branches in river

Rethinking wood in rivers to ensure ecological health and human safety

By Dan Scott, Sustainability Leadership Fellow and PhD Student in the Department of Geosciences.

Picture a healthy river: one that is teeming with life, supporting an array of plants, insects, fish, and animals. Such a river also supports you, providing beautiful views, opportunities to play outdoors, and clean water. When you pictured this river, did it have wood in it? Was it complex, with patches of different types of vegetation, some logs and brush, deep pools in some places and shallow reaches in other places? Or, was it a well-kept river that looked like it would be right at home next to a perfectly manicured lawn? It turns out that most people prefer simplified, manicured rivers and see things like wood as harming streams, and needing to be cleaned out [1]. In fact, wood is almost as important to the organisms living in and around rivers as the water that flows in the channel.

Which of these rivers looks healthier? Which would you rather live next to? To a river scientist, the messy-looking one on the right looks much better for fish and insects, and provides nutrients to the soil surrounding it, while the one on the left probably doesn’t support much life at all. Photo credit: Ellen Wohl (left) and the author (right).

Historically, the default perspective regarding wood has been that it endangers humans, increases flood risk, makes rivers harder to navigate, and acts as debris [2]. That has led to the widespread removal of wood from streams, with severe consequences for the things that live in and around rivers, including us.

Wood comes in many forms. Small bits and pieces, dispersed logs, and large accumulations known as jams all play an essential role in the physical and ecological function of rivers. When it breaks down, wood provides food to in-stream organisms like insects and material on which plants such as algae can grow. Log jams commonly create cover for fish, which allows them to grow to maturity. One of the most important functions of wood is in changing flow patterns, which in turn move sediment and nutrients across the land surrounding the river. This scours new surfaces so that trees can sprout and grow, brings older patches of forest into the river, and creates a mosaic of diverse vegetation across the valley bottom. Diverse vegetation supports a diverse array of animals that depend on that vegetation for habitat and food. Wood generally makes rivers messier, which is exactly how fish and other animals tend to like them.

Satellite imagery from Google Earth of the Hoh River, Washington. Notice the clumps of tan and brown logs scattered across the floodplain. All that wood makes the valley bottom a more complex and messy place, providing diverse habitat for fish and animals and supporting local biodiversity.

That said, wood can also pose a danger to people and property, giving it a bad reputation in the eyes of society. Flooding is essential to nourishing floodplain soil, as floods deliver sediment and nutrients over river banks and onto floodplains. However, the presence of wood in a river slows down the flow of water, which, during a flood, pushes more of that water up and over the banks, onto floodplains that may host roads and houses. Wood can also accumulate around bridge piers, causing flooding and exerting enough force to damage the bridge structure. This is especially true for bridges that aren’t built with enough space underneath them for wood to pass through. To boaters, anglers, and hikers, wood can pose a significant risk by trapping swimmers underwater. The same log jam that hosts dozens of happy fish one day could drown a whitewater kayaker the next. When we remove wood from rivers to mitigate these risks, we often simplify the river, reducing fish populations that we might depend on as a food source, or reducing how often the river floods and delivers essential nutrients to vegetation on its banks. Unhealthy ecosystems around rivers lead to decreases in native animal and insect populations, which ultimately harm surrounding agriculture, hunting, fishing, and other human activities.

A bridge in Kansas clogged by wood. This wood may cause flooding of the land surrounding the bridge or damage the bridge structure. Photo credit: Ellen Wohl

This presents a challenge: we know that wood is essential for the health of streams, but we need to ensure that wood is compatible with human needs. None of the dangers posed by wood are inherent to wood itself: wood jams don’t hunt down anglers and drown them. Managing the risks wood poses is a matter of managing how we build, act, and live around rivers. Some solutions are obvious: educate recreationalists about how to be safe around rivers, engineer bridges to be able to pass wood underneath them (i.e., make them taller), and avoid building near rivers to lessen flood damage. These solutions can be very difficult to implement, however. People like living near rivers, where their homes may flood; taller bridges are more expensive; and public education doesn’t necessarily reach all recreationalists, whereas a single death or injury can lead to drastic management actions like removing most or all of the wood along a river.

The author, portaging his packraft over a massive wood jam on the Hoh River, Washington. Photo credit: Ellen Daugherty

River scientists’ research will help balance the risks wood poses to humans with the benefits it provides to ecosystems. Currently, we are implementing a collaborative tool to study wood and create a large database of how wood behaves in rivers, called the Wood jam Dynamics Database and Assessment Model (WooDDAM). This database and the research that comes out of it will allow managers to make the most informed decision possible when considering whether to retain, remove, alter, or even reintroduce wood to rivers. How wood changes over time, especially during floods, determines the risk it poses to humans and property. By quantifying how wood behaves in rivers (e.g., moves downstream, accumulates other pieces of wood), managers can identify and retain low-risk wood and know when to remove or alter wood that poses a high risk to people or property.

Public policy and urban planning also serve an important role in managing wood. The city of Fort Collins, for instance, has instituted a policy of buying land near the Poudre River and converting that land to flood-resilient uses, like parks, sports fields, and natural areas. This reduces the need to pull wood out of the river, since floods don’t do as much damage as they used to. Education campaigns focused on informing recreationalists of the ecological importance of wood can help prevent people from manually removing wood, especially from small, sensitive streams that harbor insects and fish.

Next time you see a messy, wood-rich river, think about the important ecological functions that wood is performing. It might not be well-manicured, but a messy river system tends to be a healthy one, and wood is usually at the heart of that mess.

References

  1. Chin, A. et al. Perceptions of Wood in Rivers and Challenges for Stream Restoration in the United States. Environ. Manage. 41, 893–903 (2008).
  2. Wohl, E. A legacy of absence: Wood removal in US rivers. Prog. Phys. Geogr. 38, 637–663 (2014).

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