The Bay Area is defined by water. Not just by the bay itself, but by the Pacific Ocean and myriad rivers, reservoirs, lakes and ponds. Yet most of the bodies of water you drive past are devoid of people. No swimming. No splashing around. Not even on the hottest days.

Why is that?

To answer this question, we had to drive out to one of the few places you’re allowed to swim: Lake Del Valle near Livermore.

• Jump straight to: Places you can swim around the Bay Area

Lake Del Valle is the only reservoir used for drinking water that you can also swim at in the whole Bay Area. And that basically answers the question: You can’t swim in most lakes around the Bay Area, because most lakes around here are really reservoirs used for drinking water supplies, and California law bans “body contact” in drinking water reservoirs:

“Except as provided in this article, recreational uses shall not, with respect to a reservoir in which water is stored for domestic use, include recreation in which there is bodily contact with the water by any participant.”

That means hiking and boating are fine, because body contact is minimal, but no swimming.

A reservoir is a natural or artificial lake that stores water for flood control, agriculture or drinking water. The drinking water reservoirs are what we’re primarily concerned about here.

Back when California was building the State Water Project — the massive series of dams and reservoirs that bring water from Northern to Southern California — it commissioned a study to find out how clean all those reservoirs were. That study found non-body contact (such as hiking and boating) didn’t significantly impact the water quality, but swimming could lead to fecal coliform and other bacteria (i.e., from poop and pee).

Based in part on that report, the state developed a set of guidelines in the 1970s for our drinking water reservoirs. And swimming has been banned ever since.

It should be noted, however, that it appears a number of water districts remained somewhat unaware of the regulations until the state started to crack down much later. For example, swimming was allowed in some Santa Clara County reservoirs through 1990—until the department of health let the water district know swimming was banned in drinking water storage facilities. (The county later did a feasibility study to see if a swimming spot could be built because of public demand. But nothing came of it.)

Kurt Souza, assistant deputy director for the Division of Drinking Water, said back when the regulations were written, water wasn’t filtered first. It was just pulled out of the reservoir, treated with chlorine or other chemicals, and then sent down our taps. So there was a need for it to be very clean.

Filtration requirements started around 1990, and these days water treatment is a lot better. There are a lot more options for first filtering the water and then treating it.

That means, if we were writing the law today, we probably wouldn’t need to ban swimming everywhere.

“I’m not sure if you’d make the same law today,” said Souza. “You’d probably make it the other way, where you would require a certain amount of treatment for a certain type of recreation.”

OK, so we want our water to be clean, but what about the birds and the animals? They get the water dirty, too.

Yes, birds pooping in the water is bad, but it’s just not as bad. Although there are some reservoirs where animal fecal matter does become an issue, largely human pathogens are what get humans sick.

But you’re allowed to swim in drinking water reservoirs in other states and countries. Why not here?

Yeah, sorry, California has different rules developed in a different time. California is also a drought state, so those reservoirs can get really small and the water can sit around for a lot longer. That’s partially why you’ll see more places on the East Coast that allow swimming. Because it rains more there, the water turns over fast.  That’s also why we have different rules here for rivers and moving bodies of water.

OK, so why can you swim in Lake Del Valle? And how can I get swimming allowed at *my* lake?

When the law was written, there were specific exceptions listed — all of which come with requirements for additional treatment and testing. For example, all of San Diego County got an exemption for their reservoirs a long time ago. And all State Water Project reservoirs were exempted as part of the arrangement to get voters to approve funding for the project. Lake Del Valle is part of the State Water Project. There are no other exemptions in the Bay Area.

Now, if you want a new exemption to allow swimming at a reservoir near you, then you have to get a law passed by the state Legislature. The last time one of those got signed into law was in 2013 for Bear Lake.

Where can you swim around the Bay Area?

There are some amazing swimming locales farther afield — in the foothills or down toward Big Sur — but if you want to keep it closer to home, here are a bunch of Bay Area swimming spots where jumping in the water is allowed:

• The beach! Aquatic Park in San Francisco is a very popular swimming destination for hard-core swimmers. For a more casual experience, head to China Beach in the city, McNears Beach at China Camp State Park in San Rafael, Oyster Cove Beach at Oyster Point Park in South San Francisco, Coyote Point in San Mateo or Crown Memorial Beach in Alameda. There are also a number of more secluded beaches off Tomales Bay, like Heart’s Desire.

