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 was

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.