The Shulchan Aruch OC 160:2 rules that water with which one has done "melachah," is disqualified from netilat yadayim. Does this include water which has flowed downstream from a hydroelectric plant?
Indeed (as in your comment), in 160:5 it says that melachah disqualifies water only if it was drawn, "but not with water from a mikvah or a spring while they are still attached [to their source]." I'm not entirely familiar with how hydroelectric plants work, but doesn't the water continue flowing nonstop as it's turning the turbines or whatever? So that should allow it to be classified as still mechubar.