Answer:
Westward expansion meant that the US acquired ever increasing amounts of land, and needed people, farmers, to settle there so it could maintained without a huge military presence. This led to massive immigration in the second half of the 19th century, as mostly Europeans came here seeking to finally have their own land.
It also added vast resources to the United States, both in terms of potential agricultural production and things like gold, silver and timber.
It stimulated the construction of the Transcontinental Railroad, and added new states to the country.
It further aggravated the divide between north and south, and accelerated the onset of the Civil War.
Explanation:
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The greatest common factor is 1
Europe is not a typically dry place (unlike for exampe Mesopotamia) so it does not need rivers for irrigation.
Instead, rivers are needed more for transport - they have served as a means of communication and for the exchange of goods.
For example, Danube allowed a big-scale trade between the cities lying on it: Vienna, Budapest and Bratislava.
More than 2/3 of Central America's agricultural land is used for cattle grazing.
