Two small cities located roughly 75 miles north of San Francisco are only accessible by boat Wednesday after heavy rains caused flooding and mudslides in parts of Northern California, cutting off nearby pathways and roads.

Residents of Guernesville, which has a population of about 4,500, and Monte Rio, which has a population of about 1,200, were met with a deluge when the swollen banks of the nearby Russian River overflowed on Tuesday night, hitting low-lying areas in the Sonoma County's Russian River Valley.

The impact of the flooding was so severe that Guerneville "is officially an island," according to a statement issued by the Sonoma County Sheriff’s Office. 

Major California throughways were also badly hit, prompting the closure of multiple highways which have now been rendered virtual swamplands.

"Lake County is pretty much swimming right now," California Highway Patrol Officer Joel Skeen told the Santa Rosa Press Democrat Wednesday morning.

The Sonoma County Sheriff's Office on Tuesday afternoon issued mandatory evacuations for communities along the Russian River, which had floods levels reach 32 feet by 5:30 p.m. 

Officials expect the river to crest at 46.1 feet by Wednesday night, the highest level recorded in nearly 25 years.

The flooding has been pinned on heavy precipitation in the region caused by recent storm systems pummeling the western coast of the U.S.

Fears of mudslides also came to fruition in Monte Rio, one of many towns in the area still dealing with the effects of California's 2017 wildfires. 

On Tuesday, one mudslide trapped Monte Rio residents Kear Koch and his wife before they were rescued by first responders.

"Well, I fell into the mud when the tree fell over the top of me. It happened so fast you don't even know, you know. It's like I see an image of a tree. It's not there. It's there. You know what I mean," Koch told KGO, an ABC affiliate in San Francisco.

Firefighters in the area have had to pull people out from their cars and others stranded in their homes, many of whom were unable to evacuate from the area in time before the flooding began.

"It’s awful out here. We’re an island," Monte Rio Fire Chief Steve Baxman told the Press Democrat, describing the devastating conditions that have marooned residents in their homes and vehicles, hoping for rescue.

"We took 17 people out of cars and houses during the night. Too many people are driving into water," Baxman said.

Flood warnings for the San Francisco Bay Area and many parts of the Sacramento area will last until Thursday morning, according to the National Weather Service.

Flooding has been a persistent problem in California's valleys for years, with more than $580 billion worth of buildings and other structures extremely vulnerable to flood damage, according to the Public Policy Institute of California. 

In addition to loosened soil and debris brought on by wildfires and droughts, inadequate drainage systems in the state's urban areas make locale such as parking lots and streets extremely susceptible to pooling.

Floodwaters along Russian River A road is blocked by floodwaters on February 15, 2019 in Forestville, California. Recent rainfall in Northern California has caused major flooding and mudslides in towns along the Russian River. Photo: Justin Sullivan/Getty Images