East Bay

• Shadow Cliffs (Pleasanton): Formerly a gravel quarry, this lake in Pleasanton is operated by the East Bay Regional Park District. Lifeguards are on duty at the swimming beach in the summer, but swimming is allowed year-round at your own risk.
• Don Castro (Hayward): No swimming or boating is allowed on the reservoir lake, but the chlorinated and filtered lagoon located next to it allows swimming. Swimming is allowed there only in the summer.
• Contra Loma (Antioch): Fun fact: Contra Loma was actually the site of a dispute over whether people should be allowed to swim in water used for drinking supplies. State regulators said no, and the result is a swimming lagoon built (and chlorinated) separated from the reservoir. Open from the spring through fall, the lagoon does reach capacity.
• Cull Canyon (Castro Valley): A secondary dam was built to create this swimming lagoon with a sandy beach separate from the reservoir, but still surrounded by open space. Swimming is allowed only when lifeguards are on duty in the summer and does reach capacity.
• Quarry Lakes (Hayward): There is a swimming complex with a white sand beach, restrooms and lifeguards at this recreation area run by the East Bay Regional Park District. There is a fee and it does reach capacity. Additionally, the swimming area can be closed due to low water levels.
• Lake Temescal (Oakland): Closed until the spring, Lake Temescal does permit swimming with or without lifeguards when the water quality is good enough. Unfortunately, during the winter the water quality is often poor.
• Lake Anza (Berkeley): In Tilden Regional Park, Lake Anza has a small sandy beach open from April to November for a small entrance fee — if the water quality is acceptable. When bacteria levels are high, the lake closes to the public.
• And, of course, Lake Del Valle (Livermore): The lake is surrounded by over 4,000 acres of land used for hiking, camping and horseback riding. There are two swimming beaches, one on each side of the large lake. Lifeguards are on duty during the spring through fall, though swimming is allowed year-round at your own risk.

North Bay

• Bass Lake (Bolinas): Bass Lake is one of the few lakes in Marin County that isn’t a drinking water reservoir — and therefore one of the few you can swim at. Bass Lake can be found near the southern end of Point Reyes Seashore via a hike down the Coastal Trail from the Alamere Falls trailhead. It’s a bit of a hike and scramble down to the water, but it’s closer than another swimmable lake farther up the trail, Pelican Lake.
• The Inkwells (Lagunitas): A bit of a open secret in Marin, the Inkwells are a series of natural rock pools just as you enter Samuel P. Taylor State Park, off to the side of Sir Francis Drake. When the rain has been heavy, the water flows fast from Kent Lake down to San Geronimo Creek and through the pools.
• Spring Lake (Santa Rosa): The massive Spring Lake Regional Park, run by the county, includes a swimming lagoon filled with filtered water and open from Memorial Day to Labor Day. There’s also a  water park for kids made up of an inflatable playground.
• The Russian River: With swimming beaches and spots all up and down the Russian River, it’s hard to pick just one. Certainly, Johnson’s Beach in Guerneville is a popular spot, though it can get crowded in the summer. Same with Monte Rio Community Beach. Healdsburg Veterans Memorial Beach is easy to get to and the swimming area is created with a temporary dam on the river in the summer.
• Lake Sonoma (Geyserville): Run by the Army Corps of Engineers, Lake Sonoma allows swimming at your own risk anywhere you can safely get in. Try Yorty Creek or the Warm Springs recreation area. A warning: The lake is far more popular as a boating destination, so be careful if you’re swimming.
• Lake Berryessa (Napa County): Lake Berryessa is a reservoir, but it’s a reservoir used for irrigation and flood control. That means swimming is allowed. All swimming is at your own risk; there are no lifeguards here either. Oaks Shores and Smittle Creek are the most popular day use areas with bathrooms, picnic areas and parking.

South Bay + Peninsula

(If you remember swimming in Almaden Lake not that long ago, you might be surprised to know you can’t anymore. It’s been closed because of water pollution for a few years as the lake undergoes an improvement and cleanup project. Santa Clara County, in general, does not allow swimming in its lakes and reservoirs.)

• Garden of Eden (Felton): Another natural swimming hole that requires a hike, the Garden of Eden is just that: an Eden inside the Henry Cowell Redwoods. In the summer, you can park at the day use area off Highway 9 in Felton and cross the seasonal bridge. But in the winter, you have to walk in from the Ox Fire Road trailhead.
• Parkside Aquatic Park (San Mateo): The only way to get here is through a whole bunch of residential neighborhoods, but once at the park you’ll find a moderately warm sandy beach next to a lagoon with a roped-off swim area during the summer.
• Redwood Shores Lagoon (Redwood City): Run by the city, the lagoon serves as a stormwater retention pond, but it also allows non-motorized boating, swimming and fishing. Swimming is at your own risk and you should watch out for boats.
• Foster City Lagoon (Foster City): Foster City streets drain into this salt-water lagoon. The bacteria levels are tested regularly and if there is contamination posing a health risk then signs are posted at the beaches. Swimming is allowed at your own risk.
• Memorial Park (Loma Mar): This nearly-500-acre park includes a swimming hole where the creek crosses through the park.
• Uvas (Morgan Hill): If you’ve ever done an organized swim, triathlon or aquathon at UVAS Reservoir in Morgan Hill, then you’re probably thinking you can swim there anytime. You can’t. Swimming is, generally speaking, not allowed.

Did we miss your favorite swimming spot? Let us know. baycurious@kqed.org

*We’ve added more swimming spots based on your recommendations.