Quick google search found the article (has map) below and list some other information.

[link to academic.emporia.edu]


Regional Area
The Mississippi River Valley alluvial aquifer encompasses an area of approximately 32,000 square miles (Figure 1). It is present in seven states and is the uppermost aquifer in the Mississippi embayment aquifer system.

Major rivers that drain the relatively flat alluvial plane include the Mississippi, St. Francis, White, Arkansas, Yazoo and Quachita Rivers (Ackerman, 1996).