Wood for Salmon

In coastal watersheds throughout the northwest, salmon and steelhead populations have suffered dramatic declines. A key driver of many of these declines is the legacy of past logging practices, which have left many streams choked with sediment and cleared of the forest overstory. Mature, forested riparian buffers provide not only shade but also downed material that adds important complexity to the stream channel.

Old, downed trees play an important role in salmon survival.  When the trees fall into streams, the trunks dig out deep pools, and cause gravel to settle. The gravelly ends of pools are ideal substrate for salmon to lay eggs in winter. In the summer, young salmon use the pools to feed and grow before their outmigration to the sea.

Thus, one way conservationists and timber companies have tried to restore salmon populations is by building engineered wood structures using logs, steel cable, and pins that attempt to recreate natural wood jams. These constructed projects, however, are expensive and often fail to create the intended habitat.

Conservancy scientists started testing a simpler approach in 2009, one that mimics nature more closely. With our local partners in Mendocino County, we placed large, long trees in specific locations in rivers and streams. Some were placed loosely and others wedged against trees or rocks. As trees were carried downstream during winter flow, they formed natural wood jams.

We tested this approach over six years along 45 miles of stream, and found that this method reliably creates habitat for salmon. The woody debris dug out pools, collected spawning gravels, and provided the fish cover from predators, just as fallen trees would have. And importantly, it costs roughly one-fifth the price of the more engineered approach.

Although the more traditional engineered log structures would be the more appropriate method in some cases (such as when there are bridges or other infrastructure downstream, or in very large streams), we have shown that for many streams simply adding long logs back into the stream channel is effective.

Importantly, it demonstrates a cost-efficient way to accelerate the restoration of habitat in these watershed – which can play an important part in the recovery of these iconic fish species.