In the wake of the melting glaciers lot of the upper Americas was flooded. There was glacial Lake Agassiz that incorporated large parts of the area around upper Minnesota and Lake Superior.
But with the receding glaciers, much of that excess water was drained due to isostatic rebound. It's took a long time for the depressed earth to rise up after the trillions of tons of ice had melted.
Which is why Canada still has many trillions of tons of water in their landscape while only the upper U.S. has natural lakes not close to the Atlantic Ocean or Gulf of Mexico. It would be interesting to find out exactly how many natural lakes Canada has. Whenever I fly back from a trip to Britain (to visit the wife's kin) I marvel at the numerous lakes over the Canadian landscape I see as we pass over.
When the wife and I drive out west from Wisconsin to Minnesota and the states beyond, I can't help thinking and marveling that just 150 -200 years earlier most of the farmland I see in the midwest into S. Dakota was tallgrass prairie inhabited by only scattered Indian tribes.
But even with modern civilization most of the land is remarkably "empty" if you know what I mean. Which one of the big reasons I love going out west.
Inarguably, there were fewer resources out west, and those moved around, so you either hunted or fished for what was in season, or you moved with it. Even now, the population is relatively sparse compared to the coasts. But when the Europeans got here, there were well established and sedentary communities all over those seaboards and related watersheds, especially in the coastal regions. The same areas that were heavily populated then are heavily populated now, for the same reasons: river (highway, because the water was the most efficient means of transporting large amounts of anything that couldn't walk on its own) access, resources, fishing, trade, fertile land, timber (at the time). Few coastal areas in the US have large amounts of suitable building stone, so the common material was wood, and that does not generally persist, especially in an environment rife with termites and carpenter ants. IOW, the river terrace that seems like a meadow today may well have been a stockaded village then (We found one when I worked on an Archaeological Crew in Virginia in '78). So much of that has been built over and/or cultivated, mined for sand and gravel, etc., that the true extent of those habitations will never be known, and whatever existed in the vast Eastern/Gulf coastal plains flooded by the rise in sea level after the ice age will not likely be known unless there is another one, if then.
One other thing, is that we tend to think of dry, solid ground as the preferred route of travel in the age of the wheel and especially motorized transport. When waterways and pack animals are the best means available, and are, for the most part, more practical than wheels, the value of real estate that many would not value today increases significantly. Coastal Marshes and swamps are seen as curiosities, impediments, and hazards by those who prefer the wheel, but become transport media and resource-rich areas to those who learn them and their resources. Wildlife (food) abounds in such areas for those who know how to find it.
Similarly, the western canyonlands and badlands areas which seem so uninhabitable become a refuge to those who are not limited by wheeled access and who know the resources available and where and how to find them, despite those resources being limited.
