It's actually legal to stick a generator in a stream here in CO. Damming that stream, however, is not legal.
It's legal to run a water powered generator because the number of folks, like me, who live on a creek is incredibly minimal here....so even if we all did it...there would be no real impacts to flow/etc. The challenge would be finding a creek where the water runs fast enough and at a constant rate to make such a thing happen. Would work great during "run off"......not so great in times of drought.
and when the guy above with the Senior water right turns the crank mid-Summer and takes most of the water out of the creek for his fields....you won't be generating any power at all. For it to really work you'd need a right to that water. then you could do whatever you wanted with your 'right" to that water.
There are a few units out there but they simply don;t produce the power needed for growing dope.
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There's another one that looks like a torpedo that you tie off in the stream and the water flows through it/spins the prop/generates electricity. again though.......it's gonna take a shit ton of flow to do much...... unless there are 12V grow lights....
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good luck......