why Lewis and Clark went on the expedition?

You're welcome.

President Jefferson assigned them this expedition.

http://en.wikipedia.org/wiki/Lewis_and_Clark_Expedition

Thnx both.

Lewis and Clark embarked on their famous expedition, known as the Lewis and Clark Expedition or the Corps of Discovery, for several reasons. Here's how you can find more about it:

1. Historical Background: To understand the context behind their journey, it's helpful to first explore the historical backdrop. Start by searching for information on the early 19th century United States, including the Louisiana Purchase and the westward expansion.

2. Research Primary Sources: Read primary documents, such as Lewis and Clark's journals or letters, to gather insight into their motivations. These firsthand accounts can provide valuable information on their expedition's purpose.

3. Study Secondary Sources: Consult scholarly articles, books, or reputable websites dedicated to the Lewis and Clark Expedition. These sources often analyze and synthesize primary materials, providing a broader understanding of their reasons for embarking on the journey.

4. Official Instructions: Review the instructions given to Meriwether Lewis by President Thomas Jefferson. Research the goals assigned to the expedition, like exploring the newly acquired western territories, searching for a water route to the Pacific Ocean, and conducting scientific observations.

5. Consult Expert Opinions: Discover interpretations and viewpoints from historians who have extensively studied the expedition. Learn about their theories on the motivations for the journey and the different factors that influenced Lewis and Clark's decision.

By delving into these resources and conducting thorough research, you will gain a comprehensive understanding of why Lewis and Clark undertook their expedition, ultimately answering the question.

The short answer is President Thomas Jefferson sent them.

The long answer is that Jefferson had just bought the Louisiana Purchase and everyone wanted to send an expedition to find out what we bought and tramp around it before England or Spain or someone decided to claim it. Not only a route across the purchase and to the west coast could be walked and mapped but we would find out what sort of plants, animals, native people and soils existed across most of the country.