This Was solved by a user who said he looked all along the Mississippi from the coast up to Minnesota. the location was Lake Larto in Louisianna  

Hint #1 This image is in a state that boarders the Mississippi River

Here is a new map for you all to investigate. As you can see it is very low in elevation variability, and also heavily altered by man made processes. Good luck!

p.s. there are still 2 unsolved geo guesses