Inlets and outlets: cold pockets and the pull

A lake can look perfectly still. The places to watch are where water comes in and where it goes out. Inlets can bring a sudden patch of cold. Outlets can bring a pull you won't see from the surface. Neither is a reason to avoid lakes, but know where they are before you set off along the shore.

Inlets: where the cold comes in

An inlet can be a stream running in, a spring coming up through the bed, or a pipe or channel feeding a reservoir. The thing to watch for is the sudden change in temperature where that water meets the lake.

The Outdoor Swimming Society (OSS) describes this in flooded quarries, which are usually fed by springs or streams. It notes you may find a patch of cooler water where the spring or stream enters. In a quarry, it adds, the flow from those feeder streams and springs is the only current you're likely to meet.

The risk is cold water shock. RoSPA describes it as a response to entering water at 15°C or lower that can leave you struggling to breathe and to swim. You expect the cold on the way in. An inlet can bring it halfway round your swim, when you're not expecting it.

What helps:

• Know where the inlets are before you start. Look at a map for streams running in, and look along the shore when you arrive.
• Give them room, and swim past rather than stopping in the cold water.
• If you hit a cold patch and your breathing goes, slow down and get it back under control before you carry on. Try breathing in for three and out for six. There's more in panic in the water and cold water shock.

Outlets: where the lake drains

Lakes aren't tidal, but RoSPA points out that they can still have strong currents. At an outlet, that means a change in the flow that could draw you towards it.

A natural outlet is where a lake becomes a river. If the outflow runs over a weir, treat it as a weir. The OSS warns that circulating currents at the bottom of a weir can trap swimmers and hold them under the water. Read weirs: why you never swim near one before swimming anywhere near the end of a lake that drains into a river.

Dams, towers, spillways and overflows

Reservoirs have man-made outlets, and they're the ones to take most seriously. The OSS explains how they work. Near the dam there's usually a tower holding the pipes that draw water off, for drinking water or hydro-electric power. Some reservoirs have one pipe, others have several at different depths. Spillways let water out downstream, or let a full reservoir overflow.

The OSS says the dangers around these structures are very real, and advises keeping a wide distance from them. It also advises caution when swimming downstream of a reservoir.

Water companies describe what's going on underneath. Yorkshire Water warns of strong undercurrents caused by water being continually drawn into large submerged pipes. United Utilities warns that machinery under the water can cause strong currents. Both start below the surface.

The OSS flags one more piece of equipment. Some reservoirs use aerators to mix the water layers, and these should be marked with buoys. The air bubbles can cause a sudden loss of buoyancy, so stay well away from them.

Yorkshire Water doesn't allow swimming in its reservoirs, and United Utilities has a no-swimming policy at most of its sites, for these reasons and others. There's more in reservoirs and quarries.

Planning a swim around them

A map and a walk along the shore are a good start.

Find the flow in and out. Look for streams, pipes and channels running in, and for the end where the lake drains. On a reservoir, find the dam and the tower. Plan a route that keeps you well away from the outlet.

Read the signs and ask. The OSS suggests asking locals about the best places to swim and reading local hazard signs.

Stay near the edge. The OSS advises swimming along the bank rather than across larger lakes, so you can get out when you feel cold. The RLSS asks you to keep an eye on a fixed landmark to check you're not being carried, and to plan several exit points along your route.

Check before you go. Wild's spot pages carry recent reports from swimmers who've been in, alongside today's weather and an estimated water temperature. Take note of any report that mentions a cold patch or a pull near a pipe.

Inlets and outlets are only two of the things to check at a lake. For the full picture, including depth, algae and getting in and out, see safe lake swimming: the complete guide.

Sources

1. Outdoor Swimming Society, Understanding Flooded Quarries
2. Outdoor Swimming Society, Understanding Reservoirs
3. Outdoor Swimming Society, OSS Intermediate Tips for Safe Summer Swimming
4. Yorkshire Water, Visitor safety
5. United Utilities, Stay safe at our reservoirs
6. RoSPA, Lake and pond safety
7. RoSPA, Open water swimming
8. Royal Life Saving Society UK, Top Recommendations for Safer Open Water Swimming

This guide is general information, not a substitute for local knowledge or training. Conditions change: check them on the day, and if in doubt, don't go in. Images on the blog are illustrations, not pictures of specific spots.