At some point he was using 2/3 of the creek.

For a short stretch. The water goes back in at the end. It's a completely environmentally sustainable system.

There are at least the following environmental impacts possible:

- decreasing the amount of water available downstream.

- changing were the downstream water ends up.

- changing the speed of water downstream.

- changing the variability in the stream’s deficit the downstream.

- making it harder/impossible for animals to move along the stream.

I would think this does do the last, and, likely, also the next to last.