What great geological event explains the formation of the Great Lakes, the St. Lawrence River system, the Columbia -Snake River system, and Great Salt Lake?

Great Lakes Formation (http://techalive.mtu.edu/meec/module08/GreatLakesPastandPresent.htm)
TEACH Geography (http://www.great-lakes.net/teach/geog/)
Great Lakes - MSN Encarta (http://encarta.msn.com/encyclopedia_761575552/Great_Lakes.html)
St. Lawrence (river) - MSN Encarta (http://encarta.msn.com/encyclopedia_761570826/St_Lawrence_(river).html)

Hope these help you in some way.