Runoff in the Obed-Emory River Basin of Tennessee

RUNOFF IN THE OBED-EMORY RIVER BASIN OF TENNESSEE* Michael W. Mayfield INTRODUCTION. Despite the presence of several basin characteristics that are generally considered to contribute to relatively stable stream discharges and substantial base flow, the streams of the ObedEmory river basin of east Tennessee exhibit unusually high flood peaks and remarkably low base flows. This paper will briefly describe the runoff characteristics of those streams and will offer an hypothesis regarding the factors that lead to the peculiarities. The study focuses on the 764 square-mile area drained by the Obed-Emory river system above the gaging station at Oakdale, Tennessee , which lies about 35 miles west of Knoxville and 16 miles west of Oak Ridge. The stream system includes the Emory River, the Obed River, Clear Creek, Daddy's Creek, and a number of small tributaries. (Fig. 1) Most of the Obed-Emory basin is a part of the tabular Cumberland Plateau, which may be distinguished from the more thoroughly dissected Cumberland Mountain section adjacent to the north. Fenneman effectively describes the area as having "an undulating surface submaturely dissected by young valleys whose steepness and depth increase toward the edges." (J) This section of the Plateau is underlain by nearly horizontal Pennsylvanian strata, chiefly alternating sandstones and shales. A thick conglomeratic sandstone formation (Rockcastle conglomerate) lies closest to the surface over much of the area. Groundwater in the basin is under considerable artesian pressure, so that water levels in wells rise as much as 100 feet when an aquifer is penetrated. * I would like to thank Professor Edwin H. Hammond for his invaluable assistance in the development of this research project and in the preparation of this manuscript. Mr. Mayfield is a graduate student in the Department of Geography, University of Tennessee, Knoxville, TN 37916. He was given the award for best student paper at the Master's level for presenting an earlier version of this paper in the student honors competition during the Southeastern Division, AAG, meetings in Athens, Georgia, during November, 1978. Vol. XIX, No. 1 55 THE OBED-EMORY BASIN -UMBERLAND MTS Wartburg Oakdale Ar Crossville lichucky Collins Bas Fig. 1. The Obed-Emory Basin. The majority of the basin surface is gently rolling, with most upland slopes in the headwater sections having an inclination of eight percent or less. Toward the basin outlet, upland slopes of eight to 15 percent are more common. Prevailing surface elevations decline in a gentle regional slope from nearly 2,000 feet near Crossville to 1,300 feet at Wartburg. The major streams flow on the sandstone caprock of the Plateau in their upper reaches but become progressively entrenched downstream in gorges as much as 500 feet deep with walls that are often vertical or even overhung. Several large ridges rise above the undulating upland surface. The most prominent ridge, the Crab Orchard Mountains, is developed on the northeastern extremity of the 200-mile-long, straight Sequatchie 56Southeastern Geographer anticline. (2) Several smaller and less prominent ridges are the result of differential erosion on local thrust-faulted structures. (3) The northeastern section of the basin, drained by the upper Emory River, is occupied by highly dissected mountain terrain developed on more easily eroded shales and siltstones. This mountainous section accounts for only about 10 percent of the basin area. The seasonal runoff regime of the Emory River is much like that of other free-flowing streams in the southeastern United States. Peak flows occur during the late winter and early spring months, and discharge is least during the late summer and early fall. Although the general seasonal trend of streamflow is not unusual, the magnitude of the flood peaks and low flows associated with those seasonal changes is clearly anomalous. Winter and spring discharges are unusually high, and late summer and early fall flows are especially low. The mean discharge for March is more than 20 times as great as that for October. The degree to which the seasonal flow regime of the Emory River deviates from the general pattern of streams in southern Appalachia, or from the pattern that might be expected of a stream with its basin characteristics, can best be understood by comparing the Emory...