aerial view of a weir in the Thames

Understanding Weirs

Spot the dangers and avoid getting into trouble

Lawrence Hookham

Weirs and millraces are very popular spots for swimming, jumping and playing in water in summer. But they can lead to swimmers getting into trouble. Here we explain the dangers, how to spot them, and what to avoid.

Many people swim in the pools upstream from weirs with some well known public swimming locations – Warleigh Weir near Bath, and Farleigh and District Swimming Club – based on the lazy pools of deeper water that can exist upstream of a weir.

The top of a weir is usually safe enough in low flows, just be careful not to slip and fall down the weir.

But conditions change in higher flow, and downstream of a weir.

The advice below applies to millraces, where fast water rushes out in a stream from the mill or where there used to be a mill.


Stopper waves are vertically recirculating currents which can form at the bottom of a weir and are extremely dangerous, as they are hard to escape. They are a feature of deeper water at the bottom of a weir, so may appear when rivers are higher. Water falls over the weir, drives to the bottom of the riverbed, bounces back up and then rejoins the downward flow at the top.

These circulating stoppers can be aggressive and impossible to escape. You can try to swim down or sideways to and exit the circuit, and then rise to the surface.

Higher river flow increases the chances of a stopper in a location. Be aware they can appear at ‘safe’ locations – if locals are safely playing at the base of a weir it might be safe to join in, but never assume this. Each time you are considering getting in you have to judge, particularly on that day and in that minute.

U Shaped and Box weirs

Some types of weirs, such as U shaped and box weirs, are fatal to trapped swimmers and kayakers. This is because they are hard to swim out of – their shapes mean everything is dragged into their centre.

two swimmers slide down a shallow weir Imogen Radford

Rainfall and flow

A weir that is safe to swim above (or below) when there is low flow, can become hazardous when river levels rise. Two examples of this: first, water can start flowing over the top of the weir with force, pulling a swimmer with it. Second, with higher river levels stopper waves at the foot of a weir are more likely.

Water increases in speed and force closer to a weir.