Historical Sedimentation in West Tennessee Gullies

Southeastern Geographer Vol. 28, No. 1, May 1988, pp. 1-18 HISTORICAL SEDIMENTATION IN WEST TENNESSEE GULLIES* Michael L. Barnhardt INTRODUCTION. Human attempts to alter or modify natural ecosystems often lead to disruptions of a magnitude far greater than intended or expected; soil erosion is a prime example. The association of human cultivation with accelerated soil erosion is evident worldwide, and Butzer concluded that accelerated soil erosion has probably been an environmental problem for over 10 millennia. (1) Deforestation, followed by cultivation and grazing, is increasingly a research focus of the scientific community due to its devastating effects on major ecosystems. A decrease in crop productivity, loss of topsoil and fertility, alluviation in river channels, increased frequency and magnitude of overbank flooding, and destruction of wetlands are consequences of accelerated soil erosion. (2) Political, social, and economic policies also play a role in soil erosion. Decisions to expand into rainforests to accommodate population pressures or provide a product for export may be well-intended but are environmentally unwise. Many of our most productive floodplain soils owe their existence to normal erosion and sedimentation processes, but accelerated soil erosion implies a rate exceeding the geologic norm. Associated with historical agricultural practices, it is generally not conducive to soil development because it proceeds at a rate far in excess of soil-forming processes. The evidence supporting this historical increase in erosion and sedimentation lies in the stratigraphie record preserved in many river valleys. Knox found that historical sedimentation rates exceeded presettlement Holocene rates by two or three magnitudes in the Driftless Area of Wisconsin. (3) Trimble and Lund found similar increases associated with post-European cultivation in the same region, although the two studies differed in the timing and mechanics of the sedimenta- * This work was supported in part by the Department of Geography and Planning, Memphis State University, and by two grants from the Memphis State University Faculty Research Fund. That support does not necessarily imply University endorsement ofresearch conclusions. I wish to thank J. Bullen, J. Giblin, and K. McCarthy for field and laboratory assistance. Dr. Barnhardt is Assistant Professor ofGeography at Memphis State University in Memphis, TN 38152. Southeastern Geographer tion. (4) Trimble's extensively documented study of human-induced erosion in the Southern Piedmont carefully illustrates the importance of conservation measures in controlling soil erosion. (5) The reconnaissance erosion surveys conducted in the 1930s by the Soil Erosion Service (forerunner of the Soil Conservation Service) ended an era of land mismanagement from which some areas have not yet recovered. The purpose of this study is to examine the historical sedimentation record of several valley-floor and valley-side gullies originating in the loess-dominated bluff landscape of Southwest Tennessee. Gullies similar to these are common also in loess-covered landscapes from Iowa to Mississippi. In addition, this paper will evaluate the geomorphic status of these ephemeral gullies that owe the majority of their activity to disruptions associated with deforestation and cultivation between 1860 and 1935. The gully terminology used in this paper will follow that suggested by Schumm and Daniels. (6) A valley-side gully is a small, steep-sided, sharply incised, elongated channel on a valley side that is eroding into sediment where no significant channel previously existed. A valleyfloor gully (or entrenched stream) is incising into alluvium (channel fill) deposited in a previous valley. The geomorphic implications are important . A valley-side gully represents an expansion of the drainage network , whereas the valley-floor gully (entrenched stream) represents a more fundamental change in basin dynamics that may, eventually, lead to a major reworking of the entire drainage basin. Many gullies are difficult to classify simply because they can be defined as a valley-side gully in their headwaters and a valley-floor gully downstream. In fact, throughout the study area the recent rejuvenation of the gully system has produced a sequence of such transitional gully types. STUDY AREA. Meeman-Shelby State Park is located about 40 km north of Memphis and occupies 5200 hectares of the Mississippi River bottomland and the loess-dominated Chickasaw Bluff No. 3 (Fig. 1). Total relief within the park is about 60 m. Several of the larger entrenched stream valleys exhibit...