Tidal rivers: can you swim against it?

Where a river meets the sea, it stops behaving like an ordinary river. The tide pushes up from the sea, and the water you're swimming in can slow, stop or turn round. The advice here is UK-focused.

Questions to ask before you swim

How the tide runs up a river

The Met Office describes tides as the rise and fall of a body of water caused by the interaction between the moon, the sun and the Earth. Where a river reaches the sea, the rising tide pushes water back upstream. The Outdoor Swimming Society (OSS) describes what that means in a storm: debris coming down a tidal river meets whatever the tide is pushing up it, so run-off and pollution can arrive from both directions.

The current moves with the tide. Swim England says that in estuaries, tides are one of the biggest factors to consider, and that at different states of the tide a current can change direction and speed. The same stretch can run towards the sea at one point in the day and inland at another, or barely move around the turn of the tide. The OSS lists changing flow direction among the extra hazards of tidal rivers, along with tides and, in some places, bores.

Strength varies through the day and the month. The OSS's tides guide says more water moves in the middle two hours of a rising (flood) or falling (ebb) tide, which produces stronger currents, and that currents are stronger on spring tides than on neaps. The Met Office explains that spring tides come twice a month and bring the highest and lowest tides of the month.

Can you swim against it?

It's the right question. The OSS makes the point for any river: if you can't swim upstream against the flow, you won't get back to where you got in. In a tidal river, the flow you start with may not be the flow you finish with.

Swim England's advice is to swim out against the current first, so it gives you a push on the way back. If you do get caught in a current, it says not to fight it head on: stay calm, work out which way it's taking you, and swim at 90 degrees to it to get out of the flow.

The tide can make other hazards worse, too. Investigating a fatal paddleboarding accident at a river weir in Wales in 2021, the Marine Accident Investigation Branch found that a high river level, a fast current and low tide together created treacherous conditions at the weir. Our weirs guide explains why weirs are a danger on any river.

Checking the tide

Tide times are the starting point. The Met Office notes that published tide times come from the nearest port with a tidal station, which may not be very close to where you are, and recommends checking them every day. The OSS goes further for tidal rivers: the tide doesn't always come at the time predicted, and a local tide table might not apply exactly to your spot.

Don't stop at the table. The OSS suggests learning how your stretch behaves from people who use it, such as boaters, anglers and experienced swimmers. In some places, it says, the best time to swim is the slack water either side of high tide; in others it's at low tide. Its tides guide puts slack water at about an hour either side of high or low tide in many places, while warning that actual slack times can differ from the official tide times.

Wild shows tide times on its beach spot pages. Our guide to reading a tide table explains the numbers, and swimming against the tide looks at the same question at sea.

The rest of the tidal package

The OSS says tidal rivers and estuaries come with all the hazards of rivers and of the sea, plus a few of their own: silty or muddy water, very muddy exit points, long gaps between places to get in and out, and brackish water. Work out your exits along the whole route, not just where you start.

Boats are common on tidal water. The OSS's advice is to keep clear of them, make sure they can see you with a bright swim hat and a tow float, and use mooring ladders only with great caution and never when boats are around. Some tidal rivers ban swimming or allow it only with special permission, so check the local rules before you go.

After heavy rain, things get more complicated, so read river swimming after rain too. For everything else about swimming in moving water, see the river swimming guide.

Sources

1. Outdoor Swimming Society, Understanding Tidal Rivers and Estuaries
2. Outdoor Swimming Society, Understanding Tides
3. Outdoor Swimming Society, Understanding Wind and Rain
4. Outdoor Swimming Society, Is It Safe?
5. Swim England, How to deal with currents while swimming outdoors
6. Met Office, Tides
7. Met Office, Beach forecast and tide times explained
8. Marine Accident Investigation Branch, Report on the investigation into the stand up paddleboarding accident that resulted in four fatalities at Haverfordwest Town Weir, Wales on 30 October 2021

This guide is general information, not a substitute for local knowledge or training. Conditions change: check them on the day, and if in doubt, don't go in. Images on the blog are illustrations, not pictures of specific spots.