I'm an historian of the colonial Americas. I agree with the comment below about 1491 being a great guide to this question. The very short answer is that there was an active effort throughout the 19th and 20th centuries to establish a myth of an untouched wilderness that settlers "expanded into" rather than confronting the reality of conquest.
The longer answer involves the above, plus a lot of population demography and source analysis regarding the effects of "virgin soil" epidemics. Crosby's Ecological Imperialism is an excellent guide to that stuff. Even the very earliest European accounts of populations in the interior, like the expedition of de Soto, [1] are describing the aftermath of massive depopulation, since many diseases were spread by an advance guard of European domesticated animals before any Europeans actually showed up on the scene (or at least, any literate ones did).
The longer answer involves the above, plus a lot of population demography and source analysis regarding the effects of "virgin soil" epidemics. Crosby's Ecological Imperialism is an excellent guide to that stuff. Even the very earliest European accounts of populations in the interior, like the expedition of de Soto, [1] are describing the aftermath of massive depopulation, since many diseases were spread by an advance guard of European domesticated animals before any Europeans actually showed up on the scene (or at least, any literate ones did).
[1] https://www.neh.gov/humanities/2004/septemberoctober/feature...