Fallen logs invigorate stream life
December 4, 2024 | By Declan McCabe | The Outside Story 
Illustration by Adelaide Murphy Tyrol
For 12 summers, my Vermont colleagues and I offered guidance to high school student and teacher teams who conducted research on streams as part of a National Science Foundation EPSCoR program.
These teams received training in July, and for the rest of the summer and early autumn, employed their new skills by taking water samples and flow measurements and making observations about aquatic macroinvertebrate populations. They also took note of the size and abundance of fallen logs in the water.
I remember students expressing curiosity about this last measurement. Why, they asked, should we bother to note the presence of logs in the water? A good way to answer that question is to consider the historical practice of clearing and straightening streams – and the damage it has caused.
People have often regarded streams as primarily drainage features: ways for rainfall to efficiently exit the landscape. From this perspective, a meandering stream needs fixing. In our region, for the past several centuries, people have “improved” streams by straightening them, removing wood and other obstructions, armoring banks with riprap, and, where streams approach roads or other busy areas, diverting water into underground pipes.
The cumulative effect of these actions is that many watersheds have less capacity to absorb and slow water than they previously did. As water flows quickly, it erodes streambanks and cuts into streambeds. Trapped within these deeply incised channels, water can’t spread into upland landscapes and instead rushes downhill. After heavy rains, the downstream impacts can be devastating. Excess surface water can rapidly build into powerful floods, capable of damaging homes, roads, and other infrastructure.
Torrential floods disrupt aquatic life but straightened and cleared streams harm aquatic ecosystems in other ways, as well. We learn in grade school that a wavy line between two points is longer than a straight line; for the same reason, a meandering stream holds more water and offers more space for habitat than a straight one can. And straight, monotonous flows offer poor living conditions for many species. For example, many species of fish require slow eddies, backwaters, pools, or rocky riffles for food, shelter, and reproduction. Many insect larvae, as well as young amphibians, require pools and submerged shelters where they can hide and feed.
As in forests, in streams, messiness is good. By slowing a stream’s main flow, logs can push water up over banks into soil, and eventually help straightened streams return to more natural, meandering paths through the landscape.
This change reduces flooding downstream and also sets the stage for more and more diverse aquatic habitats. As the upstream sides of logs – especially those with branches still attached – accumulate fallen leaves and other debris, these surfaces offer shelter and food for macroinvertebrates and other organisms.
Some fallen trees form partial dams and with these, deep, substantial pools and sand bars. In a complex underwater habitat of submerged branches and trunks, piled sand, and other features, macroinvertebrates and fish can take shelter in areas of slack water.
Wood also serves as a filter, removing gravel, sand, and silt from the water column. Sediments and organic materials that accumulate among branches on stream beds store nutrients that might otherwise contribute to downstream eutrophication. In this way, the presence of wood in streams helps protect downstream lakes from algal and cyanobacteria blooms.
Submerged wood is also important as a direct food source. More than 20 different species of aquatic invertebrates in our region rely directly on wood for food. These so-called “miners” tunnel into underwater logs and branches, consuming wood as they go. They include the larvae of beetles, mayflies, caddisflies, stoneflies, and true flies, and collectively play a critical role in the aquatic food web.
So, why document logs in streams?
Because the presence of wood in the water is a key indicator of stream health. Wood is so important to aquatic systems that stream restoration practitioners often focus on adding wood features as a way to heal damaged waterways. Although this work requires expert planning and permits – no one reading this article should rush out to drop logs into streams – there are ways all of us can help promote natural restoration.
We can avoid the temptation to “clean up” stream banks, and rather allow trees, broken branches, and leaves to accumulate in water naturally. We can also encourage friends and family to reconsider the old aesthetic preference for straight, fast-flowing streams, and distinguish between true trash and natural debris.
By all means, remove tires and abandoned shopping carts from your neighborhood stream, but when given the choice, let sleeping logs lie.
Declan McCabe teaches biology at Saint Michael’s College and is the author of “Turning Stones: Discovering the Life of Water,” (Down East Books, 2024). The Outside Story is assigned and edited by Northern Woodlands magazine and sponsored by the Wellborn Ecology Fund of New Hampshire Charitable Foundation.