So you need to Pick a problem: stop a river

So you pile dirt across it.

1. But the water climbs behind your pile, spills over the top, and washes it all away.
2. So you build in thin, hard-packed layers on a wide base, because water pushes harder the deeper it gets.
3. But water seeps right through dirt.
4. So you put a core of clay in the middle, and a sand filter behind it.
5. But the water sneaks underneath, through cracks in the bedrock.
6. So you drill deep holes and pump in cement grout: a grout curtain.
7. But if water ever pours over the top, it eats the dam from the outside.
8. So you give the extra water a safe way down: a spillway.

Spin a turbine with the falling water, pave a road across the top, and you just built a dam.