Fallen logs invigorate stream life

For 12 summers, my Vermont colleagues and I guided high school student and teacher teams researching streams as part of a National Science Foundation EPSCoR program. These teams received training in July and employed their new skills for the rest of the summer and early autumn by taking water samples and flow measurements and observing aquatic macroinvertebrate populations. They also noted 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 exit the landscape efficiently. 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. Water trapped within these deeply incised channels 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. 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. Straight, monotonous flows provide poor living conditions for many species. For example, many fish species require slow eddies, backwaters, pools, or rocky riffles for food, shelter, and reproduction. Many insect larvae and young amphibians require pools and submerged shelters where they can hide and feed. 

As in forests and streams, messiness is good. By slowing a stream’s main flow, logs can push water up over banks into the soil and eventually help straightened streams return to more natural, meandering paths through the landscape. This change reduces flooding downstream and sets the stage for 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, trunks, piled sand, and other features, macroinvertebrates and fish can shelter in slackwater areas.

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 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 essential 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 the stream – there are ways all of us can help promote natural restoration. We can avoid the temptation to “clean up” stream banks and 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 a choice, let sleeping dogs 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). Illustration by Adelaide Murphy Tyrol. The Outside Story is assigned and edited by Northern Woodlands magazine and sponsored by the Wellborn Ecology Fund of New Hampshire Charitable Foundation: nhcf.org.