Coal is more portable than hydro electricity, so you can burn it where you actually need heat/electricity.

What do you do with unused hydro output? You can waste it or you can power bitcoin. This could be arbitrary big numbers in terms of KWh, but the opportunity cost and carbon footprint would be zero.

Since bitcoin mining is so perfectly competitive, it will be done where the cost of energy is lowest, which is zero for the most places - where energy would be otherwise wasted. And since other consumers pay considerably more than zero, they'd get oil/gas/coal delivered to them instead of wasting them on bitcoin miners.

Hydroelectric power destroys aquatic ecosystems [1]. Many technologists are prone to resorting to “clean hydro” power as their way of dismissing concerns about carbon footprint. I'd much prefer the story around Bitcoin's carbon footprint shift to one of offsetting rampant consumerism through disincentivizing spending.

[1]: http://www.elwhafilm.com/

that's ridiculous.

You are forgetting (among many) about the impacts of a decentralised currency for the billions of people without access to bank accounts or a stable currency.

Billions? The bitcoin network is doing less than 7 transactions per second. It's less than useless for anything but a handful of people trading it as speculation.

How is Bitcoin helping people that have a need for a bank and a stable currency?

The largest mines in china don't use coal, because coal is more expensive than hydro.