When you imagine a healthy stream – whether it’s one that runs through an urban neighborhood or weaves through a natural area – does that picture include large pieces of wood scattered across the water? While streams naturally contain obstacles like fallen trees, boulders, and log jams, studies show that people often perceive wood in waterways negatively. They may view it as a safety hazard, a barrier to recreation, or simply a messy aesthetic. But, as it turns out, this “mess” is all part of nature’s plan to create a healthy ecosystem.
Trees that grow along streams tend to fall into the water over time. This happens due to natural causes like wind, death, beaver activity, or shifting of the banks that once held roots in place. In areas with lots of human activity, these trees are often removed from waterways to allow for recreation or navigation, or to protect infrastructure. But removing fallen trees also removes an important component of the stream ecosystem.
In recent decades, scientists have learned a lot about how wood in streams benefits the aquatic ecosystem. In fact, the benefits are so apparent that many restoration projects now include adding wood back to the stream to mimic natural conditions.
Still, many well-intentioned people remove wood from streams thinking that they are reducing barriers for fish, facilitating water flow, or improving the beauty of the stream. Understanding the role large wood plays in streams can help us reshape our view of what’s beautiful and notice the many benefits that some woody clutter provides.
What counts as “large wood” when we’re talking about streams?
In the field of stream ecology, large wood is anything that is at least four inches wide and six feet long. This includes whole trees that have toppled across the stream, log segments that have traveled downstream, large branches that have fallen from streamside trees, and clumps of roots. It may appear as a single fallen tree or pile up into a larger log jam. Depending on the type of wood, it can remain in the water for decades!
Wood in streams improves wildlife habitat.
Fish are some of the biggest beneficiaries of large wood in streams. While some people perceive logs and woody material as barriers to fish movement, they’re important for creating the habitat conditions that fish thrive in. Wood slows water down, making it easier for migrating fish to move upstream. As water backs up behind logs, it creates pools of varying depths and temperatures where fish can rest, hide from predators, or take refuge when water levels are low. The wood also provides shade, helping to keep the water cool.
Fish aren’t the only ones who benefit – reptiles and birds can use exposed parts of logs for basking and perching. Larger animals use the downed trees as pathways for crossing streams and rivers. A wildlife camera set up by Oregon State University researchers captured a wide variety of critters, including cougars, bobcats, and deer, making use of a downed tree to cross a stream.
Aquatic food webs are more robust in areas with large wood.
Wood in streams plays an important role in the food chain. It captures organic matter like leaves and provides a surface for algae to grow on. Aquatic insects and macroinvertebrates, including mayflies, caddisflies, and crayfish, feed on the algae and plant material. In turn, they are eaten by larger organisms, like fish and amphibians.
The more complex a food web is, the better able the habitat is to support a wide variety of species. Increased biodiversity helps ensure that the ecosystem, and species within it, are more resilient to disturbances.
Logs and root clumps help stabilize stream channels.
Wood also helps maintain the healthy structure of a stream channel. It slows water down, which reduces erosion of the stream’s banks. While erosion and movement of streambanks is a natural process, the stability of the stream can be disrupted if it happens too quickly. When wood is removed from a stream channel, water moves through the channel faster, causing soil to slough off the streambanks at a higher rate. Slower moving water also allows sediment, like sand and gravel, to settle to the bottom of the stream. A healthy stream channel has a variety of sediment types, each providing distinct benefits to aquatic species.
Embracing the “messy” side of nature benefits wildlife habitat.
Large wood in streams isn’t the only case in which nature’s “mess” has positive impacts. Mimicking what we see in nature is an effective way to improve the conditions of our surroundings, whether in a natural area or in backyards and community spaces. Similarly, by leaving fallen leaves on the ground as shelter and food for insects or allowing dead trees to stay standing as homes for birds, we can provide wildlife with the resources they need to thrive.
So, next time you find yourself looking at a stream, see what evidence you can find of woody debris and think about all the wonderful ways it is contributing to a vibrant stream ecosystem!