BC Hydro is a net importer of Power, not a net exporter. BC Hydro manages to make a cash net gain by spinning up Idle Hydro Power Generators quickly (100s of megawatts per axle in seconds) to meet peak demand in the USA. During low demand periods BC Hydro imports cheaper power generated by USA Nuclear and "Steamer" plants that cannot be dialed back easily like Hydro. Nuclear power stations are designed to run at a constant power level and "Steamers" take the better part of a day to reheat. Hydro Dams and Pumped Storage offer a way to smooth out electrical supply and demand, whether from nuclear, coal, gas, oil, solar or wind.
-- 
http://www.cbsnews.com/news/enron-traders-caught-on-tape/
"If you took down the steamer, how long would it take to get it back up?"
"Oh, it's not something you want to just be turning on and off every hour. Let's put it that way,"
"Well, why don't you just go ahead and shut her down." 
http://spectrum.ieee.org/green-tech/wind/norway-wants-to-be-europes-batte