Water is heavy
I just did an interview with ABC Radio Lismore about the latest proposal to divert water from the Clarence River to the Murray Darling Basin. Apart from the environmental effects (disastrous according to the studies I’ve seen) proponents of ideas like this seem to be unaware of a crucial fact. Water is heavy. A megalitre of water (worth maybe $200 as bulk supply to irrigators) weighs 1000 tonnes. Pumping that much mass over even a low mountain range is prohibitively expensive. You can overcome that with tunnels if the gradients are steep enough, but the Great Dividing Range is pretty broad in Northern NSW.
And, don’t get me started on the really crazy projects like Colin’s Canal.