When we think of rivers, our minds often conjure images of flowing water, lush vegetation, and diverse wildlife. But one crucial element that often goes unnoticed is wood—fallen trees, branches, and logs that make their way into the water. Far from being mere debris, wood plays a vital role in maintaining the health and biodiversity of river ecosystems. In fact, many of CPRW’s river projects, including post-fire restoration, involve putting wood back into the river. Sometimes, we even use salvaged wood from our wildfire mitigation projects to complete river restoration, connecting uplands work to the riverscape.

In this blog, we'll explore why wood is so important to river health and functionality and how it supports the intricate web of life in these aquatic environments.

Schematic illustration of the benefits derived from mobile wood. Infographic by *Wohl et al., 2023*.

1. Creating Habitat Diversity

Wood in rivers creates a variety of habitats that support different species. Logs and branches that fall into the water can form natural barriers, slow down water flow, and create pools and riffles. These features provide shelter and breeding grounds for fish, amphibians, invertebrates, and other aquatic organisms. Different species thrive in different types of habitats, and wood helps create the diversity needed for a healthy ecosystem.

For example, deep pools formed by large logs offer refuge for fish during dry seasons or high water events. Smaller debris creates nooks and crannies where insects and small invertebrates can hide from predators and strong currents. This diversity of habitats increases the overall biodiversity of the river, making it more resilient to changes and natural disturbances, such as wildfire and flooding.

2. Nutrient Cycling

Wood is a critical component of nutrient cycling in rivers. As it decomposes, it releases nutrients like carbon and nitrogen back into the water, which are essential for the growth of aquatic plants and microorganisms. These nutrients support the entire food web, from tiny plankton to large fish and mammals.

In addition to providing direct nutrients, wood also helps trap organic matter, such as leaves and small branches, that would otherwise be swept downstream. This trapped material becomes food for decomposers like fungi and bacteria, further contributing to the nutrient pool and supporting a diverse array of life.

A beaver dam in the North Fork of the Cache la Poudre River Watershed provides many ecological benefits to the riparian ecosystem.

3. Stabilizing Riverbanks

Erosion is a natural process in river systems, but excessive erosion can lead to the loss of habitats and negatively impact water quality. Wood plays a significant role in stabilizing riverbanks by slowing down water flow and reducing the force of the current against the banks. Large logs and root wads that are partially submerged can anchor soil and sediments, preventing them from being washed away.

Moreover, wood can act as a buffer during floods, dispersing the energy of fast-moving water and reducing the likelihood of severe erosion. This stabilization helps maintain the structure of the river channel and protects the surrounding landscape, ensuring that the river can continue to support diverse habitats.

4. Supporting Aquatic Food Webs

Wood in rivers is a cornerstone of aquatic food webs. As it breaks down, it provides a substrate for biofilms, which are communities of microorganisms that form a slimy layer on submerged surfaces. These biofilms are a primary food source for many invertebrates, which in turn become prey for larger animals like fish, birds, and mammals.

The presence of wood also influences the distribution and behavior of fish species. Some fish, like trout and salmon, use the cover provided by submerged logs to hide from predators and to ambush their prey. The complex structure of wood in rivers creates a dynamic environment where different species interact and thrive.

5. Enhancing Water Quality

Wood contributes to the improvement of water quality in several ways. By slowing down the flow of water, wood allows sediment to settle out of the water column, reducing turbidity and improving clarity. This settling process also helps remove pollutants that are attached to sediment particles, such as heavy metals and excess nutrients, thus improving the overall health of the river.

Additionally, the organic matter provided by decomposing wood supports microbial communities that can break down pollutants, further enhancing water quality. These microbes play a crucial role in processes like denitrification, which reduces the levels of harmful nitrates in the water.

At Upper Elkhorn Creek, we built post-assisted log structures (PALS) in an abandoned beaver meadow to mimic the work of a beaver, helping to improve water quality and ecological functioning by providing opportunities for sediment storage, nutrient uptake, and enhanced aquatic and riparian habitats in a burned area.

6. Promoting River Connectivity

In natural river systems, wood helps create connectivity between different parts of the river and its floodplain. Large woody debris can form jams that divert water into side channels or onto the floodplain, reconnecting these areas with the main river channel. This connectivity is essential for the movement of species, nutrients, and sediments throughout the river system.

Floodplain connectivity, in particular, is vital for the reproduction of certain fish species, the dispersal of seeds, and the maintenance of wetland habitats. By promoting these connections, wood helps maintain the ecological integrity of the entire river system.

Conclusion

Wood is far more than just dead trees and branches in a river. It is a dynamic and essential component of healthy river ecosystems, contributing to habitat diversity, nutrient cycling, riverbank stabilization, aquatic food webs, water quality, and river connectivity. Protecting and restoring wood in rivers is crucial for maintaining the health and resilience of these vital ecosystems. As we continue to manage and conserve our water resources, understanding and appreciating the role of wood in river health is essential for ensuring that our rivers remain vibrant and full of life.

Additional Resources

Why wood should move in rivers (Wohl et al, 2023): https://onlinelibrary.wiley.com/doi/full/10.1002/rra.4114

Working with wood in rivers in the Western United States (Ockelford et al., 2024): https://onlinelibrary.wiley.com/doi/full/10.1002/rra.4331

“Messy Rivers are Healthy Rivers” w/ Dr. Ellen Wohl (webinar): https://youtu.be/2RlYdzx3chA?si=f2yBhxaKZwUWSaov

The River Health Assessment Framework (RHAF) – Wood Regime Indicator: https://www.poudrewatershed.org/rhaf

Wood is Good: The Vital Role of Wood in River Health