Water, Sediment and Wood: the Three River Cornerstones
Ancient Earth-centered view of the universe. Earth, covered by water, is in the center, surrounded by air, then fire. Next come the moon, planets and stars, in the realm of “quintessence” (the fifth element”) or “ether.” From Wikipedia: Peter Apian’s Cosmographica, Antwerp, 1539.
Ancient Greek philosophers, trying to make sense of the physical world, thought that all matter would naturally arrange itself in layers or shells with the earth as it center. Soil, rock and other ”earth elements” formed the innermost shell. Next, like layers of an onion, came water, then over the water, the air. Fire formed a shell outward from the air. And enveloping all of these things, was an unknown substance , the “quintessesnce” (“fifth element”) or “ether,” the stuff of the heavens. This ordering made sense, because rocks would sink to the bottom of water, while air would bubble to the surface. And fire would send flames and smoke ever upward, striving towards the sun. In this scheme, plants, animals, and humans resided somewhere between earth and air.
We now know that this ordering of matter is not a property of things themselves, but a consequence of the relentless force of gravity. Gravity tends to pull everything down towards the earth, but denser matter is pulled more strongly, ending up on the bottom. Rivers are part of this scheme. They are the way that the Earth’s gravity pulls liquid water downward. And as the water moves downward, it drags rocks and soil and animals and plants, living and nonliving, along with it.
This is one of the most important concepts in river science: rivers don’t only move water. They move soil, and the constituents of soil: sand, gravel, cobbles and boulders, collectively called “sediment.” They move whole trees, and logs, which we call “large wood.” As these things move, they interact to form the framework of the river channel. This concept has been called the River Triangle.
The River Triangle. Water, Vegetation and Soil/Sediment interact to create the river channel.
This framework includes the channel boundaries: the river bed, and banks. The banks are held together by the roots of trees and shrubs that are nourished by the water itself, and by nutrients carried by the water. Sand and silt carried overbank onto the floodplain by river water creates and renews the floodplain soil that harbors the trees and shrubs. Carcasses of fish that migrate up the river from the ocean bring a rich supply of ocean-derived nutrients that feed animals and fertilize the trees, and that nurture the insects that become food for the offspring of those fish. The river then carries these offspring downstream, speeding their journey to the ocean feeding grounds when they are ready.
Many of the trees and shrubs that grow on the floodplain or on the river’s edge, and which hold those banks together with their framework of roots, sprouted from seeds carried in the current or airborne seeds that land on bare sand deposits created, and watered, by the recent flood. Banks with bare soil or only grass cover are vulnerable to rapid erosion by the water current. Tree branches and shrubs and exposed tree roots create a barrier that slows the current and slows or stops the erosion. The soil grains are bound together by a meshwork of roots and tiny rootlets. This is what is meant by the “framework” of the soil.
Whole trees or logs, individual or in clusters (logjams), form the “framework” of the watery part of the river channel. The water current loses speed and energy as it is forced around logs, creating pockets of low-speed water that become refuges for fish and insects seeking rest and seeking cover under the logs or attached branches. Slow water can’t carry gravel and sand, so these materials drop to the bottom, forming gravel bars and sand bars. Where water is forced over a log it plunges into the bed downstream, digging out a deep spot, a pool, a place often obscured from the view of predators by surface turbulence. The gravel removed from this pool or passing through it from upstream forms a bar at the downstream end of the pool, called the “pool tail-out zone.” This transitional area, from the deep pool to the shallow riffle downstream, is a favorite spot for fish to deposit their eggs. The tail-out zone often has loose gravel of just the right size. Water percolates into the streambed here, carrying oxygen to nurture those eggs.
Logjams play an important role in the way the river channel shifts course from time to time, a process known as channel migration. A logjam may block the main channel, forcing the water going around it onto the floodplain, where it can erode a new channel. The (old) main channel can then become partially filled with gravel and sand deposits, branches and leaves, and eventually, vegetation, becoming a side channel that only carries a small portion of the flow, and may not flow except during exceedingly high water. Side channels are important refuges for fish during high flow. Meanwhile, the new main channel, with its freshly created streambed and banks, may evolve rapidly, changing its configuration of gravel and sand bars, causing trees to fall in, creating newly-formed logjams and freshly-exposed roots as it stabilizes. This newly-formed fish habitat is often highly productive, in the same way that newly exposed soil is a productive place for new plants to grow, free from competition of established vegetation.
