Southern Illinois the Ozarks of Missouri, is chiefly in southern Jackson, Union, northern Alexander and Johnson counties. The eastern area, more closely related to the Wabash Valley Seismic Zone, is mostly in northern Pope, southern Saline, Gallatin, eastern Johnson and southern Williamson counties. The Shawnee National Forest covers a large territory, including seven wilderness areas: Garden of the Gods, Bay Creek, Clear Springs, Bald Knob, Burden Falls, Lusk Creek, and Panthers Den. Of southern Illinois' rivers, only the Mississippi and the Ohio are navigable for modern commerce. The Big Muddy River, Marys River, Saline River and Cache River run their courses in
Big Muddy Creek (Missouri River tributary) Fort Peck Indian Reservation. It joins the Missouri west of Culbertson. It was explored in 1805 by the Lewis and Clark Expedition, who called it Martha's River in their journals and noticed on their return voyage in 1806 that it had changed its mouth on the Missouri. Along with the Milk River and the Poplar River, it is one of three waterways in Canada that drain into the Gulf of Mexico. Big Muddy Creek has also been known as: Big Muddy River, Little Yellow River, Martha River, Martha's River, Marthas River, Marthy's River and Park River. Big Muddy Creek (Missouri