Hello,
Your question states:
What was the main reason the Mississippi River was so important to the growing nation?
Your answer would be:
It gave a route for trade to move faster and even transportation.
Explanation/Reasoning:
They used the Mississippi River for transportation to get all over the United States, it was a faster way for trade routes too.
You have a wonderful day now:)
Hope this helps!
~Rendorforestmusic