ANSWER: THE MISSISSIPPI RIVER
Island Number Ten: An Impediment to a Union Attack
Confederate engineers had made wise decisions in selection of their defense sites. The bottomlands around Island Number Ten and New Madrid were naturally swampy.
Read more about it:
The Confederate Navy | A Book About Ships and Harbors by H.D. Ingles
A Stroll Through American History | U.S. History Milestones by H.D. Ingles