The first true victims of American Imperialism were the Native Americans. As settlers slowly pushed West, native peoples were killed, forced out, or forced to assimilate into White American culture. Land would be set aside to "house" these many peoples, but even that would soon be settled. Native lands were slowly chipped away until they look as we see them. Many groups went from hundreds of thousands of miles of territory to a few hundred acres. Sometimes the land they were "given" wasn't even in their ancestral homeland, but thousands of miles away. Around the same time Americans would rob many Mexicans of the lands that they had settled and farmed for generations.
The rivers led to fertile soil and protection for people in the area so it was a prime place for settlement. Example; Egypt and the Nile.