That's not a river, that's the 101 freeway in Montecito. It's flooded, and a mudlside on Topanga Canyon Boulevard, and Maricopa Highway 33

Above, the Northbound 101 freeway in Carpenteria

 Above the Maricopa Highway 33

Above Topanga Canyon Boulevard.

above, Burbank on Country Club Drive

http://santamariatimes.com/news/local/update-flooding-causes-destruction-on-south-coast-dead-southbound-hwy/article_111f3acb-b966-5e2e-bdce-d1264986769c.html
https://www.nbclosangeles.com/multimedia/January-Southern-California-LA-Storm-Rain-Weather-Wildfire-Flooding-468320873.html
http://www.latimes.com/visuals/photography/la-me-fw-mudslide-20180109-htmlstory.html

For a before and after satellite photo of the area, before the fire, and after the flood, see the compared images at https://earthobservatory.nasa.gov/IOTD/view.php?id=91573