By Kevin Davis, USFS
Picture a healthy river or stream with cold, clear water running over boulders and cobbles through a shady corridor of tall evergreen trees with overhanging shrubs lining the banks. That picture in your mind might be your favorite fishing stream; or it might be the view from a bridge that you drive over on your way to town.
Picturesque it certainly is, but there may be something missing. What does a tree become when it tips over? Well yes, dead, but when a tree falls in a river or stream it begins a second very important ecological phase, as a log.
Wood is as vital in streams as the gravels and substrate that fish spawn in, especially in the heavily forested north woods. You might even say that rivers evolved with wood for thousands of years.
When a tall white pine or cedar fell in the channel it would form a major obstruction in the water’s path. Depending on how the tree fell in the channel, the water either diverted around it, scoured under, plunged over, or backed up above it as more wood lodged in around it.
All these features create incredible fish habitat, and the more habitat present in a stream, the more fish it can support.
Wood is the home for aquatic invertebrates such as caddisflies, stoneflies, and mayflies. The decomposing logs form cavities and aquatic insects feed on the cellulose and add to the organic material in the water. Wood, especially trees with roots attached, can remain stable even in high flows that move large boulders. Logs and log jams help to store and sort gravels that form gravel bars where streamside vegetation grows.
Wood helps develop spawning beds that have the right sized gravels where trout and salmon look to lay their eggs. A river system that is lacking wood, for whatever reason, usually has a decreased potential to support aquatic insects, and therefore does not support as many fish. Without the logs and log jams to create pockets to accumulate gravels, there may not be a healthy riparian plant community or gravels available for spawning fish.
Log jams also dissipate energy when streams are flooding. Rushing water gets backed up behind big log jams and the water’s destructive power is reduced. If stable wood and log jams are present through the entire length of the river flood waters pinball through and never quite reach that critical energy to blow out of the river banks and wreak havoc on everything in the floodplain.
This is not to say that having enough wood in the system will prevent flooding. Flooding is a natural way a river dissipates energy and that is why maintaining healthy floodplains adjacent to rivers, with plenty of vegetation and logs, is so important; they help reduce flood damage downstream.
So why would we take wood out of a river if it performs so many valuable functions? Years ago, when fish biology was becoming a field of practice, they thought log jams presented barriers to fish migration, so they cleaned it out. Foresters and loggers salvaged wood out of rivers and streams to send to the lumber mill. Engineers took wood out of the channel since it posed a threat to washing out downstream structures like roads and bridges.
On popular boating rivers, rafters and guide services asked to have dangerous logs and jams removed to make the river safer for boating. Wood is removed from streams for many reasons and has been for many years.
In the past couple decades, however, many natural resource practitioners have come to realize the importance of wood, and where it is appropriate the Forest Service is utilizing wood in natural configurations to do stream restoration projects.
Engineered log jams are used to create fish habitat, enhance hydrologic functions and protect infrastructure such as roads and bridges. Rootwads are used in bank stabilization projects where roads were lost in a flood.
Rivers and streams in North Idaho may never be the fisheries that they once were, but if we manage them for all their natural components, including wood, they will at least be set on a trend to maintain clear, cold water and healthy fisheries for years to come.
Kevin Davis is a hydrologic technician with the U.S. Forest Service. He works all over Bonner and Boundary county on national forest land doing watershed related projects involving culvert replacement, road maintenance and fish habitat improvement.