"This river is Hammawi ancestral ground where they once lived and historical and cultural artifacts will be forever lost."

Then you can thank the draining of the river for helping to reveal artifacts that were otherwise lost in the submerged riverbed.

"I don't understand why someone would do this."

You don't understand the need for water? Water to grow food and feed livestock? Water to drive non-polluting hydroelectric generators so that less oil can be burned?