Some functions of large woody debris

Large woody debris (LWD) is more important than one might think:

LWD creates obstructions that encourage sediment deposition, especially in small- to medium-sized streams. The sediment stored behind individual log “steps” or jams can be quite substantial; removing it can dramatically increase downstream sediment loads.

Log steps create abrupt changes in channel slope (i.e., waterfalls) that cause scour pools. These pools are an important habitat element for aquatic insects and fish. On many smaller streams, virtually all the pool habitat may be associated with LWD. In the absence of LWD the stream becomes all riffle, offering little habitat value.

The presence of LWD can either enhance or reduce channel stability by deflecting or directing flow to or from erodible banks.

u LWD traps organic material traveling downstream, making it available to instream organisms. In anadromous fish streams, this includes trapping salmon carcasses that are an important source of nutrients.

Many aquatic insects attach themselves to LWD either permanently or temporarily while they reproduce. The succession of colonizers on LWD depends on its age and state of decay.

Wood is crucial in creating and maintaining the complexity of habitat for anadromous salmonids. LWD-created pools provide fish with a quiet environment to rest, feed and avoid predators. Gravel bars created by LWD are used by salmonids as spawning habitat. Juvenile salmonids are particularly dependent on LWD for escape cover and refuge during high wintertime flows. Studies have shown fewer salmonids in streams where LWD is either naturally limited or has been reduced.

LWD affects the streamside forest that is its source. LWD creates low velocity areas where sediment and organic material deposit, speeding soil development and providing nutrients for streamside forest development. Downed logs trap sediment and provide areas for seedlings to establish. Woody debris shields seedlings from scouring during high flows. Established streamside forests eventually contribute LWD back to the stream during disturbance events.