Spatial Patterns of Channel Instability Along an Ozark River, Southwest Missouri

Abstract

In the Ozark Highlands of Missouri, unstable river reaches that display rapid planform change are described as active reaches. While active reaches can be part of the natural morphodynamic regime, accelerated gravel bar deposition and bank erosion have been linked to historical and recent anthropogenic activities. Relationships between geomorphic controls and specific forms of channel instability are poorly understood in the Ozarks. The objectives of this research were to (1) develop an active reach classification scheme that provides an objective means of identifying and evaluating longitudinal patterns of instability in this and other Ozark rivers, (2) identify active reaches and different forms of instability along 80 km of the Finley River in southwest Missouri, and (3) investigate physical controls on active reaches. Historical aerial photographs and geographic information system (GIS) analysis showed that active reaches occur along 21% of the length of the main stem, preferentially in valleys with confinement ratios between 10 and 30 and near major tributary confluences. Four active reach forms were identified with proportion of length as follows: extension (8%), megabar (6%), cutoff (5%), and translation (2%). Depositional megabar-type active reaches tended to form directly downstream of erosional extension-type active reaches, probably due to excess gravel supply by reworking of historical floodplain deposits. The lack of a dominant active reach form, along with a stable main stem sinuosity over time, suggests that the Finley River is a semi-stable, self-organized system in balance with watershed inputs since at least 1955, the date of the earliest photo series.