South Puget Sound Salmon Enhancement Group

Engineered Log Jams (ELJs) and Large Woody Debris (LWD)

Traditionally, streams functioning in a natural setting accumulated wood in the form of branches, root wads, and large logs. These woody structures known as log jams or large woody debris (LWD) provided critical habitat for fish, amphibians, and invertebrates through the formation of pools and the creation of cover.

Historically, we (the human population) did not know that LWD provided habitat to stream inhabitants. The general consensus was that wood in the streams blocked fish passage, as it prevented our boats from freely traveling up or down the waterway. It became common practice to remove wood from streams. Today some logging practices, road construction, and dams continue to contribute to a reduction in the amount of LWD found in rivers.

New knowledge has changed the perspective on LWD in streams, from an unwanted stream component, to a desirable stream component. Current scientific studies show that LWD enhances fish, especially salmonid, habitat by providing resting pools for adult spawners, promoting sediment deposition, and creating places of cover for juveniles. Now restoration biologists are trying to put back, what was historically taken out.

The Engineered Log Jam (ELJs) arose as a man made substitute for natural LWD. Essentially, an ELJ is made up of large pieces of wood strategically placed by engineers and construction workers in streams to mimic LWD. There are four main functions of ELJs as described by Herrera Environmental Consultants, Inc. (2006).

1) Habitat Enhancement: ELJs reestablish diverse and complex habitat through the creation of pools and cover

2) Bank Protection: ELJs direct flows away from stream banks, helping to prevent erosion

3) Bar Apex: ELJs in the middle of a channel, divide the river into multiple channels, increasing channel length, depth, cover, and number of pools.

4) Grade Control: ELJs help to retain sediment in the form of gravel for spawning salmonids as well as soil along the edges of the stream enriching the riparian zone.