In summer there are low-tide mornings when the San Francisco Bay water is only knee-deep and the soft, slippery mud sucks hungrily at my feet. Wading into my usual swimming spot on the edge of the Bay, I wallow along until I give up and flop on my belly to float in the shallow water, digging my hands into the bottom, crawling along like an amphibian through the slimy eelgrass until I get way, way out to where it’s deep enough to kick.
Low tides are heaven for tidepooling, hell for swimming. If you think summer is the prime time for swimming in San Francisco Bay, think again. The season can be a slog: It brings cold wind, fog, and those persistent morning low tides. And what has always puzzled me is that the extreme low tides tend to happen in the morning in summer. That’s the time of day when I like to swim—it’s less windy, plus it starts my day right.
The timing of tides changes daily, so the fixity of morning low tides has been a mystery to me. Why oh why do the lowest of each day’s tides only happen around my favorite swim hour? In search of answers, I got in touch with the National Oceanic and Atmospheric Administration.
NOAA oceanographer Todd Ehret started me out with some tide basics: San Francisco Bay generally has two high tides and two low tides a day, each of a different height. Year-round, each tide arrives an average of 50 minutes later than the previous day.
Yes, yes, I say, I already know this: In a typical June, the lower low tide will show its muddy face on, say, a Monday before dawn, on Tuesday at breakfast time, and Wednesday at coffee break. But it never shuffles on into the afternoon as it should. The next day, it shows up again in the early morning, mucking up my a.m. swim.
Swimming in the Bay
A four-part series by Susan Kuramoto Moffat on what you can learn about our local waters from diving in. See her Summer 2025 Issue column for resources on how to get started swimming outside.
Past columns
Fall 2025: Salty Autumn
Winter 2026: Winter Clean
Spring 2026: Swimming with the Fish
How can this be?
Ehret patiently tells me to keep my eye on the sun. Yes, the sun as well as the moon contributes to the height of tides.
The gravity of both the sun and the moon pulls the oceans toward them in a bulge, and centrifugal force creates another bulge on the opposite side of earth. Alignment matters. When the moon and sun are lined up and pulling in the same direction twice a month, the resulting “spring tides” cause extreme fluctuations and my beach narrows under a high tide or widens into a mudflat. These tides “spring up” around the full and new moons (the word “spring” has nothing to do with the season)—and fall down just as sharply.
And when the moon happens to be at the closest point of its monthly elliptical orbit around the earth—its perigee—at the same time the tide is “springing,” gravitational forces are amplified even more in a “perigean spring tide,” which happens six to eight times a year.
In addition, because of the tilt of the earth relative to the sun, tides are more pronounced around the time of the summer and winter solstices.
And yet, all of this doesn’t explain why the lower of each day’s two low tides tend to happen in the morning in late spring and summer, with conditions at their worst in June—which is what really matters in a universe revolving around my personal swimming schedule.
Finally, Ehret gets to the answer that has been eluding me: The tilt of the earth relative to the sun affects not just how high the tides are, but their daily rhythm. The sun’s gravity always pulls on the earth’s oceans, boosting tides on both sides of the earth (those twinned bulges). But at our latitude, we feel the effect of both boosts only around the equinoxes in March and September when the sun is directly over the equator. In these months the lower low tide marches obediently through all the hours of the day and night.
As the earth tilts to its extreme at the solstices, however, the bulges caused by the sun get tilted (imagine a skewed hula hoop) so that the far side of the bulge is down around Australia. We are affected only by the side of the bulge that hits the northern hemisphere, which brings more frequent extra-low morning tides in late spring and summer. The same thing happens in the other direction in winter, causing the lower low tides to get stuck in the evenings. Who cares, though, because I’m not swimming at night!
But I curse the witching hour when the lower low tide aligns with the perigean spring tide around the time of the summer solstice! It yields a powerful brew of yucky conditions. On these muddy mornings, I just tell myself I’m getting a full-body lesson in astronomy.
