The power comes from where it's generated. If hydro generation is stopped for some reason, the power will come from somewhere else, and that's why there is interconnection. The legislation is silly because it's unenforceable. That is, of course, unless the customers are willing to isolate themselves from backup sources and chop the interconnections with the rest of the grid so that 100% of the power comes from non-fossil fuel plants.

In a dry year or two with rationed power (a-la California in 2000) the state residents will have an epiphany, reconnect to the west coast grid, and live happily ever after until the next ecology target is found and it starts anew.