It was called Lake Missoula, a freshwater inland sea, more than 600 meters deep at its deepest point, holding more water than Lake Ontario and Lake Erie combined.
These are the largest collection of freshwater lakes anywhere in the world by surface area that pretty much act in a similar way as an inland sea does like the Mediterranean or the Baltic.
The perfect ships for moving across the calm waters of an inland sea like the Mediterranean but pretty terrible for moving across the unpredictable and often harsh waters of the open ocean.