Large woody debris (LWD), also called coarse woody debris, is important to the structure and function of southern Appalachian streams. In forested watersheds, LWD is a major link between the forest in the riparian zone and the adjacent stream habitat. Large woody debris is defined as woody debris > 10 cm (about 4 inches) diameter and 1 m (3 feet) long.  Woody debris plays several roles in streams. It contributes structure and hiding cover for fish, maintains physical stability of the stream, and provides a range of habitats for stream organisms. 

Very large pieces of LWD, with a diameter greater than 50 cm (20 inches) and a length that exceeds the width of the stream, are particularly important in streams. Longer pieces of LWD are often material from fresh blow-downs, and over time they are broken up into shorter pieces. Large-diameter pieces of LWD can only be produced by mature forests. Woody debris that is longer than the stream width is more stable and unlikely to be moved even by bankfull flows. These pieces of LWD, like large boulders, are roughness elements that work with flowing water to carve out pool habitat for trout and other organisms. Pools are areas in the stream that are deeper and have slower moving water. Sediments and organic debris may be deposited in pools. Thus, LWD is also important in mountain stream hydrology, channel geomorphology, and nutrient dynamics, as well as in providing food and shelter for organisms. Trout are more likely to be found in pools that have several pieces of LWD (Silsbee and Larson 1983, Flebbe and Dolloff 1995, Flebbe 1999).

Streams that flow through old-growth forests in the southern Appalachians generally have more LWD than streams in second-growth forests. Well-developed, mature riparian forests can provide a sustainable supply of LWD for streams, particularly the most critical very large pieces (Silsbee and Larson 1983, Flebbe and Dolloff 1995, Hedman and others 1996, Flebbe 1999).

Streams with mid-successional riparian forests generally have less LWD than do those in old-growth forests. Trees in these areas are not yet as large as those of old-growth forests. The amount and size of LWD in streams is a function of the age and size of woody material that the riparian forest can contribute. Many of these systems are in a transition stage where carry-over debris (left over from the time of logging) is disappearing, and the riparian forest has not yet generated significant new debris. Where significant amounts of carry-over debris remain, LWD in streams can be similar to that in old-growth forest streams (Hedman and others 1996).

One such stream with a mid-successional riparian forest is the subject of research on the role of large woody debris for fish habitat.  Where large woody debris is lacking, often because it was previously removed, large woody debris can be added to improve stream structure and function. 

Most smaller first- through third-order streams have low stream power (Leopold and others 1964), very high channel roughness (Chow 1959), and shallow, narrow channels that are easily obstructed. These features enhance the retention of coarse particulate organic matter (CPOM) such as woody debris and leaves within these channels (Sedell and others 1978, Naiman and Sedell 1979, Bilby and Likens 1980, Wallace and others 1982, Cummins and others 1983, Minshall and others 1983).

In these small headwater streams in forested regions, woody debris not only is a potential energy source, but also serves an important structural role (Swanson and others 1982, Harmon and others 1986). These types of streams are common in the southern Appalachians (Wallace and others 1982, Webster and Swank 1985, Golloday and others 1987, Golloday and others 1989). Woody debris has many roles in high-gradient streams (Harmon and others 1986): (1) it contributes to stair-step profiles that result in rapid dissipation of the streams energy (Bilby and Likens 1980); (2) it aids in retention of other particulate organic matter (for example, see Bilby and Likens 1980, Molles 1982, Speaker and others 1984, Golloday and others 1987), which may influence both trophic and nutrient dynamics (e.g., Bilby 1981, Molles 1982, Newbold and others 1982, Melillo and others 1983, Webster and Swank 1985, Webster and others 1990); (3) it provides fish habitat (Triska and Cromack 1981, Sedell and others 1982); and (4) it provides a substrate for some stream invertebrates (Anderson and others 1978), and food for some aquatic invertebrates that may be xylophagous (Anderson and others 1978, Anderson and Sedell 1979, Dudley and Anderson 1982, Pereira and others 1982).

Like many watersheds in the southern Appalachian Mountains of North Carolina, Wine Spring Creek has a history of land use that includes harvest of trees to the streambanks and removal of large wood from the stream to facilitate transport of logged timber during the early years of the 20^thcentury.  This stream has rainbow trout and a small population of brook trout.

Large woody debris (LWD) is an important link between streams and the adjacent riparian forest. In streams like Wine Spring Creek where the riparian forest has been removed, it may take many years for a new riparian forest to grow and for trees to die and fall into the stream. Compared to two reference streams in North Carolina old-growth forests, Wine Spring Creek has less LWD, evidence of conditions associated with mid-successional riparian forests.

Large woody debris and large boulders add complexity to the stream and create a diversity of habitats for trout spawning, rearing, and feeding. On average, about 71 percent of pools and riffles in Wine Spring Creek are occupied by trout, compared to about 90 percent in the two reference streams.

But there are differences within this 9.8 km stream system. Some sections of Wine Spring Creek, with either a boulder-dominated substrate or LWD amounts comparable to those in the reference streams of old-growth forests, have relatively high abundance of trout. In these reaches, riparian forests are relatively old. Other sections with less mature riparian forests lack these forms of complexity; they have cobble and gravel substrate, no LWD, and reduced trout populations (Flebbe 1999).