[blam]

I wonder why he didn't just let the water flow on the surface of the land instead of putting it in pipes?

The Blanco River in Texas just rushes out of the ground and forms the Blanco River.

Spent many fine summer days lazing in the Blanco River.

[SunkenCiv]

It just sets there, so the pipes presumably are there to pipe it out. There's a large "fossil water" deposit under the western desert in Egypt as well.