Shut-in (river)


A shut-in is a type of rock formation found in Ozarks streams where they carve through a mountain range, causing a complex of pools, rivulets, rapids and plunge pools. Shut-ins are inherently confined to a narrow valley or canyon, with the river valley widening out both above and below the formation. Because the rock resists downcutting, streams typically descend at relatively steep gradient through shut-ins, with the downstream terminus of the formation often marked by a very large plunge pool. The river becomes unnavigable at shut-ins even by canoe due to the rapids and narrow channels.
Johnson's Shut-Ins State Park in Missouri, with its hard rhyolite and a diabase dike that divert the Black River into many small streamlets following a complex joint system, is the most well known example. More than ninety other shut–ins occur within and around the St. Francois Mountains region of southeast Missouri.
The term has an origin in Appalachia, where it was used to refer to a narrow river gorge confined by resistant rock layers. In southern Illinois, the Burden Falls Wilderness area includes a narrow canyon below a waterfall that is confined by a resistant sandstone layer. The gorge is referred to as a shut–in, following the Appalachian usage for the term.