How to estimate the depth of a pool of water from a distance above?

In a few episodes of "Man vs. Wild", survival expert Bear Grylls was seen jumping off a cliff into a pool of water below. He made some rough estimation of the depth of the pool before making the decision to jump. I am puzzled as to how he managed to do it with just quick visual observation. Any wrong judgement could lead to a broken leg or even death.

If the terrain becomes too difficult, a jump from the cliff to the pool of water below might be the only way to make any progress. How would a person go about estimating the depth of the pool and the height of the jump by quick visual observation? Is there a proper guideline for jump height versus pool depth?

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What's your backup plan if you're partway down and realized you misjudged the depth? –  Don Branson Aug 25 '13 at 13:07
@DonBranson, sometimes we need to take calculated risk when we are trapped between a rock and a hard place. Isn't this part and parcel of survival? –  Question Overflow Aug 26 '13 at 10:23
I don't think so, no. –  Don Branson Aug 26 '13 at 12:12
Why do you think he was basing on his observations? His films are so planned, he has a team, and finally he can go down with camera turned off, measure it, go up, and make a shot –  ВГДЕЖЅZЗИІКЛМНОПҀРСТȢѸФХ Aug 30 '13 at 5:49

Firstly, Bear Grylls is notorious for not doing things the way they look on television, but actually conducting extensive planning, multiple attempts, using alternative methods to get better camera angles, and in some instances, apparently going to stay in a hotel!

But with regard to your question, there are a lot of things you can do to gain a good assessment of water depth:

• If the water is relatively clear, find an object on the bottom and see how it moves through parallax when you move along the cliff
• Watch for shadows, if the sun is high enough. If you can see the height of the object casting the shadow you can estimate the depth
• If the river is fast flowing, but at the section you want to jump in it is relatively smooth and flat, the implication is that it is deep
• Throw in a large rock or even a tree trunk or long branch
• Immediately downstream of a waterfall two things happen- a very deep section, where the falls carve out the bottom of the river, followed by a shallower section where debris washes up. Waterfalls have many dangers, but you can often expect a deep pool

remember, a jump from 80 feet is pretty easy to stop - you don't need much depth of water to be safe. If I had 20 feet of water under me I'd be confident I could stop before hitting the bottom, especially with baggy clothing.

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I understand that he took some short cuts during filming, but that's besides the point. I am more interested if there is any objective way to get some numbers. Can you elaborate on your first and second point? It is hard to visualize without a diagram. Point 3 can be quite subjective. Point 4, is a good point. I would probably tie a rock to a rope or some vines and lower it down to get the wetted length instead of throwing it down where I want to jump. Sorry, looks like I am asking too much :[ –  Question Overflow Aug 26 '13 at 10:19
To estimate distances/heights/depths, parallax is very useful. It requires you to move sideways and estimate the difference in what you can see from each point. Basically you use a bit of pythagoras theory to get a close approximation. 3 subjective - yes, but pretty useful in those conditions. –  Rory Alsop Aug 26 '13 at 10:23
Don't know how many times I've thrown a rock into a pool of unknown depth, watched it disappear and said, "Well it is deeper than that rock..." ;) –  LBell Aug 30 '13 at 1:58

The only way to judge the depth of water with any certainty is to get in it, swim around, and dive repeatedly to the bottom searching for objects (trees, boulders, the body of the last person who leaped before the looked.)

Sure there may be ways that can help you estimate depths to degrees (see Rory's answer) but the odds you have a good visual on anything but the clearest of still-water lakes are pretty slim.

If the terrain becomes too difficult, a jump from the cliff to the pool of water below might be the only way to make any progress.

Death (or paralyzation) is less progress than re-tracing your steps.

Of course, if you are in a life-or-death situation (eg, jump or the raging wildfire will consume you) then play the odds.

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