SAN FRANCISCO, Calif. (KRON) — Flooding from back-to-back storms doesn’t happen very often in the San Francisco Bay Area. That’s especially apparent on social media, where several videos have surfaced showing Californians playfully splashing and paddling around in flood water. “Street surfing” has also emerged as a thing, using a tow rope, surfboard, and car on waterlogged streets.

“It’s The Bay” posted a video on Instagram of a shirtless man tubing along the streets of East Oakland, and another video of two women paddle-boarding through a San Francisco neighborhood. More than five inches of rain fell on San Francisco is less than 24 hours on New Year’s Eve.

Floodwater is murky brown, and doesn’t look very clean. But what exactly is in the water?

San Mateo County Health Officer Dr. Scott Morrow said, “Floodwater can contain all kinds of dangerous materials. Raw sewage, animal waste, toxic substances, chemical compounds, fungus, bacteria – things you would rather avoid. It is obviously best to avoid any contact with the water.”

Morrow warns that floodwaters pose health and safety risks for people who come into direct body contact with it. “You need to take precautions to avoid illness or injury,” he said.

KRON4 asked Santa Cruz County interim director of environmental health Andrew Strader whether he would consider playing in flood water. He replied, “Absolutely not.”

Storm water runoff flushes everything on the ground, including fecal material and bacteria, into a street or creek until the water can eventually drain into the ocean.

Pam Welty, who lives on Arguello Boulevard in Pacifica, said sewers “bubbled up for hours” when her neighborhood flooded over the weekend. “The sewer (poops and TP and who knows what else) flowed directly into the storm drain.”

In addition to exposure to unsanitary materials, when the water is deep enough, drowning is also a hazard. “If it’s flowing water and you don’t know how deep it is, you can get swept away,” Strader said.

The New Year’s Eve weekend storm dumped several inches of rain within a short time window and overloaded storm drain systems.

Strader advises residents with flooded homes to evacuate until flood waters recede and an environmental cleanup crew can assess damages.

An atmospheric river will usher in another round of heavy downpours on Wednesday. Flooding this time around will be similar, or possibly even worse, compared to New Year’s Eve.

The National Weather Service issued a Flood Watch for the entire Bay Area beginning late Tuesday night and extending through Thursday afternoon.