In many UK lakes, I see signs prohibiting swimming with the explanation DANGER: Deep water. This link contains an example. I don't understand those signs. Why is a 30 metre deep lake more dangerous to swimmers than a shallow 3 metre deep lake?

Is it because deep lakes may be colder (which depends on mixing)? Currents (should be less, not more)?

Or does it really just mean If you can't swim, you may drown?

Perhaps they're referring to Kraken, Nessie, or other flesh-eating critters living in unseen depths?

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    +1 Just for the last sentence. I can't say, but Aravona's suggestion is very likely in my opinion: The trend goes to anyone is getting sued by everybody for liability of anything - so everything gets a sign stating the obvious. I am waiting for the sign: Danger, water is wet, you could ruin your hair-style. – imsodin Jun 22 '17 at 11:04
  • Comments are not for extended discussion; and especially not for vildly off topic discussion. This conversation has been moved to chat. – Rory Alsop Jun 24 '17 at 23:26
  • I found a list of warning signs at rospa.com/rospaweb/docs/advice-services/leisure-safety/… - but no advice as to how deep water needed to be to justify a "deep water" sign. I've recently seen the sign near water that was probably only 1m deep - which is deep enough to be a real danger to small children. – Michael Kay Jun 25 '17 at 18:24
  • I always thought that if it is over my head I don't care how deep it is. A friend at work said he was in the Navy and they had a swim day when they were far out at sea. Lots of the sailors participated, but then it was remarked they were over the Marianas trench, the deepest point of the ocean. Many sailors hurried back to the boat. – Ross Millikan Jun 26 '17 at 3:45
  • @RossMillikan Note that my question is specifically about lakes, not seas or oceans. The latter are an entirely different beast considering waves, currents, wildlife, etc. – gerrit Jun 26 '17 at 8:04

10 Answers 10


Well, there's the obvious issue that the bottom may shelve very steeply, so that non-swimmers trying to paddle may drown.

Beyond that, I think there's a specific point and a more general point.

Specifically, deep water is more dangerous in terms of cold water shock.

In the first 3-5 minutes after entering cold water there can be a gasp reflex or muscle spasms that might cause intake of water into the lungs. Also, blood pressure and heart rate are raised, which can cause heart problems or stroke.

Deep water will remain colder than shallow water. And if the swimmer gets into trouble, they will likely be out of their depth. Also, as has been pointed out in the comments, the banks will often be steep and harder to exit.

The accidents tend to happen to young men with a macho approach to cold water. According to RoSPA (The Royal Society for the Prevention of Accidents):

Cold water shock can be prevented by people getting in slowly and acclimatising themselves to the water

But many of these young men tend to jump in and swim out 50 metres (150ft). All of a sudden, cold water shock kicks in.

It has nothing to do with how strong a swimmer you are - if you look at triathletes, they wear wetsuits to protect themselves against the shock of the water.

More generally, it seems that Local Authorities are increasingly taking a US-style legally defensive attitude to any risky activity. Increasingly their reflex is simply to ban swimming to cover themselves. So from this perspective the notice may simply be an excuse for a ban. Here's the response of an official to an Outdoor Swimming Society petition for access to a lake:

Only if the Outdoor Swimming Society can strictly control and manage swimming on the lake, including providing and paying for the appropriate signage, safety measures, public liability insurance and cover of lifeguarding facilities all year round - and this council can be satisfied that all the necessary safety precautions are put in place and be indemnified against any incidents that may occur - will the council reconsider its position.

Good grief - talk about over the top! This was in a lake where sailing and canoeing were allowed, and the Society was asking why swimming had been singled out. This prejudice against outdoor swimming seems to be an English thing - you don't see it so much in Scotland or the Continent.

So what are they going to ban next - sea bathing, perhaps?

