Unicorns drained the Murray

In Australia, the Murray River is a very large river system that has so much water sucked out of it by irrigators that it just doesn't flow at all by the time it reaches the sea. It is a major political issue as the river crosses several states.
Her: Mum, I know why the Murray River doesn't flow!
Me: oh?
Her: Yes! Unicorns drink it! Big herds of unicorns come down and drink all the water!
Me: I'll let the Government know so they can do something about it.
Her: You can't kill unicorns! Unless you stab them under the left eye.