Why are dams thicker at the bottom than at the top?
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Although the ocean contains a lot more water than a lake, it is the depth that determines the weight of water directly over a diver that defines the amount of pressure the diver experiences. Therefore, a diver who is 20 meters below the surface of a lake will experience more pressure, in fact, twice the pressure, than the ocean diver experiences at 10 meters. Saltwater is denser than freshwater, but the increase in pressure caused by the increased density is small in comparison to that caused by the difference in depth.
Dams hold back bodies of water, and water pressure increases with the depth of the body of water, so the pressure from the water pushing horizontally on the dam is greater at the bottom than at the top. If holes were bored near the bottom, middle, and top of a dam, the longest horizontal stream of water would fire out through the bottom hole because the water pressure is greatest there.