There was a time (dunno if it's still true) when in non-peak usage hours BC Hydro was storing potential behind it's dams and buying dirt cheap from AB to meet BC demand.  Then in US peak hours BC Hydro was using its stored potential behind the dams and selling to the US at peak prices.

The reason being at that time that AB couldn't just shut down its coal or gas fired generation plants, whereas BC Hydro could reduce output by holding back water.