What happens when it fills up?...................
There are underground rivers all over the place, most of them go into large underground basins and are eventually pumped to the surface.
Before there was such a large population there were many more above ground rivers because the basins would get full.
It is pretty much just like the rivers above ground.
When you dowse for water you find what you think is the best place and then you follow the stream in and out of the area to get the best place.