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 panorama

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.[1] More than ninety other shut–ins occur within and around the St. Francois Mountains region of southeast Missouri.[1]

The term has an origin in Appalachia, where it was used to refer to a narrow river gorge confined by resistant rock layers.[1] 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.