There are a range of hazards that are inherent in outdoor swimming in a river. Make yourself aware of them for a safe and enjoyable swim.
Jumping and diving into a deep river pool is great fun if it’s done with some knowledge and with care. If you can see the bottom, don’t jump, it’s too shallow. If you can’t, don’t jump – there could be anything underneath. So if you do want to jump or dive in, get in and test the depth of the area and ensure it’s clear of objects first. Especially beware of undercuts where a swimmer might become trapped beneath a ledge under the water.
Weirs are generally to be avoided – some types in particular (box weirs) are fatal to trapped swimmers and kayakers. In some places you will see people swimming happily upstream in the pools formed by weirs (their purpose is to deepen the water), that’s probably safe in low flows. Don’t be tempted to slide down the face because that’s where the biggest danger lies – in the stopper at the bottom of the falls. Here, circulating currents pull you back towards the falls and under the water. In some cases it’s impossible to escape.
Also be aware that natural stoppers can be found on many upland rivers, sometimes in relatively low flows. Learn how to spot them, and if you do get caught, know how to get out.
Rocks can be both a hazard and a useful shelter against a strong current. Obviously check for rocks before jumping in, and take care when swimming; it’s all too easy to bash your ankle or knee on a submerged rock. They also tend to be extremely slippery, so bear that in mind when clambering along or in rivers.
Check for obstructions before you get in. Fallen trees, for example, can act as “strainers” and in tandem with the force and weight of water can hold a swimmer under the water. Many a kayaker has died in such circustances.
There are other hazards such as undercuts and tree roots that might trap you in a similar way. Banks are more likely to be undercut on the outside of bends which is where the current flows fastest, and the water runs deepest. So if you do venture to the fast and deep water, be very aware of what’s ahead and bear in mind the increased force and weight of water.
This kayaking article on river hazards demonstrates the various features to be wary of, and includes some nicely-drawn diagrams.