The state park is located on the southern edge of what once was the Great Willow Swamp, a marsh area covering the low-lying areas between the Mississippi, Rock and Green rivers, and is believed to have contained one of the most concentrated and varied wildlife populations in the central part of North America.
(The state park; is located; ) (a marsh area; covering; the low-lying areas between the Mississippi, Rock and Green rivers)
The state park a marsh area
is located covering
the low-lying areas between the Mississippi, Rock and Green rivers