There is not as much pressure near the top of the dam, so it is not as broad there. Concrete arch dams use an arch design to hold back the water. They are built inside narrow, steep canyons. The side that faces the water arches in. When water pushes on it, the dam just pushes into the canyon walls.
Arch dams can be very thin because they can hold back a lot of force with their arch design.
If water finds a way to seep through a dam, the pressure will not let it stop. The hole will get larger, and will require costly repairs. Also, when rivers are stopped, wildlife and vegetation can die off. Areas behind the dam are generally flooded, causing more harm to the environment.