Hydrologic Response of Watersheds of the Cumberland Plateau, Tennessee

Southeastern Geographer Vol. 26. No. 1. May 1986. pp. 36-54 HYDROLOGIC RESPONSE OF WATERSHEDS OF THE CUMBERLAND PLATEAU, TENNESSEE Michael W. Mayfield Streams draining the tabular section of the Cumberland Plateau in Tennessee exhibit much more extreme flow characteristics than most streams in adjacent physiographic provinces; peak flows are very high and low flows are especially low. The mean annual, 5-, 10-, and 25-year floods of Plateau streams are much higher than the averages of comparably sized drainage basins in the state. An analysis of variance procedure performed on the standardized mean annual flood values for streams within the Plateau against all other Tennessee streams shows a probability of only .0165 that the two groups are from the same population . Such streamflow regimes are all the more unusual in that these watersheds exhibit low drainage density, gentle to moderate slopes, heavy forest cover, and less intense rainfall than some of the surrounding areas. (J) Regression equations developed by statewide studies for the prediction of peak flows seriously underpredict flood flows for streams of the Plateau. For example, a study by the U.S. Geological Survey predicts 2-year floods of 68 m3 s_1 for Richland Creek and 530 m3 s^1 for the Emory River, compared to observed values of 113 and 1240 m3 s_1 respectively. (2) A similar study, which considers more basin characteristics , indicates a 2-year flood of 480 m3 s_1 for the Emory. (3) It is clear that the flow rates of these streams do not conform to those of other streams in the East Tennessee region. Floods of Plateau streams have been a major problem from time to time for the cities that are built at their outlets to the adjacent Ridge and Valley Province and Highland Rim. Such cities include Spring City on the Piney River, Dayton on Richland Creek, and Oakdale and Harriman on the Emory River. (4) At the same time, the extremely low summer and fall flows of these streams preclude their use as water supply streams unless they are impounded. One study showed that water-intensive industry is conspicuously absent in Plateau counties, Dr. Mayfield is Assistant Professor of Geography at the University of North Carolina at Greensboro, NC 27412. Vol. XXVI, No. 1 37 presumably because of the difficulty of obtaining sufficient supplies of water. (5) In this paper, the nature of streamflow in Cumberland Plateau basins is examined in order to determine the degree to which individual streams conform to the general pattern of high peak flows and minimal low flows. Thirteen gaged streams with drainage areas of 125 to 2000 square km are studied (Fig. 1). Smaller basins are not considered beFig . 1. Study area. 38Southeastern Geographer cause of the extreme variability in hydrologie response of very small watersheds. THE PHYSICAL SETTING. The Cumberland Plateau in Tennessee is a broad upland which extends in a northeast to southwest direction from the Kentucky border to Alabama. The Plateau does not stop at the political boundaries, but it does change substantially in character to the north and south, with the transformations in character roughly coincident with the state boundaries. In Tennessee, the Plateau is a true tableland over most of its extent, having an extensive gently-sloping upland. To the north the surface is much more dissected; to the south it is lower and discontinuous. The two largest river systems of the Plateau are the Clear Fork-Big South Fork and Obed-Emory. The main stem of the former is more than 140 km long, while the latter extends 100 km. The other stream systems of the Plateau differ from these in that they have much shorter channels; each must descend the 300 meter drop from Plateau upland to the Tennessee Valley, Sequatchie Valley, or Highland Rim in 50 km or less. Not surprisingly, their channels are much steeper. The stratigraphy of the Plateau consists of a thick sequence of Pennsylvanian shales, sandstones, conglomerates, and coal underlain by massive Mississippian limestone units. Over the majority of the area, all but the lower Pennsylvania beds have been removed by erosion. Most of the Plateau surface is developed on one of the more resistant units such as the Rockcastle...