Oregon and Washington generate far more hydro-electric power than most states. How does all this hydro-electric power help keep our electric bills low (half the cost of electricity in many states)?

Answer 1

http://planetsave.com/2016/03/09/hydropower-costs-renewable-energy-hydroelectricity-costs-vs-renewable-fossil-costs/

Answer 2

Water power is cheaper than coal power. Although building the dam is expensive, the water is considered free there from then on ( I could argue at length about that, being into eating salmon for example but anyway....). Coal must be dug up, oil must be pumped up, gas must be drilled for, all expensive forever (ignoring the black lung disease etc).