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    Where I live in Germany there is a big freshwater lake where people bathe in summer. The councils put up platforms and rafts for people to have fun with. Lately they have taken them all away again and put up No Swimming signs, because they are worried about insurance and predatory lawyers. – RedSonja Jun 22 '17 at 11:25
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    @RedSonja - sorry to hear that the virus is spreading... – Tullochgorum Jun 22 '17 at 12:52
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    In the cases where it's immediately very deep, it's also much harder to get out of water safely without anything to stand on. That's also probably the point when you release the bank is steeper than you'd thought, loose gravel isn't a great climbing surface and your tireder/colder than you'd thought. – Nathan Cooper Jun 22 '17 at 13:38
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    if the swimmer gets into trouble, they will likely be out of their depth...pun intended? – Rocky Jun 22 '17 at 18:41
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    I wonder how long it will take people (or legal systems) to realize that lakes aren't generally a danger because of the owner of the lake. – Dronz Jun 22 '17 at 23:40

In the UK, many of these signs are in old disused and flooded quarries and mines or reservoirs. The issue with any of these sites is they are typically incredibly deep (like 100m~) and become deep very fast. I know of at least one reservoir where within 2m of the shore it drops to the point where you can no longer see the bottom (I'm not even sure how deep it is).

When it's hot it's easy to jump straight into the nice cool water. The problem is that the water being very deep takes a lot of energy to heat up, so even near the shore it can be tens of degrees Celsius colder than the air temperature. This can lead to cramp, stitches or even cold water shock (see quote below from wikipedia).

In humans, cold shock response is perhaps the most common cause of death from immersion in very cold water,1 such as by falling through thin ice. The immediate shock of the cold causes involuntary inhalation, which if underwater can result in drowning. The cold water can also cause heart attack due to vasoconstriction;[2] the heart has to work harder to pump the same volume of blood throughout the body. For people with heart disease, this additional workload can cause the heart to go into arrest. Inhalation of water (and thus drowning) may result from hyperventilation. Some people are much better able to survive swimming in very cold water due to body or mental conditioning.1

In any of these responses it can become impossible to swim. If the water gets very deep very quickly, you don't have to be far from the shore to get into serious issues.

Most lakes, etc. in the UK don't have life guards or typically anyone near with a boat, etc. so it's difficult to help people in these situations. This, historically, has led to groups of people (mostly teenagers) drowning as when one gets into trouble, people rush to help and end up drowning too.

It's become a particular issue in the UK because we have lots of old (Victorian) industrial sites, many of which are flooded. For example, living in North Wales where I live is literally covered in quarries, and mines like this:

enter image description here source

A couple of headlines from this article on the BBC:

381 people drowned in accidents across the UK in 2013

Gullet Quarry, in Worcestershire, claimed the lives of two swimmers during July 2013

The body of Conor McColl, 16, from Luton, was recovered from a former quarry in Clophill, Bedfordshire

Headlines like the above have led to a lot of legitimate and silly (that add no benefit), signs being put up by councils and local authorities. They simply don't want to get asked "Why didn't you do anything about this hazard" when/if someone dies.

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    "[water] even near the shore... can be several 10s of degrees Celsius colder than the air" That seems like a dubious use of "several", to me. I wouldn't use "several" for anything less than three and I find it hard to believe that there can be anywhere that has freezing water and an air temperature of 30+C (85+F). There's certainly no such place in the UK, where air temperatures in that range typically only happen on a few days each year. – David Richerby Jun 22 '17 at 15:45
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    @DavidRicherby "You go from 28C (82F) outside to 10C (50F) in the water. You can get muscle cramps and stitches." also bear in mind that in a flooded quarry/mine head the water can drop 100m instantly I mean literally one step from the shore. So this water is not going to get warm, pretty much ever. – user2766 Jun 22 '17 at 15:48
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    @Liam 28C to 10C isn't a change of "several tens of degrees Celsius." It's not even two tens. – David Richerby Jun 22 '17 at 15:52
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    @Liam Yes. As I said, "That's a dubious use of 'several'." Words have meanings so, if you say "several" when you mean something other than "several", then people will misunderstand you. If you don't think that's important, then *shrug*. – David Richerby Jun 22 '17 at 15:58
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    The moral here seems to be that if someone is drowning and is falling below the surface as a result it is far easier to pluck them off the floor of a 3 m deep body of water than swim to the bottom of a 100 m deep body of water. The latter being basically impossible. – Brad Jun 22 '17 at 17:37

I cannot swim, so if I am expecting something to be shallow enough to stand in and then it turns out that it is much deeper then I will die.

