Magnitude-Frequency Relationships of Alluvial Landforms of the Cumberland Plateau, Tennessee

Southeastern Geographer Vol. XXIX, No. 2, November 1989, pp. 106-117 MAGNITUDE-FREQUENCY RELATIONSHIPS OF ALLUVIAL LANDFORMS OF THE CUMBERLAND PLATEAU, TENNESSEE* Michael W. Mayfield INTRODUCTION. A long-held tenet of many fluvial geomorphologists is that most "geomorphic work" is accomplished by frequent floods (recurrence interval of 1/2 to 1 year) rather than by infrequent floods of great magnitude. This school of thought is based upon research by Wolman and Miller who examined the amount of work done by streams in relation to prevailing flow rates. Wolman and Miller suggested that the geomorphic effectiveness of extreme events and commonly occurring events can be compared "in terms of (1) the relative amounts of 'work' done on the landscape and (2) in terms of the formation of specific features ofthe landscape." (J) They argued that the suspended load of a stream is an accurate measure of the amount of geomorphic work accomplished by a stream, while channel geometry and changes in individual fluvial landforms are examples of specific features formed in the landscape. Later research has tended to agree with Wolman and Miller in that events of extremely high magnitude did not move large volumes of sediment nor modify fluvial landforms to a degree comparable to their extreme magnitude. (2) To be specific, the Wolman-Miller principle holds that frequently occurring flood events have the greatest importance in accomplishing overall geomorphic work and in modifying fluvial landforms because the greater power of extreme events is more than compensated for by their rarity. More recently, studies have shown that some streams draining areas which produce occasional flood events of extreme value transport such large volumes of sediment during those events that ". . . the infrequent hydrologie event is most significant. . . ." (3) Baker demonstrated that * This research was supported by a summer research grant from the University of North Carolina at Greensboro. I would like to thank Carol Crutchfield, Rob Stephens, Ashby Lynch, and George Wallace for assistance in the field and John Morgan for making numerous suggestions for improvement of an earlier version of the manuscript. Dr. Mayfield is Assistant Professor of Geography and Planning at Appalachian State University in Boone, NC 28608. Vol. XXIX, No. 2 107 the second element of the Wolman-Miller principle (formation of specific features) "requires modification for application to stream channels in certain climatic and physiographic settings." (4) He concluded that in regions with climatic conditions which favor infrequent floods of great magnitude, separated by long periods of quiescence, the Wolman-Miller principle is not applicable. Harvey reached similar conclusions based on research in an area of semi-arid climate dominated by cloudbursttype storms in Southeastern Spain. (5) The purpose of this research project is to evaluate the effectiveness of frequently occurring floods in modifying fluvial landforms in relatively small (250-1500 sq km) upland basins in an area with a humid climate. Thus, the applicability of the second aspect of the WolmanMiller principle, the modification of fluvial landforms, is evaluated. No attempt is made to investigate the amount of sediment transported by events of various recurrence intervals. Previously described exceptions to the Wolman-Miller principle have been in regions of arid climate with infrequent, but extreme, flood events. THE STUDY AREA. The Cumberland Plateau of Alabama, Tennessee, and Southern Kentucky is a true tableland, with broad undulating uplands only slightly to moderately dissected by streams incised 100 to 300 m below the plateau surface. The Cumberland Plateau surface truncates various Pennsylvanian sandstone and conglomerate units at elevations of 450 to 600 m. (6) The area receives 1200 to 1650 mm of precipitation per year, resulting in 600-800 mm of runoff annually. (7) Despite gentle slopes, dense forest cover, low drainage density, and permeable soils, the streams of the Cumberland Plateau produce floods of unusually high magnitude for the region. (8) The 25-year flood for Whites Creek is equal to 3.35 m3/km2, and the equivalent value for the Emory River is 1.73 m3/km2. (9) Such high flood values are attributable to a dearth of storage; soils are quite thin on the uplands and the bedrock is largely impermeable. (10) Alluvial deposits along Cumberland Plateau streams consist almost entirely of coarse cobbles and...