Large Woody Debris

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.