Rivers and trees and woody debris


Rivers are dynamic and breathtaking things. The actions of physics combine with biological processes to create some of Earth’s loveliest and most important ecological systems. Rivers, their processes, and their associated vegetation directly or indirectly support a majority of the organisms on earth for all or part of their life-cycles. Riparian vegetation, including forests, is so closely integrated with the river itself that ecologically it is difficult to consider them as separate.

Rivers and streams exist because there is water on Earth, because of the force of gravity acting upon that water, and because of geomorphic conditions that cause water to accumulate and flow aboveground.

Groundwater and rivers, while they influence each other, are vastly different systems. Groundwater moves slowly through the earth, while rivers are decidedly more swift. And groundwater and streams feed each other depending upon the geology of a given area. We have gaining streams and losing streams: gaining streams are fed by groundwater sources, while losing streams feed the groundwater. Gaining streams are more common, due to the accumulation of available water that occurs when a larger area of land drains to a given stream. Losing streams are most common in areas of particular geologic fingerprint, such as Karst systems, and in arid regions.

Floods, considered by humans to be some of the most despised and destructive acts of nature, are important events that can generate major changes in the earth and its ecosystems – and, despite what we think, for the better. Technically, a flood is simply the response of a stream to a larger-than-typical input of water to the system. Flood flows, or storm flows, are what shape the river and its ecosystem. Smaller, more frequent floods form the active channel, or what we recognize as the river’s bed and banks. Larger, rarer floods, while powerful, do not exert a consistently significant force on the shape of the river. Quite the opposite: the river will withstand the force of larger floods and maintain its equilibrium condition if it is in balance, similar to the way an overfilled glass will maintain its shape and shed the liquid it cannot hold.


I encountered a result of a flood that occurred recently on a tributary to the Napa River. The above alder tree was alive (and apparently in good spirits) when it was ripped from the ground and pulled downstream by the relentless force of a spring 2016 flood flow (in fact, despite its horizontal state, it was still alive when I found it). The flood was so pervasive that the original site of the tree could only be discerned from the opening it left in the canopy. The trunk is now facing downstream, where previously unreachable regions of the tree can be examined from the comfort of the ground. Different species of lichen stratify with the height of the trunk. Already the stream appears to be assimilating it, as one might arrange a rug to accomodate new furniture in a room. The rootball is perfectly positioned to create a beautiful scour-pool already forming beneath it, and another small tree has been caught crosswise in the stream, helping it adjust to the new change in bed elevation. In an equilibrium stream, such things are minor adjustments to the overall poetry of the water moving through the landscape. Downed trees are not disasters; there will always be more trees, and the fallen merely create sunny openings where new trees can sprout and grow. The water finds its way, as it always does. The geometry of the stream remains as it ever was – the channel is as broad, the thalweg as deep, the substrate as well-distributed as it should be.


And there’s this incredible step-pool that’s developed downstream of the second downed tree, with the rest of the water migrating around the down wood. Eventually, this reach will break down and envelop this wood, possibly cover it entirely – and find a new microequilibrium in this twenty linear feet of stream reach. Woody debris is as critical to the health and structure of these streams as gravels and cobbles are. More so than boulders, woody debris, usually in the form of fallen trees from the riparian forest, is responsible for the creation of pools – critical habitat for native salmonids and many other aquatic species. Pools are resting places and refuge from predators. A commonly known koan of stream geomorphologists is “Riffle, Run, Pool, Glide”, referring to the four stream features identifiable by their form within the stream channel and frequently found in equilibrium streams. Riffles are fast-flowing and shallow, a major source of oxygen in the water. Runs, originating downstream of riffles – as all fisherpeople know – are where the food is and where the fish can be found feeding. Pools are important resting habitat. And glides are a transition between pool and riffle, shallower than pools but deeper than riffles, and where the river most often looks to be moving as a single water molecule. It’s all very cool to observe in nature.

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When the forests around streams are logged, it affects the stream ecosystem in deep ways. The principal process by which woody debris becomes available to the stream is tree senescence and death. If the forest is logged improperly – if only the old-growth trees are taken – this will limit the availability and quality of woody debris in the stream spanning into decades later. This shift in the forest can actually change the relationship of the stream to its fish populations. A normal distribution of tree ages, a universal law of nature, is a gradually descending curve with high numbers of young trees and low numbers of old trees. This makes intuitive sense – an area cannot accommodate as many large trees as it can small trees. A forest, like all ecosystems, is limited by space and resource availability.  The first photo in this blog post is a very old, dead conifer (probably a Douglas-fir) that is gradually sinking into the stream channel and creating a scour-pool beaneath it. The stream absorbs the impact of this woody debris and shifts its microhabitats (the riffle-run-pool-glide sequence) to accommodate the new feature.

The understanding of the relationship between forests and rivers by humans has, luckily, led to a dramatic change in the way riparian forests are managed. In California, most plantations now treat riparian areas as completely off-limits to logging. This is in response to regulatory directives from the Department of Fish and Wildlife regarding fish habitat and endangered species protection, which in the northern part of the state includes Northern Spotted Owl habitat (these owls nest exclusively in old-growth forest, and are an indicator species for the old-growth status of the forest). It’s easy for a trained forester to tell if a forest has been improperly logged, but most members of the general public think of forests as densely vegetated places with many small, closely clustered trees. Unfortunately, the forest creatures we still have with us co-evolved with a very different structure of forest habitat, and the decline of many of our native species is associated with our own ignorance of what forests should be – as is the increasingly common occurrence of hugely destructive, unstoppable wildfires. Vast swaths of forest with tons of small trees and very few or no large old trees are not natural and are not beneficial for many species of wildlife – or, as it turns out, for the humans living near them. More in a future post- we’re getting into fire season here in California and it’s likely to be a hot one.