Large Organic Debris in the Lower Tar River, North Carolina, 1879-1900

Southeastern Geographer Vol. XXXI, No. 2, November 1991, pp. 55-66 LARGE ORGANIC DEBRIS IN THE LOWER TAR RIVER, NORTH CAROLINA, 1879-1900* Jonathan D. Phillips and Gerald R. Holder INTRODUCTION. Regular commercial navigation of the lower Tar River began in 1840, and during the last halfofthe 19th century the river carried considerable traffic. The U.S. Army Corps of Engineers was charged with maintaining the channel and undertook many operations to remove large organic debris (downed trees, logs, stumps, and woody snags), which constituted the major hazard to navigation. Historical records of these activities from 1879 to 1900 provide an opportunity to evaluate the role of large organic debris in the Tar River fluvial system, in effect providing a 20-year experiment on the effects ofdebris removal. These data are significant because a large body of work in fluvial geomorphology suggests that organic debris accumulations in stream channels may be necessary for system equilibrium in forested drainage basins. If, as studies on smaller streams suggest, large organic debris is critical in dissipating energy and maintaining some form of fluvial equilibrium , then one would expect system dynamics to provide an ongoing supply of downed trees and branches to the channel. Thus, removal of debris from the channel should trigger feedback responses which would work to resupply the channel with new organic debris. Hack and Goodlett from a primarily geomorphological perspective and Likens and Bilby from an ecological perspective have argued that streams and surrounding forests coevolve to a mutual equilibrium. (1) Furthermore, studies on forested mountain streams by Marston and Heede have found woody debris in dams or log steps to be necessary for stream channel slope adjustment. (2) This suggests that there may be an * Jim Cox unselfishly shared the fruits of his exhaustive examination of obscure federal records and his knowledge of Tar River history. Leo Zonn assisted in fieldwork. Work reported here was to be the first phase of a broader study of the historical physical geography of the Tar River by the coauthors. This was cut short by the untimely death of Gerald Holder in April 1991. This work is dedicated to his memory. Dr. Phillips is Associate Professor ofGeography at East Carolina University in Greenville, NC 27858. At the time of his death, Dr. Holder was Assistant Professor of Geography at Longwood College, Farmville, VA 23901 . 56Southeastern Geographer equilibrium quantity of large organic debris (LOD) in the forested river channels and that the quantity of debris in the channel should be an indicator of system stability. This contention is supported by documentation of significant changes in flow regimes and channel morphology associated both with increasing LOD supplied to channels and with removal of in-channel debris. (3) An obvious way of testing for this phenomenon is to remove the woody debris and observe the response. If LOD indeed plays a critical role in Tar River energy dissipation, its removal should be followed by feedback processes which return LOD to the channel. Such an experiment on any but a very limited scale would not be feasible now in the Tar River because debris removal ("snagging" in the local vernacular) is restricted by numerous regulations and policies. The historical record of channel-clearing operations, however, provides insight into channel response to debris removal and enables a post facto "experiment." Historical records of U.S. Army Corps of Engineers navigation projects for the 1879—1900 period and a contemporary survey were used to assess the response of the Tar River system to the removal of debris. Fluvial geomorphological theory suggests that LOD is a critical part of the energy dissipation regime of forested drainage basins. As long as there is a continuous supply of woody riparian vegetation, increased bank erosion following removal of debris should increase the rate of LOD supply and replenish the in-channel woody debris. The historical records were used to determine whether such replenishment actually occurred. ORGANIC DEBRIS IN RIVER CHANNELS. The historic record and contemporary surveys indicate that there are significant quantities of woody material obstructing the lower Tar River channel. This large debris (defined as having a diameter of 10 cm or more) plays an important role in fluvial processes in some...