Heavy rain in the last 36 hours? Read this before a river swim

A river that was gentle last weekend can be high, fast and murky after a wet couple of days, and not every problem is visible from the bank. The advice here is UK-focused.

The question to ask

The 36 hours is our own rule of thumb. We couldn't find an official waiting time, and the Outdoor Swimming Society (OSS) says there isn't a simple answer, because it depends on the river and rivers vary a lot. What it suggests instead is looking at the river itself: you can generally see whether the flow has dropped from the level.

Faster, higher water

The Environment Agency says that after rainfall, river levels and flows may be higher and faster, which raises the risks for swimmers. It also warns of dangerous flows after rain and thunderstorms. The OSS explains the basics: more water means more flow, so the river runs faster and deeper.

Where the rain fell matters as much as how much. The OSS describes upland rivers as flashy, going into flood or spate quickly after heavy rain, and notes that it can take many hours for floodwater to work its way downstream. A dry, sunny morning where you are doesn't mean the river has settled.

Faster water makes everything harder. The OSS says currents get stronger after rain, and that if you can't swim upstream against the flow, you can't get out of the way of things downstream, such as bridges and trees. Its advice after rain is to swim upstream first where you can, and to take extra care finding safe exits so you aren't washed past them. Flooded banks can also change the exits you know, so a familiar spot may not have the same way out.

Our weirs guide explains how much a weir can change when the river rises.

Debris you can't see

Floodwater picks things up. The OSS lists logs, branches, litter, street signs and rocks, enough to make a river you know well feel unfamiliar. Heavy rain can also wash sediment off fields, which turns the water cloudy.

The bigger danger is what you can't see. The OSS warns that you can't always spot branches under the water, and that being pushed against one by a strong current is extremely dangerous, because you can be trapped under the water.

Why pollution goes up after rain

Rain washes things into rivers. The Environment Agency lists agriculture, urban drains, roads and highways and sewage discharges among the sources, and says pollution is more likely to get into rivers after rainfall. The Swim Healthy guidance from the Environment Agency and the UK Health Security Agency says heavy rain can wash harmful bacteria off farmland.

Storm overflows are part of the picture. Combined sewers carry rainwater and waste water in the same pipe. The Environment Agency explains that overflows were developed as relief valves, to reduce the risk of sewage backing up when heavy rain pushes those pipes past their capacity, and that what overflows in heavy rain is diluted sewage. Government policy says these discharges should only happen in exceptional circumstances, such as unusually heavy rainfall. Every storm overflow in England now has a monitor fitted, and since 1 January 2025 water companies have had to publish their storm overflow data in near real time.

The OSS suggests avoiding swimming just downstream of sewage and water recycling discharges after rain. It also notes: "Heavy rain after a long dry spell is more of a problem." Our guide to river water quality and sewage explains how designated river bathing waters are tested and rated.

Before you go, and while you're in

Wild's river spot pages show recent rainfall and the river level, so you can check both before you set off. The Environment Agency's service for river, sea, groundwater and rainfall levels on GOV.UK is useful too, including for gauges upstream of where you plan to swim. Then look at the water when you arrive. The OSS's advice is to check it very carefully on the day.

The Environment Agency's own advice on reducing risk includes not swimming after recent rainfall. If you do swim, Swim Healthy recommends covering cuts with a waterproof plaster, trying not to swallow the water, and cleaning your hands with soap and water afterwards, especially before you eat. If you become unwell, seek medical help and tell them you've been open water swimming.

Rain is one part of reading a river. The river swimming guide covers the rest, from finding your way in and out to locks, gates and bridges.

Sources

1. Outdoor Swimming Society, Understanding Wind and Rain
2. Outdoor Swimming Society, Understanding Rivers
3. Outdoor Swimming Society, Understanding Currents And Eddies
4. Outdoor Swimming Society, Understanding Weirs
5. Environment Agency, Helping you choose where and when to swim in open water
6. Environment Agency, Environment Agency and partners warn Berkshire river users to stay safe in heatwave
7. Environment Agency, Combined Sewer Overflows Explained
8. Department for Environment, Food & Rural Affairs, Storm overflows: policy and guidance
9. Environment Agency and UK Health Security Agency, Swim healthy
10. Environment Agency, Find river, sea, groundwater and rainfall levels

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.