Poverty Point was the largest earthworks structure in the Americas in the period between 1,700 - 1,000 BC.  Built by Native Americans as a full-scale city, it is thought to be the center of trade for the Americas at the time and is sited just 25km from the Mississippi River.  This site pairs well with the Cahokia Mounds site in Southern Illinois about 900km to the north - also near the Mississippi River.  

While there isn't a lot to see today, you can get the idea of how the mounds created a giant "C" shape with easy river access that would facilitate trade.  The remains of a dock are even still there.  The small museum on site helps to better understand the site but the best way is to look at an aerial view - either on the site's website or Google Earth.  

Logistics:  I visited Poverty Point in September, 2014, just after it received World Heritage status.  It was a 4.5 hour drive from my home in Dallas but Monroe, Louisiana is the nearest airport with decent air service.  I agree that you'd need a car in order to visit this site.   For US$4.00 you can have the run of the place - possibly even alone - as this site is VERY far off the tourist track.  The site is managed by the Louisiana State Parks.  Nearby you can find the American Civil War battlefield site of Vicksburg - just 50 minutes to the southeast and the San Antonio, Texas Missions World Heritage site is an easy 9 hours drive to the southwest.   You'll need to go to either Vicksburg, MS or Monroe, LA to find the restaurants and hotels of any substance.