Among the most common official reasons given for not swimming in reservoirs, lakes and quarries is the water; allegedly it’s freezing, even in high summer. In fact, these often alluring stretches of water aren’t usually colder than any other and they conform to the laws of physics. Larger bodies of water do, however, behave differently from smaller bodies of water and moving water.
In bodies of water such as reservoirs and lakes in temperate climates, in spring and summer the surface layer will begin to warm due both to the rise in air temperature and solar radiation. The latter only penetrates so far. Warmer water is less dense and will float, provided nothing – such as a storm – causes the water to mix. Generally this layer varies between 18 to 24℃ in a warm summer in the UK. The surface thermal layer can be anywhere from one to twenty meters deep, and interestingly will tend to be deeper in larger bodies of water because they are subject to greater wind action which mixes the warmer water to a deeper level.
The water in the depths will vary from 4℃ to 7℃ (water is at its densest at 4℃).
The narrow (around 1 meter deep) thermal layer between the top and bottom layers is called the thermocline, and here the temperature drops rapidly from around 18 to 7℃. It’s worth knowing about this layer because if you do jump in you might encounter colder water rather suddenly.
When the air temperature begins to drop in Autumn, at some point the thermal layers will mix again as increasingly dense, cooler water sinks below the surface. Because water at 3℃ or cooler is less dense, this water will float on the surface which is why bodies of water freeze from the surface down rather than from the bottom up. So in winter, the surface layer might well be the coldest layer, and aquatic life can survive in the warmer depths.
The point then is to be aware of the likely temperature, to understand that in summer there might be a layer of much cooler water below the warm surface, and to feel the water and judge temperature before you take the plunge.
Be especially aware of the potential for cold incapacitation, which can strike even fit, expert swimmers. Swim parallel to the shore unless you’re certain you are sufficiently acclimatised and experienced to cross a lake, reservoir or quarry. Suitable boat or kayak cover is a good extra safety measure if you do want to swim a distance far from shore.