Has the seven year cycle of the great lakes ever been explained?

My grandfather once told me in the 70's that the great lakes rise steadily for seven years then fall for seven years in an endless cycle. I witnessed this years later when I returned. Lake Erie had risen enough to submerge approx 200 lineal feet of beach and the waves were lapping at the seawall. I've heard that the increased population in the region has affected this normal cycle and that it has not been as predictable as it was decades ago. It was considered a mystery back then ( in the seventies ) but I hoped that someone has figured it out by now ... signed, Forever Curious