Lake Agassiz

You think the Great Lakes are great? Not long ago, during the Pleistocene there were other giant bodies of fresh water that left their mark on the land. Lake Missoula. Lake Lahontan (remnants still exist as Pyramid and Walker Lakes). Lake Bonneville, of which Utah's Great Salt Lake is but a tiny fraction.

But the biggest of them all was glacial Lake Agassiz, larger than all five Great Lakes put together.
(Inspired by a recent comment at CalPundit.)