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Dead wood : geomorphic effects of coarse woody debris in headwater streams, Great Smoky Mountains.

Article Excerpt
ABSTRACT--Coarse woody debris (CWD) is an important component of headwater streams. This study compares the distribution, residence time, and geomorphic function of CWD between two headwater streams, one located in an area of the Great Smoky Mountains that was logged 80 years ago and the other in an old growth forest zone. Coarse woody debris channel obstructions are more frequent along the old growth-area stream (3.0 per 100 m) compared with the logged-area stream (1.4 per 100 m). Differences in CWD frequency and size between the two reaches have important consequences for observed variations in channel morphology and sediment storage. Coarse woody debris stores approximately 25 times more sediment along the old growth reach than along the logged-area reach. In addition, variations in channel width and longitudinal profile are more closely tied with variations in CWD frequency along the old growth stream. The greatest effect of CWD on channel morphology occurs where debris slides have delivered multiple trees to headwater streams creating 'log jams.' One log jam consists of over 50 trees and stores ~1200 [m.sup.3] of sand and gravel. Future investigations should focus on monitoring sediment release from large log jams since such releases may dramatically affect downstream channel morphology and basin sediment yield.

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Coarse woody debris (CWD) has been recognized as an important component of headwater streams (Harmon et al., 1986). Coarse woody debris controls organic matter transport (Bilby, 1981), promotes sediment storage (Megahan, 1982), and causes channel bank erosion (Keller et al., 1995). Disturbances such as logging and mass wasting affect the rate at which CWD is delivered to streams. Spies et al. (1988) and Hedman et al. (1996) showed that CWD production is greater in old growth forests compared to younger-aged stands. In the Great Smoky Mountains, Silsbee and Larson (1983) found that CWD was larger in diameter in old growth-area streams than in logged-area streams. Landslides scour hollows and deposit large pulses of CWD into headwater streams, which may remain in place for decades (Pearce and Watson, 1983; Perkins, 1989)

While several studies have highlighted the function of CWD in maintaining aquatic habitat in the southern Appalachians (Coulston and Maughan, 1983; Fleebe and Dolloff, 1995; Hilderbrand et al., 1998), little is known about the effects of CWD on channel morphology and sediment storage in...

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