Serving parts of North West London and the Home Counties.

Dangers of swimming in reservoirs


We do not allow swimming at any of our open water sites.

The water industry are proud custodians of many sites of special scientific interest and areas of outstanding natural beauty, this includes bodies of open water, such as reservoirs, that are used as storage areas for water prior to treatment and supply.

According to the National Water Safety Forum, during 2017, 255 people lost their lives in open water due to accidental causes.


Dangers of swimming in reservoirs

There are dangers associated with reservoirs and other bodies of water which we are urging people to take heed of.


Cold Water Shock

This is the body’s cold shock response when entering cold water. Even water as warm as 25oC can cause hyperventilation leading to drowning. At 15oC blood is diverted to vital organs causing a strain on the heart and failure of co-ordination due to muscle tensioning meaning that even strong swimmers can be unable to reach the shore or grasp throw lines.

Some reservoirs and other bodies of water remain cold all year round, others may have a variable temperature profile meaning that parts remain cold all year round. This may not be obvious to a visitor or observer from the bank side.


Localised Currents

Reservoirs contain machinery and pipework beneath the surface. This can start without warning and generate currents that can act as a draw on a swimmer.


Deep Water

This might seem obvious but some reservoirs are very deep and may have shelves or areas where the depth changes suddenly. This may not be apparent from the shore.


Mud and Silt

There may be an accumulation of silt on the bottom of the reservoir. This can be a particular hazard when jumping in as it can trap the swimmers feet preventing them from resurfacing.


Blue-green Algae

Some reservoirs are prone to blue-green algae. These are naturally occurring but toxic organisms that can cause serious illness to humans and animals. Contact and ingestion can cause abdominal pain, vomiting, diarrhoea, skin rashes, eye irritation and allergic reactions. Very severe contact and reactions can cause liver injury and death


Access and egress

Dams, walls and other manmade structures around the bank side can be slippery and lead to a person falling into the reservoir. There may be concealed or hard objects beneath the surface causing a risk of injury.

In some areas the banks may be steep or sheer, in others slippery. This can prevent a person getting out of the reservoir.



Whilst their geographical setting is often a large part of their appeal as places to visit, it also presents challenges when emergency services are required. We would recommend that people do not place themselves in danger and stay out of the water.