Answer: the access to natural resources.
Explanation: At the beginning of the 19th century president Jefferson sent a group of Americans led by Lewis and Clark long across the Missouri River. The objective was to find a route across the continent and also to have a look at the natural resources of the West. The discovery of natural resources such as water, gold and coal drove many people to settle the area because they saw the possibility to flourish there. The natural resouces meant not only food and water to survive but also the manufacture of goods, power streamships, railroad engines, etc.