[7][5] The flooding occurred at the end of one of California's worst droughts on record, and much of the state was unprepared to handle the huge volume of rain and snow.

The precipitation helped to refill surface water supplies, including many major lakes and reservoirs, but had limited impact on groundwater reserves.

[10][11][12] The dipole basically describes the wintertime stationary waves over North America, which contribute to the mean temperature difference between the climatologically warmer western U.S. and colder eastern half.

[14][15] In early January 2017, the Russian River in Sonoma and Mendocino Counties rose 3 feet (0.91 m) above flood stage, inundating about 500 houses.

[17] The community of Forestville and the nearby Laguna de Santa Rosa flooded, blocking roads and agricultural lands.

[18] The American River east of Sacramento reached record flows, although property damage was limited by the protection provided by Folsom Dam.

The resulting flooding along Coyote Creek forced the evacuation of 14,000 people in San Jose[26][27][28] and caused $73 million in damage.

[30] The Puerto Suello Hill Tunnel along the Sonoma–Marin Area Rail Transit line was partially damaged by mudslides; this delayed pre-revenue testing of the system for three weeks.

However, due to heavy storm runoff into Lake Oroville, dam operators were forced to continue using the concrete spillway, eventually destroying the lower half of the chute.

The Feather River Fish Hatchery was flooded with turbid water, and several million juvenile salmon had to be evacuated from the facility.

[36] Multiple landslides and bridge collapses in the Big Sur area closed a long stretch of the Pacific Coast Highway, isolating coastal communities.

Five people drowned in the Greater Los Angeles urban area as heavy rainfall flooded highways, created sinkholes and cut power to 110,000 households.

In Orange County, three people were safely rescued from the Santa Ana River, while in Thousand Oaks one man drowned and three others were injured in a flash flood in Arroyo Conejo Creek.

The flooding also partially refilled Lake Cachuma, an important local water source which had essentially dried up in the preceding drought.