Through ScienceDaily: A new study by Baylor University geology researchers shows that Native Americans' land use nearly a century ago produced a widespread impact on the eastern North American landscape and floodplain development several hundred years prior to the arrival of major European settlements.

The actual article is: Stinchcomb, G.E., T.C. Messner, S.G. Driese, L.C. Nordt, and R.M. Stewart. 2011. “Pre-colonial (A.D. 1100–1600) sedimentation related to prehistoric maize agriculture and climate change in eastern North America.” Geology 39 (4) (April 1): 363 -366.

I don't have access to the article, but you can read the abstract linked above and the ScienceDaily article. And here's a similar story about pre-Columbian influences on the the Amazonian forests. If you want to read more about the Americas before Columbus, I would recommend reading 1491: New Revelations of the Americas Before Columbus by Charles Mann. It describes decades of research looking into the size of pre-Columbian indigenous populations, how far they go back in time, and how they influenced their natural environment.