Normally I would expect that water gradually get deeper but in the case of a steep increase in depth, like a quarry, it may not be so obvious.

I appreciate the deep water signs as well as not being dead.

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    If you cannot swim it is extraordinarily rash to make such assumptions about the water depth. – PJTraill Jun 22 '17 at 22:12
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    In a filled in quarry the depth often goes from 5 feet to 100+ feet in one step. – Joe S Jun 23 '17 at 11:12
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    Most natural water starts shallow and gets steadily deeper the further 'in' you go. So wading into the sea isn't too dangerous, as long as you don't go too deep. – Sobrique Jun 23 '17 at 12:55
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    @Sobrique - 'depends on how strong the undertow is. – Mazura Jun 23 '17 at 15:14
  • This is the answer. It's not the extreme depth of the water that's dangerous, it's the fact that you can't paddle in it. – Valorum Jun 23 '17 at 21:49

One thing not yet mentioned is quickness and ease of rescue.

In my younger years, I was a trained lifeguard, and though my certification is long expired, I remember most of the concepts.

A struggling swimmer on the surface is one thing, but a swimmer that has been witnessed going down, or worse, reported missing by a companion, is quite another. For such a situation, in water up to 10 or 12 ft. (3 or 4m), the lifeguards have a chance of doing something. They line up near the last known position, dive down, look and feel along the bottom as long as possible, come up for a breath or two, and repeat until the victim is found or until exhaustion. Beyond that depth, even the most fit have trouble remaining down as long, or even being able to get all the way down, and reach exhaustion much quicker.

At one lake where I worked, there was a buoy line. We never took any disciplinary action if anyone went beyond the line, but it was also clearly marked on signs that the lifeguards would not attempt rescue beyond that point. All we were even allowed to do, per our employer's rules, was call 911. Odds are the county would have sent out a dive team, and it would have been mostly a recovery operation, not a rescue.


Depth affects safety because you can't breathe if the water level is above your nose, when your feet are standing at the bottom. After that point, it doesn't matter much; there is no difference between 2.5 meters and 25 meters.

Some people cannot swim very far before they get too tired and need to stand on the bottom to recover.

Warning signs are for the general population, not for the English-channel-crossing Olympians. They apply to everyone from age 3 to 103. "Swimming", for many people, means wading around, and making a few bold strokes here and there in between standing. Deep water doesn't accommodate that activity very well.

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    And lets not forget the 'climbing out again' element. It's considerably harder to get out of water if you have no footing. – Sobrique Jun 23 '17 at 12:58
  • Your 2nd paragraph explains why your 2nd sentence is wrong. If I am unwantingly underwater, I'd rather need to muster up enough energy to swim up 2.5 meters to at least inhale again, than to need to muster up enough energy to swim 25 meters. – TOOGAM Jun 25 '17 at 12:13

Some commentary focuses on quarries and industrial lakes. I am not of the opinion that that makes a difference: what's down there (ie, machinery, rock ridge formations, etc), and what's in the water (ie, chemicals) is more important than how deep the water is. In this case, signs should scream "Danger! Keep Out!", rather than "Danger! Deep Water!"

I'm also not of the opinion that cold water temps at depths are a reason for the signs, although I do think it might play a role in swimmability, which pertains to your initial question. But experienced swimmers know this, non-swimmers do not. And at depths where temps make a difference to non-swimmers, the depth - and not the temps - is a more troublesome issue for the non-swimmer.

