How have humans altered the landscape of the United States?

Question

surana · Answer

Putting up dams, bridges, cutting down forests and trees, industrialization, digging of mines, building of roads and cities.

anonymous · Answer

A few clear examples: Mount Rushmore is not a natural formation. Neither is the Crazy Horse sculpture. The Missouri River has been highly "channelized" so that it does not wander over the river bottom land like it used to, and like other rivers it has been dammed in many places to control flow, prevent flooding, and provide recreation. Some rivers have been diverted, or added to (the Hennepin Canal in Illinois) for various reasons, often to aid trade or provide drinking water to urban areas. Prairie grassland was heavily farmed and created the Dust Bowl in the 1930's due to drought and high winds. Much of the farming in Nebraska and Kansas relies on water pumped from below ground to happen, or there would be no farmland, just grazing land. Marsh lands have been drained to create more farm land. Check a map of Boston in 1776 and Boston today...much of the bay has been "reclaimed" or "dewatered" for building land. The Mission District of San Francisco is the same, and in one earthquake it suffered greatly since reclaimed land "liquifies" in that situation, causing great damage to structures and threatening people. The Golden Gate Bridge is not a natural phenomena. The Saint Lawrence Seaway is not a natural phenomena...the falls at Niagara and other area used to prevent boat traffic to the Great Lakes. Great cities are natural developments, and neither are interstate highways. The lock and dam system on the Mississippi allows boat traffic more readily and safely than the "wild river" every did. Levees keep New Orleans dry, when it would mostly be underwater without them. Air conditioning has allowed the population of places like Tucson and Phoenix to grow...without it they would be much smaller towns. Hills get flattened for development, ravines get filled in for development, and forests are cut for development (though a lot less these days.) 
I hope these show the breadth of the impact of humans on natural geography for you.