Why is a dam built much thicker at the bottom than at the top?
Water pressure grows with depth, P = ρgh, so the deepest water pushes hardest on the wall.
Water pressure grows with depth, P = ρgh, so the deepest water pushes hardest on the wall. The base must therefore withstand the greatest force, which is why dams are wedge-shaped — thin where the pressure is low near the surface and thick where it's highest at the bottom. Notice the pressure depends only on depth, not on how wide the reservoir is.
Key point
P = ρgh — pressure rises with depth, so the base takes the largest force.
Common mistake
thinking a bigger reservoir means more pressure on the wall.
Memory tip
only depth (not area) sets the pressure on a dam wall.
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