STREAM INCISION, PIRACIES, AND LANDSCAPE EVOLUTION OF THE EL DORADO VALLEY ON THE NORTHWESTERN FLANK OF THE OZARKS

El Dorado Valley, which previously has been referred to as the McCarty-Horse Creek lineament, is located on the northwestern flank of the Ozarks in eastern Bates, southwestern Cedar, eastern Vernon, and northeastern Barton counties. It is an abandoned valley, approximately 40 km long, 2.5 km wide, and up to 30 m deep.

Historical accounts indicate that the original location of the village of Montevallo overlooked a "mountain valley." A short segment of this abandoned valley was mentioned in early geologic reports from Vernon County; lacking adequate maps, investigators considered it to be simple stream piracy by McCarty Creek. Subsequent geologic mappers regarded the entirety of this feature as structural lineament, perhaps related to monoclinal folding. Investigations with new remote sensing imagery, shuttle radar topography mapping (SRTM) and digital elevation models (DEM), together with provisional field studies, reveal the striking lateral continuity of El Dorado Valley. The age and precise origin of this feature, however, remains obscure.

Quaternary stream valleys essentially are superimposed on this feature. Several misfit streams flow along the axis of the abandoned valley for short distances, but at least five stream piracies occur where modern streams cut through narrow gaps along the margins of the valley. Walnut Creek interfluve, which branches northwestward from El Dorado Valley, as well as Wilson Branch Valley and Rocky Hollow, were possible tributaries of El Dorado Valley, which would indicate southerly directed flow, if this feature is fluvial in origin. If this interpretation is correct, the location of El Dorado Valley near the saddle of the divide between the Spring-Arkansas River and Osage-Missouri River systems is notable. Pleistocene ice-damming along the Missouri River could have resulted in development of periglacial lakes in the Osage River drainages, and El Dorado Valley may have been a possible spill-over between the Missouri and Arkansas drainages.

Alternative hypotheses include parallel faults or fractures or exhumed Pennsylvanian paleovalleys. El Dorado Valley is preserved in a large down-dropped block between the Chesapeake and El Dorado North fault systems, but currently, available evidence does not support the structural hypothesis.