What Open Water Swimmers Need to Know About Tides

Living on a spherical planet influenced by the gravitational pull of another spherical celestial body—the moon—means that larger bodies of water on Earth rarely stay in one place. Instead, these bodies of water, particularly oceans and seas, feature a range of tides. Some are dramatic, some are barely noticeable, but they can all have an impact on your ability to swim.

How Do Tides Work?

As the moon orbits Earth, its gravity creates a tidal force that causes the Earth itself and the water covering it to bulge out slightly on the side closest to the moon and the side directly opposite. The areas at roughly 90 and 270 degrees from where the moon’s pull is felt most directly squash a bit in response to that tidal force.

In theory, those bulged areas experience high tides while the flatter areas see a low tide. But Earth isn’t one uniform ocean; landmasses get in the way and continents, water depth, and even the time of year can affect how much of the moon’s pull can be felt in a particular area.

The Bay of Fundy in Canada features the world’s greatest tidal range at 53.5 feet. The Bristol Channel between England and Wales experiences an extreme tidal range of about 49 feet.

By contrast, some places in the Mediterranean, as well as the Baltic and Caribbean seas, experience tidal differences close to zero. These so-called amphidromic points typically occur in the center of an ocean basin where there’s very little vertical movement from tidal action. Southwest Australia is a site of one of these points and the difference between high and low tide can be barely noticeable.

Here in North America, many beaches on the East Coast have noticeable tides that can impact when and how you’ll be able to get in a good swim.

For example, here in the Boston area, one of the beaches we frequent, L Street, sits inside Dorchester Bay, an inlet off the Boston Harbor that’s very shallow. There are vast differences between high and low tide that cycle throughout the year depending on the season and the phase of the moon.

During high tides, swimming is no problem. But during low tides, the water can be so low that it uncovers mussel beds, a mucky section of bottom, and various other debris that isn’t fun to walk over or swim through. Thus, we schedule swims to start and end within two hours of high tide to avoid all that gross stuff.

Less than a half-mile around the curve of Dorchester Bay, however, is another beach called Pleasure Bay. This enclosed area is calmer, and we don’t have to dodge any boats. It’s also suitable for swimming at low tide. The bottom isn’t ideal, but we’re less likely to cut our feet on shells, and the tidal difference isn’t as noticeable. We can swim there at dead low tide quite happily, though on very low tide days, a loop might be a few yards shorter than it would be on day with a very high tide.

When it comes to understanding tides and how they might impact your swim, it’s all about what the local conditions are where you’ll be swimming. Get a tide chart for your local beach (there are free apps aplenty for that these days) and talk to locals who frequent the waterway. People who fish the area and recreational boaters tend to have loads of insight about how local tides and currents work and can help you understand how to navigate the area safely.

River Tides

Although tides are typically thought of as a marine phenomenon, they also can affect rivers and even lakes to some degree, though most lakes are too small to register these differences. The Great Lakes have a tidal range of about 5 centimeters, so your local pond is unlikely to offer any noticeable indication of the water’s diurnal movement.

But on some rivers, tidal influence can be striking. There are about 100 rivers around the world that produce tidal bores, a sudden wave of water from the incoming flood tide that travels up the river against the river’s current.

The largest tidal bore occurs on the Qiantang River in East China, where the water rushes into the river in excess of 15 miles per hour and can reach 4 meters high and 3 kilometers wide. The roaring of this water has been dubbed the Silver Dragon, and every year on the date of the biggest bore, locals celebrate the return of the dragon with a festival.

Some particularly brave (or reckless?) surfers have been bold enough to surf that bore and others around the world; in March 2016, Australian surfer James Cotton earned a Guinness World Record when he surfed the Kampar River bore in Indonesia for 17.2 kilometers.

Whether you’re surfing or swimming, it pays to be extremely cautious when dealing with bores. And even for other tidal fluctuations in rivers, it’s also helpful to know the conditions and what to expect; you may be happily swimming along with the river’s current until the tide shifts and then suddenly find you can’t make any forward progress. This happens in other tidally influenced locations too. It can be downright dangerous if you’ve misjudged tidal timing and get stuck swimming in place for hours until the tide slackens before you can finally push through.

No matter where you choose to swim, the key is to always know what you’re getting into before you go. Check the local tides and weather conditions, and swim with knowledgeable folks who respect how powerful the water can be.