The Mississippi is a 2,340 mi river down the center of America, early American settlers used the river's connection to the Gulf of Mexico to enter America. Steamboats were used to transport enslaved people up the river into America for labor, this method of transportation led to the populations of enslaved people in America being focused along the Mississippi, especially in states like Louisiana and Mississippi. Many of the towns along the river therefore relied heavily on slave labor for their agriculture based economies.
Here's a video with a few facts about the Mississippi River, the man talks a little slow so feel free to speed it up for a faster watch.