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.

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.

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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.)