Some advice and info for rivers and lakes. The main issues are pollution, debris and obstructions, currents and flooding, and wind.
Please note that I have included information and advice that I believe to be accurate and helpful, but I take no responsibility for swimmers. All outdoor swimmers must be solely responsible for making their own assessment as to the risks of any sort of any particular swimming or related activity.
See also Understanding Wind and Rain on the Outdoor Swimming Society website, https://www.outdoorswimmingsociety.com/understanding-rain-windy-weather/
Wind
Anywhere can be affected by wind, with trees blown down and branches falling off, trees in the water, roads blocked, and travel disrupted. Heavy winds in summer and late summer of the most dangerous, because trees are full of leaves, and if there has been rain as well that even heavier and more likely to fall down.
There is no easy way to tell which trees might fall down beyond this, so be very careful, especially if the winds are force five and above.
Wind will make it colder both in and out of the water, and will cause waves, especially in a lake but also in a river, which needs to be factored in if you are going to be swimming against it or pushed along by it.
Feeling unwell
If you feel ill after swimming in open water, especially if it has been raining heavily, consider seeking medical advice and explain that you have been swimming and in what circumstances.
(Please note that there is no truth whatsoever in the idea that drinking Coke has any beneficial effects if you are affected by a swim in polluted water.)
(The latest research from the World Health Organisation concludes there is no evidence of COVID-19 transmission to humans through sewage, but non-infectious traces of the virus can remain in the sewer system, and researchers are using this to study the disease, https://www.water.org.uk/covid-19-information-for-customers/)
Rivers
Can be affected, especially after heavy rain, by pollution, agricultural run-off, discharges from sewage/water recycling plants, pollution from roads. You can work out from maps or local knowledge whether there are nearby roads, livestock in fields, and there are maps available showing the locations of and discharges from sewage/water recycling plants. It is generally best to avoid swimming just downstream of these places after rain, or if you do choose to swim keep your head out of water and be very conscious of hygiene after swimming, avoid swallowing the water and be sure to cover up any cuts. See https://www.outdoorswimmingsociety.com/is-it-clean/, which includes a link to the Rivers Trust map (note: very broadband-heavy) of discharges and their sources. There is no need to stop swimming everywhere after it rains: most swimmers can use their senses and judgement.
Heavy rain and strong flow can also stir up sediment or wash it off fields (especially arable fields) into the rivers directly or via tributaries, streams and drainage ditches. This will make the water cloudy and is classed as pollution.
Debris and obstructions are real danger in rivers. These can include logs, branches, litter, street signs, rocks washed down, and all can change the river from a familiar place to one that is unfamiliar. You can’t always see branches, and being washed against them by a strong current is extremely dangerous as you can be trapped under the water. See https://www.outdoorswimmingsociety.com/river-hazards/ and https://www.outdoorswimmingsociety.com/currents-and-eddies/.
The current can be much faster, and river banks can be flooded. Be extremely cautious if you do consider swimming, make sure you know exactly where you get out and have a backup point, be careful to check what the current is like and if you have any doubt don’t go in.
Some places can be a little less dangerous, in particular many mill pools or river pools that have much shallower stretches downstream, so if you were washed down it would be to shallow water. But be careful in and around the weirs and mill races: white water and eddies can be hazards – see https://www.outdoorswimmingsociety.com/understanding-weirs/.
How long should we wait after heavy rain before swimming in a river? There is no simple answer or set time, as it depends on the river. Generally you can see whether the flow has diminished by the level of the river. In some river spots lower downstream the water can take a while to go down, as it flows into the river off fields and down tributaries first. It depends how flat the land is. Livestock runoff depends whether the cattle can actually get to the river or are kept back, but there might be streams and drainage. If you don’t already know the area, looking on satellite maps can help.
Consider whether there is sufficient water to dilute any pollution to levels that are unlikely to be unsafe. Heavy rain after a long dry spell is more of a problem.
This article has some useful advice on this, and the issue generally, https://totalimmersionuk.blogspot.com/2020/07/as-swimming-pools-and-many-open-water.html
You can look up river levels on several websites including https://flood-warning-information.service.gov.uk/river-and-sea-levels; https://riverlevels.uk/
How does this work in tidal rivers? It seems likely that you might need to think about runoff pollution or debris being pushed upstream as well as down twice a day. A stronger current and higher tides will push it on faster but also churn up sediment more. General advice about swimming in tidal rivers on this website and on Understanding Tidal Rivers and Eastuaries on the Outdoor Swimming Society website.
Lakes
(Including ex-quarries and reservoirs etc)
These vary. They can be spring fed, river fed, or rainwater fed. They might or might not be affected by pollution, agricultural run-off, discharges from sewage/water recycling plants, pollution from roads, but are generally much less likely to be affected than rivers.
Debris can be a problem, especially with high winds, but is probably less of an issue than in rivers because there isn’t a current. But look before you get in.
Flooding might affect access points.
Winds – see above.
Other advice
This piece by swim coach Salka Hintikka gives advice on swimming and water quality, including after rain.
The Outdoor Swimming Society includes in its ‘Am I Safe‘ web page includes a guide to Understanding Weather, as well as understanding water conditions and your own capability and acclimatisation – all important in autumn and winter.
This video on Autumn swimming for beginners gives advice https://www.youtube.com/watch?v=nI2GY1UpU78.
This is a shorter post on wind and rain in the Survive section of the Outdoor Swimming Society website.
Please note that I have included information and advice that I believe to be accurate and helpful, but I take no responsibility for swimmers. All outdoor swimmers must be solely responsible for making their own assessment as to the risks of any sort of any particular swimming or related activity.