Although the question is broad, you did give an example of a lake. In this case, things like currents or animals/fish playing a role in the "deep" part of the "danger" is less likely.

The word "deep" doesn't even have legal definition. After all, pools are often labelled "2 feet deep", "12 feet deep", etc. Is 2 feet really "deep"? I think not. So a sign that says "deep water" is simply playing to fears, forcing people to be careful.

And that is a valid context: to play on fears, as well as to avoid legal liability. For example, you mention a lake. Lakes are not "x meters deep"; rather, they have variable depths. Someone who cannot swim, perhaps a child, may walk into a shallow area of a lake, walk around, and suddenly find themselves walking at depths they can no longer walk without having to swim. Is it deep? Yes, for that person, it's too deep for them to walk. Is it deep for a diver? No, of course not - but a diver is used to temperature changes as well as knows how to swim. The signs, therefore, must pertain to those who cannot swim.

Further, you see signs at public pools which indicate how deep that part of the pool is - but you never see signs that warn of "deep water", even at the 4 meter end. There's a pool near me which has a 20' depth for scuba instruction and practice. No signs. Why? Why does a lake with variable depths get signs, while a pool with a known depth not get a sign?

And that's the answer: the depths vary, and will be too deep for non-swimmers. The water isn't more or less swimmable at varying depths, it is that it is less walkable at varying depths.

And beaches: why do beaches never have signs indicating that water is deep? The depth most definitely varies. There are currents. Temperatures more assuredly change. There are fish, some really nasty ones like sharks and jellyfish. Yet, there are not universally placed signs warning people of these dangers, right? Well, not true: It would take away from the beauty of the beaches to have these signs all over the place; but in fact, they do place signs there. And they do tell you of the dangers: deep water, rip tides, shark sightings, rocks, buoys, and so on. They're just usually not placed right in the middle of the water where one might swim (or walk) into one. And they're very specific about the danger that lurks.

As to liability: That there are signs at all is probably to avoid legal liability, but only in limited cases. The reality is that lakes are typically owned by government, and suing a government entity is a herculean task. I think, then, liability is less of the impetus to place a warning sign then it is to help people know that waters are probably deeper than they are tall. In private lakes, perhaps a warning sign has more legal implication, but your question didn't distinguish between private and public bodies of water. Rather, you asked if depth made water more or less swimmable.

By the way, the example link you provided struck me as odd (and comical, with very young children playing around a scary looking danger sign). I'm from the US, and am wondering if my definition of bathing and paddling are different than in the UK. I'm imagining that bathing has a connotation that there is a lot less formality with regards to swimming, and deep water can affect your bathing. A lost bathing suit can be most embarrassing if you can't swim down to retrieve it - but that hardly warrants a "danger" sign. As to paddling, well, you're in a boat; the article mentions that swimming and navigation are two separate distinctions in UK law, and that might mean the difference. Paddling, also, is dangerous to swimmers who could become injured by the craft in which you're paddling, or the paddles themselves.

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    "After all, pools are often labelled '2 feet deep', ... etc. Is 2 feet really 'deep'? I think not." This in no way invalidates the phrase "two feet deep". In expressions such as "two feet deep", "deep" just means "having the stated depth," – David Richerby Jun 22 '17 at 15:50
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    A normal lake has a gentle entrance slope: you might need to go ten meters out to get two meters of depth. An abandoned quarry is much steeper: you might get a hundred meters of water below you at the very edge. – Mark Jun 22 '17 at 21:50
  • "a normal lake" isn't normal, not over any reasonably large geographic area. Crater lakes can be just as sheer sided as abandoned quarries, and much deeper (hotter too if the volcano's not dead). Lakes that form along fault lines also have one side almost vertical (and anything up to 1km deep) and the other side fairly shallow sloping. – Leliel Jun 23 '17 at 1:19
  • To fill a quarry in the UK you need to fill it and keep it filled. By the very nature of gravity that usually means that the water going in is filtered and the machinery is meters below anywhere a sensible swimmer would go. We do not frack raw materials and don't have filled quarries that contained anything other than shale and coal ore so we don't really have potential for a poisoning problem in the UK. – user1901982 Jun 24 '17 at 11:59
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    "Paddling" in British English means wading (for fun) in water up to about knee level as in a paddling pool -- a very shallow pool for children to play in. – ymett Jun 26 '17 at 7:06

