Answer:
1. St. Louis, Missouri
When Zebulon Pike was sent to locate the source of the Mississippi, he started off from St Louis in the year 1805. A year earlier, the Lewis and Clark expedition also departed from the same city in Missouri.
2. Columbia River.
The Lewis and Clark expedition came in contact with the Columbia river to the Northwest of what is now the United States and used it to get to the Pacific Ocean.
3. Gulf of Mexico
The Mississippi is the longest river in North America and stretches from Minnesota in the north all the way to the South of the United States where it empties into the Gulf of Mexico.