It wouldn't surprise me if we were at 90% at one point, but at least according to this: https://wdfw.wa.gov/conservation/habitat/planning/energy/hyd..., we're presently only at 75%. Your point still stands - we have a lot of hydro here, and an increasing amount of wind power, especially in Eastern Washington. I went to look into it because I'm always surprised how much of my own (Pacific Power) bill is from coal / non-renewable sources, given that I live within an hour drive to the Columbia and am surrounded by wind farms. I'm told at one point in history (1970s?) Grant county in WA and Umatilla county in Oregon had the cheapest power in the country and were on some kind of 99-year price freeze, all from hydro, but I don't know the specifics.