Botanical Evidence for the Age of Oxbow Lakes: A Test of Harper's Hypothesis

Southeastern Geographer Vol. XXXI, No. 2, November 1991, pp. 67-74 BOTANICAL EVIDENCE FOR THE AGE OF OXBOW LAKES: A TEST OF HARPER'S HYPOTHESIS* David Shankman INTRODUCTION. The southeastern Coastal Plain is occupied by lowgradient meandering streams that have formed wide floodplains and complex patterns of abandoned channels. Roland Harper, a botanist who had traveled extensively in the southeastern United States, hypothesized in 1912 that the minimum age of an oxbow lake could be estimated by determining the age of the oldest bald cypress (Taxodium distichum) growing in it. (I) He based this hypothesis on his observations that bald cypress is usually absent along the banks of large streams but typically is one of the dominant species along the margins of oxbow lakes. It seemed, therefore, that regeneration could begin only after the channel had cut off. (2) Harper noted that bald cypress cannot tolerate long periods of total submersion and suggested that along the active channel of large streams seedlings are killed by the following years' floods. When a lake cuts off from the river, however, its seasonal water level fluctuations are immediately reduced, and, therefore, conditions at the margins ofthe lakes become suitable for bald cypress regeneration. The purpose ofthis investigation is to test Harper's hypothesis that the ages of bald cypress can be used to determine the minimum age ofabandoned channels along a southeastern Coastal Plain stream in western Tennessee. STUDY AREA AND METHODS. The Hatchie River was chosen for study because it is one of the few non-channelized, unimpounded, and freely meandering streams in the southeastern Coastal Plain for which there is a series of historical maps that can be used for reconstruction of former channel positions (Fig. 1). The Hatchie River has dissected a loess-covered plateau creating a floodplain typically 3 to 5 km wide and bounded by late Pleistocene terraces. (3) The alluvium of the active floodplain consists of eroded loess and sediments from underlying * Thanks to David A. Bell for his assistance in the field and Laurel G. Drake and Gerald R. Webster for reviewing an early draft of this manuscript. Dr. Shankman is Assistant Professor of Geography at the University ofAlabama in Tuscaloosa, AL 35487. 68 Southeastern Geographer ARKANSAS Kilometers Fig. 1. Location of the Hatchie River in western Tennessee. Coastal Plain formations. Average precipitation is about 120 cm/year, and precipitation, average discharge, and flooding frequency are greatest in the late winter and early spring. Comparisons of U.S. Geological Survey topographic maps produced at different times during the last 60 years (1931, 1933, 1939, 1948, 1961, 1977) were used to determine the approximate age of all oxbow lakes less than 60 years old along the lower 70 km of the River. Within the study area, nine oxbows were identified as having been created since 1931. Twelve additional oxbows had formed before 1931. The exact ages of the latter are not known, but they probably cut off within the last 100 years (between 1891 and 1931). This age estimate is based on the reported rates of sedimentation within the lower bottomlands of the major streams in western Tennessee. (4) The sediment load of streams in western Tennessee, including the Hatchie River, is among the highest in the southeastern Coastal Plain. Coastal Plain streams flood most years, and often there is more than one flooding event. Overbank sediment deposition causes the oxbows to fill in rapidly, and, therefore, they normally hold water continuously for only a few decades after cutoff. All twelve of the oxbows in the study area that had formed before 1931, however, still hold water throughout the year. The active channel and all oxbows along the lower Hatchie River Vol. XXXI, No. 2 69 were visually examined to determine if bald cypress were present. Few bald cypress occur on the river banks or in the youngest oxbow lakes. Their presence in older oxbow lakes was used to determine the approximate length of time between channel cutoff and their successful establishment . RESULTS AND DISCUSSION. No bald cypress occurred along the margins of the nine oxbow lakes formed since 1931; instead, the former channel banks were dominated by young black willow (Salix nigra). In contrast to the...