Swimming in your local river can be a magical experience, especially if you're lucky enough to live in an area where the water is clean. You'll get to know the resident wildlife. Perhaps kingfishers or dippers frequent your spot; you might if you're very lucky see water voles, otters, or even a beaver or two. The changing seasons are especially evident to those who float and view the world from water level.
It pays to know and understand your local rivers which will of course vary according to geography, conditions and time of year. Upland rivers tend to flow fast over rocky terrain. Because their catchment area is steep, heavy rain will result in large amounts of water entering the river which quickly goes into flood or spate. Such rivers are known as flashy rivers.
Downstream, where a more mature river meanders through a flatter landscape, conditions are very different and it might take many hours for flood water to make its way down.
Increased volume means increased flow, ie faster and deeper water.
Many rivers flow into the sea and become tidal where the salt and freshwater mix with the ebb and flow of the tides. These are very special areas for wildlife particularly birds and fish, and are often subject to extreme currents. There might also be deep mud to navigate at lower tide levels.
Navigable rivers also have boat traffic, so ensure you're visible to it, and know the rules. Don't swim through a waterskiing area, for example. That sort of traffic won't be expecting to see you.
Whatever the character of a river, in flood it presents a range of significant dangers, whether from currents and features that can trap and hold a swimmer (or a boat) underwater, or from flood water pollution. Don't go into white water - this requires expert knowledge and skill.
It's especially important in rivers to understand the way the water behaves even on a day when conditions are good, because the water flows along them and you will be taken along with the current. Bear in mind that you need to get in - and out - safely.