The concept of the River Triangle is not new. But in a recent research article, two river scientists, Janine Castro and Colin Thorne, combined the River Triangle concept with river channel classification and the forces at work sculpting the river channel to create a new, broader way of organizing our knowledge about “how rivers work.” They called this new concept the Stream Evolution Triangle.
The Stream Evolution Triangle. Each corner represents a building block of the river channel: Biology (vegetation, including trees and logs); Geology (the rock and soil making up the bed and banks), and; Hydrology (moving water). Biology dominates where energy of moving water (stream power) is low, and vegetation (especially trees) is strong. Geology dominates where erosion resistance is high (e.g. bedrock, boulders) and vegetation is sparse. Hydrology dominates where there is lots of water moving rapidly (high energy), and erosion resistance is low (e.g. sandy soil, poor root structure, no large wood). Drawings are aerial views of different types of river channels, placed according to the relative importance of the three cornerstones in forming them. Arrows show flowing water; dark patches are sand or gravel bars, or vegetated islands if heavier stippling is present; dashed areas are boulder or cobble steps that span steep channels. From Castro and Thorne, 2019.
In a nutshell, the Stream Evolution Triangle is a way to show where a given river channel lies in the balance between the three dominant components: Biology (vegetation, logs); Geology (bedrock, rocks, soil), and; Hydrology (force of flowing water). A stream channel is placed close to one of the corners if that component dominates its formation. Channels formed with the three components well balanced are placed near the center of the diagram.
Looking at each of the three building blocks, individual factors making up that component can be arranged in order from having a strong influence (near the component’s vertex) to being relatively weak (near the opposite side). For example, in a previous topic, we introduced the concept of an “alluvial” versus a “non-alluvial” channel, that is, a channel where the banks and bed are made of sediments (gravel, sand) easily movable by flowing water, versus one that has a bed or banks made of bigger, heavier, erosion-resistant rocks (or bedrock). The Stream Evolution Triangle arranges these channels from the most resistant to the force of water, near the “Geology” vertex, to the most easily eroded and moved, near the opposite side.
Expansion of the “Geology” cornerstone dealing with material making up the bed and banks. From Castro and Thorne, 2019.
Logs (including logjams) and whole trees make up an aspect of the “Biology component. This aspect, too, can be arranged in a sequence from dominating the river channel form and behavior to only weakly influencing it, as show below:
Relative influence of the large-wood “Biology” cornerstone, with situation of greatest control near the (top) apex, and least influence near the opposite side (bottom). From Castro and Thorne, 2019.
Finally, an example of factors that are part of the “Hydrology” component is the fraction of the watershed covered by impervious surfaces, like roofs, parking lots and roads. Greater amounts of impervious surface is a feature of urbanization. This factor causes rainfall to quickly enter the stream channel, rather than percolate into the ground where it would move more slowly. More water and faster water in the channel means greater hydraulic energy to erode the streambed and banks. Where this component dominates what happens, moving the streambed gravel more frequently and overcoming the ability of roots to hold the soil together, the channel erodes, forming a deep gully. This is known as "channel incision,” and is a common feature of urban streams.
Impervious, or impermeable, surfaces create a condition where the energy of moving water, the “Hydrology,” dominates the stream channel. This is a feature of urban watersheds. By contrast, in a natural forested watershed, rainfall is slowed and detained as it falls through vegetation, and a much greater proportion percolates into the ground, moving much more slowly towards the stream channel. The result is to spread the runoff over a longer time, with a smaller, less extreme peak flow, and less erosive energy.
In summary, this “conceptual model,” the Stream Evolution Triangle, helps us in two ways. It allows a synthesis of the many factors affecting how rivers behave and what they look like into a single, logical image. The various ways of classifying river channels, both those that are based on appearance and those based on physical processes (such as the distinction between alluvial and non-alluvial) can be merged with the relative importance of the factors influencing the channel shape, providing insight on why the river looks the way it does.
Finally, the Stream Evolution Triangle reminds us that rivers are much more than just moving water. This fact alone represents perhaps the most revolutionary improvement in our ability to understand, respect and manage our rivers.
References:
Castro Janine M. and Colin R. Thorne, 2019. The stream evolution triangle: Integrating geology, hydrology, and biology. River Research and Applications, 2019;35:315–326. https://doi.org/10.1002/rra.3421