alerts on the go, and click here for full coverage on El Nino.
What's the weather like #whereyoulive? Share your photos and videos on Twitter, Instagram, or Facebook by tagging them #abc7now. Your photos may be shown online or on-air!