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The water in the reservoir is on the straight up-and-down side of the dam.
As the water gets deeper, it pushes harder on the dam, so the bottom of the dam
needs much more concrete than the top part of the dam -- a triangle is the
best shape!
If you look at a gravity dam from the front, where there is little or no
water, you see a solid flat wall of concrete -- a rectangle.
Can you trace a rectangle on front of the dam using
your mouse?
If you look at a gravity dam from the back, you can't see much because that's
where the water is being held.
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