The first Euro beings that spread across the continent were microbes, which reduced the human population by half -- sometimes more. By the time Lewis and Clark crossed the continent, all but Clark had caught some form of European venereal disease, some of it coming inland from ships on the Pacific and otherwise traveling on boats up the Mississippi/Missouri complex, sometimes with the deliberate goal of being lethal. Most effectively it traveled through the fur traders who were mostly in Canada. For a while there was rivalry between the Hudson's Bay Co. factors in posts across the high prairie versus the various forms of the American Fur Company on the US side. In those days Western Canada was not part of the nation so much as a franchise awarded by the English King to the mercantile company named Hudson's Bay.
Trapping out all the beavers quickly left their ponds untended so water could travel faster. The white people of the time didn't seem to notice. They were setting about the killing of all bison, whose bones ended up in huge stacks waiting for the railroad to take them to England for "bone china" cups and saucers. The many boxcar loads of bones are inconceivable today, but photos prove there were indeed so many.
Today, as we know global warming is a result of many small changes added together through the Industrial Revolution, we are more willing to see what these various demographic forces did to the continent. Prairie grass had responded to many hooves, much grazing, and occasional fires set by indigenous people to move bison herds and renew the plants. Now grass grew tall and rank. The flow of the rivers was more intense and diminished earlier in the season, after rampant flooding when it warmed in Spring.
It's suggested that the loss of crop-raising in the warm climates allowed wild growth, as in tropical jungle, to reclaim the open spaces, bury structures and change the oxygen content. There were no census counts, so the facts are dependent on records at the time, as well as genomic analysis.
". . . the Franciscan friar Fray Toribio de Benavente wrote that “At this time [Mexico] was extremely full of people, and when the smallpox began to attack the Indians it became so great a pestilence among them… that in most provinces more than half the population died.”
" . . .Some authors have suggested that the total Native American population size decreased by more than 90%, whereas others have argued that significant population losses were more localized and the magnitude of the reduction relatively modest. . .
"We find that indigenous Americans experienced a significant contraction in population size some 500 years before the present (ypb), during which female effective size was reduced by ∼50%, thus suggesting that the impact of European colonization was both widespread and severe."
At about the same time that Columbus struck America, volcanoes around the world threw so much dust and sulphur into the atmosphere that crops failed for lack of sunlight. This happens periodically and can have harsh effects on people. Probably the impact on the early Euro contacts was daunting, esp. on those trying to establish northern colonies. Drought is one of the most dreaded weather events and evidently is what cleared the SW cliff-dwelling people. Both mean the ends of food production.
We can only guess at what the oceans were doing in this period, but their impact is always there when the currents change due to melting or freezing of continent-sized masses of ice which affects the salinity of the water, which dictates what sinks and what rises. We know how powerful the alternation is between El Nino and La Nina. Some people suggest a cycle of 1500 years that controls weather. We now appear to be approaching a tipping point when the rise and acidity of the ocean will affect cities. In pre-written history days we see "written" in the evidence that villages have been engulfed. Of course, the submergence of Beringia was a drastic change. If we look at the planet over the long term, change is the theme.
The effect of disease, war, enslavement, and even climate was the basis for Euros escaping the guilt of wiping out whole civilizations by claiming that the land was "empty" -- no one lived there. This justified the use of what seemed limitless land to be a refuge for oddballs, explorers, those wanting land, and so on. European culture was dictatorial, "root-bound", so individuals were motivated to escape westward long before the beaver/bison economic drives.
Another demographic impact was in the North American south where African people had been defined as a kind of domestic animal, a wealth that could produce crops while reproducing themselves. Occasionally individuals would escape West, or even in the East would slip away to the persisting tribes of Florida, for instance.
Blackfeet were apart from much of this ferment and change because they were so far north, backed up in the corner of the Rockies and the Canadian border, even in its mostly theoretical terms before it became the Medicine Line. Not until they acquired horses and gunpowder did they began to interact aggressively, to defend the last remnants of the bison, to define who was "in" and who was "out," mostly in terms of allegiance to the felt group.
The basis of the tribe's subgroups was in relation to the center point, usually a strong-minded father of many, rather than an attempt to enforce a known border like a river or a divide. Most of the newcomers were men with vigour and imagination who contributed to the group and "married" the tribal women, producing children who tied them to the whole. Trappers who were Scots, French, Mohawk, Samoan, Mexican -- cross-national -- both passed through and came to stay, increasing the vitality of the groups that had formed in both what would be Montana and what became eventually Canada. Most of this went unrecorded. Early missionary writing might be accessible to multi-lingual scholars with access to Vatican records.
The People of the Eastslope of the Rockies are part of the world dynamics that control what happens to everyone. The Beaver, the Bison, and the Blackfeet are all players who might be powerless in the singular, but become world-determining in the aggregate.