I'm always tempted to suggest that if they can put up a sign they can put up a life ring, and sometimes they do. But the rate of vandalism and theft of rescue equipment, while small, is real. Of course such equipment wouldn't be much use without someone capable of using it. So it's easier just to warn people off.

The risks at many sites are high and include contamination and steep uneven banks. The policy of just warning people off only really becomes a problem when it becomes a blanket policy. There are still quite a lot of places where it's possible to swim in fresh water, but they can be a long way away.


Despite the many answers here, I do not see one that I personally experienced; and, in fact, I see the mistaken idea that "Once it's deep enough there's no difference; 2.5m and 25m are equivalent." I disagree.

When I was younger and was still learning to swim, I decided to go "swimming," thinking that I could do so. That was my intention, at least.

I decided to leave the area where I was supposed to be wading, and I walked over to an area where the water was much deeper. How deep, I'm not sure, but it was much higher than I was tall. I quickly found out that I could not really swim like I thought I could.

Unfortunately for me, I have never been able to float well; I seem to be one of those "negative buoyancy" people, even more-so when I was younger. So I sank to the bottom, and I was not able to get back to the surface since I could not swim. I was terrified at first, but then I realized that if I kicked off of the bottom hard I could get a reasonable distance up, and if combined with my previously futile flailing I could actually break surface for a moment, just long enough to breathe, before sinking back down.

I did this up and down almost-swimming a few times and realized that I could keep it going enough to stay alive, so I started trying to slowly work my way back while doing this. It was very slow going and I went top to bottom back to top, gasping what air I could each time I inched my way, many times before I got to safety. I was probably running on adrenaline for part of that, and I recall being exhausted and barely being able to pull myself out of the water.

I am not sure how deep the water was, but I would guess maybe 3 to 5 meters. If it had been 1 to 3 meters deeper, maybe I could still have just barely reached the surface, maybe not; I don't know. So something in the 4 to 7 meter range I'll guess was survivable for me. If it had been 30 meters, I might not be alive now to provide this answer.

So yes, there can be a difference between "way over your head but only somewhat deep" and "wow that is way freaking deep." That is, assuming you cannot swim. If you can swim, then the difference just became much smaller, but still present according to some other answers.

Now that I am much older and can swim fine, I have swam in water deep enough that I don't know where the bottom is. As such, my first instinctive reaction to OP's question was "There is not really a difference," but then I quickly remembered my incident back when I could not yet swim.

Of course, if there had been a sign saying "Danger: This water is way freaking deep" I might have still "swam" there anyway, since I was under the false notion that I could swim. But that is a separate matter and is only one aspect of the question.


There are at least two distinct sources of danger, one that is a threat to non- and weak swimmers, and one that can affect even good swimmers.

The obvious danger is that non-swimmers can drown in water that is more than chin high. They can even drown in shallower water if they get their foot caught in the ground or otherwise "trapped" in a way that pulls their head below water, and there's no one to rescue them.

There is a less obvious danger to stronger swimmers. Shallower water is warmer, or at least absorbs heat better; deeper water is colder. The circulation of cold, deep water could cause cramps,shock or other problems to good swimmers, causing them to drown.


I think it's due to water pressure. I don't have to tell you that with every meter you go down, there's another meter of water above you. After a certain point, that could kill you.

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    That would be pretty far down. I am not convinced this is the right answer. – gerrit Jun 24 '17 at 19:58
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    Regular scuba can go down 100 feet in the ocean, deeper in freshwater. The issues have to do with breathing at depth, and don’t apply to freedivers. – JDługosz Jun 25 '17 at 19:19

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