By Paul Bakke, Geomorphologist, Washington Fish and Wildlife Office
Although humans have been altering river channels for many years, the science of rivers, that is the study of how rivers form their channels and how these channels evolve over time, is quite new. The influential American scientist, Luna Leopold, was one of the pioneers in this field. During the second half of the twentieth century, he directed a large number of field investigations and authored dozens of scientific articles about rivers. With the help of his colleagues, he gathered these data into a body of theory that established the field of fluvial geomorphology as a science, independent from engineering. That is, he helped discover natural laws which describe the form or shape of river channels and the physical processes which create the river’s form. Previously, these natural laws were incompletely developed and poorly known among the engineers charged with managing, and often modifying, our rivers.
The writings of Luna Leopold, like those of his perhaps more famous father, Aldo Leopold, are well known among those who study geomorphology and environmental science in the U. S. Unfortunately, they are less well known in the engineering community here, and are even more unfamiliar to most of the rest of the world, which means that river habitat continues to be damaged by practices that do not conform to local natural processes.
Luna Leopold started his career trained as a civil engineer. He then studied meteorology and geology, with an emphasis on hydrology, the science of water and its occurrence on the earth’s surface. His career spanned nearly 7 decades, including rising to the position of Chief Hydrologist with the U.S. Geological Survey and later becoming a professor at the University of California, Berkeley.
In 1997, at the age of 82 years, Leopold gave a speech entitled,“Let Rivers Teach Us.” The ambitious goal of his speech was, in a very few words, to recount his vast experience with the science of rivers and his struggles with engineers and government agencies that were too slow to adopt the new science of fluvial geomorphology in their project designs or policies. In short, he wanted to advocate for letting the natural form of rivers dictate how we manage them, and for collecting the field measurements needed to accurately describe the natural processes at work shaping the river channel. He wanted to emphasize the importance of ongoing field data collection and data availability, to an audience of non-scientists. Although this was clearly a daunting task Leopold’s goal of disseminating the science of how rivers function in nature remains an urgent one for the future of rivers here and abroad. It’s a task worth repeating, though perhaps at a more elementary level to reach a wider audience. This is the task I would like to begin in this, and subsequent, Word from the Wild blog posts.
Let’s start with the fact that there are two types of rivers. First, there is the river that forms its channel, its watercourse, by means of erosion of the earth’s surface. Rivers concentrate the flow of water from rain or from melting snow or ice and this flow is a force, a source of energy, which can excavate the soil or even carve its way into bedrock, forming the channel. In general, this type of river is encountered in steep terrain or in the mountains. A river formed in this way tends to be not much narrower than its valley bottom, filling the valley from side to side. Scientists call this type of river“non-alluvial,” and the process which dominates its form or shape, and its evolution, is erosion.
The second type of river, the “alluvial,” is encountered in places less steep, in the bottom of wider valleys, valleys that have flat bottomland (the floodplain) bordering the river. An alluvial river is distinguished by the materials which form its riverbed and riverbanks. These rivers flow in channels constructed from sediments which the river itself has deposited, sediments which were excavated from erosion occurring in channels and valleys further upstream. The distinguishing characteristic of an alluvial river is that it constructs its own channel with materials carried in its current of water. Both erosion and deposition, equally, are processes which dominate the form and evolution of alluvial rivers. And the flat bottomland bordering the river, its floodplain, consists of soils derived from sediments deposited by the river over thousands of years. These rivers are called “alluvial” because they are constructed from water-borne, or“alluvial,” sediments.
The Humptulips River (left) is a typical alluvial river, while Rush Creek (right) is an example of a non-alluvial river. Both are in Washington State.
I want to talk briefly here about some terms which are used frequently in river science. I’ve already mentioned fluvial geomorphology. Geomorphology is the science of how the surface of the earth attains its shape, that is, how landforms such as plains, valleys or hills are sculpted by processes such as erosion, landslides, or sediment deposition by moving water. “Fluvial” geomorphology pertains to the way rivers shape the earth. In geomorphology, there is an important distinction between “form” and “process.” The “form” of a river channel refers to its shape at one point in time. Form is like a snapshot of the river channel that shows all of its dimensions and characteristics, such as width, depth, and curvature. “Process,”by contrast, refers to something in motion, a force or an action which, over time, changes the form. Process is more like a video than a snapshot. For example, the erosion of the streambed is a process that creates a river channel with a particular form, having vertical banks and a rough streambed surface made from coarse rocks that are resistant to erosion.
The alluvial rivers, then, represent a balance, an equilibrium, between two opposing processes: erosion and deposition of sediment. As sediment is picked up and carried off of the streambed or stream bank, sediment from upstream moves in to take its place, replenishing the streambed and building new stream banks by piling up sediment as a gravel or sand bar. The form of the river channel adjusts itself, through erosion and deposition, until this equilibrium is established. If either process is altered the form of the river channel will change, going through a sequence of evolutionary stages that bring the two processes back into balance. The end result of this evolution might not resemble the original river channel, however.
For example, erosive forces can be increased by increasing the amount of water flowing in the channel, particularly the high flows that occur during storms. Forest fires, which remove water-absorbing vegetation and create water-repellant soil layers, can cause this to occur. Urbanization, which results in vast areas of roofs, roads and parking lots (impervious surfaces) that do not allow rain water to soak into the soil are another cause of increased water runoff. Increased erosive energy of this water erodes the streambed, forming a narrow, deep gully. The high stream banks of the gully are unstable, however, since the soil is not strong enough to resist gravity when exposed in a tall vertical cut. So the stream banks slide down and the flowing water carries the sediment away. In addition to there being more water, the water in its newly-formed gully is deeper, which gives it more hydraulic force. This is because water that would have spread over the floodplain is now concentrated in the gully, increasing erosive potential. As the stream banks fail the channel widens, which causes the flow to become shallower, until it is too shallow to further erode the streambed. As erosion slows down, sediment begins to accumulate along the sides of the channel, reducing the steepness of the stream banks and slowing their retreat. Eventually, sediment eroded from upstream begins to be deposited within the new channel bottom, creating a narrower channel and a new floodplain at a lower elevation than the original. This evolution once again brings erosion and deposition into balance, but creates a new landform, that is, a river channel and floodplain inset within a former floodplain at higher elevation.
Stages in the evolution of a river cross section in response to artificially straightening the channel (stage 2). Straightening results in a shorter, steeper channel. Because it is steeper, the erosive forces are greater, and the channel must go through stages of adjustment in order to bring erosion back into balance with deposition. A very similar sequence of adjustment happens when erosion is increased by enhanced water runoff, as from urbanization, forest fires, etc. The thick arrows show direction of change for the streambed and banks. The height of the top of bank, h, changes from typical, stable floodplain or “bankfull”height, to heights too large to remain stable, then returns to bankfull, as a new floodplain develops. The old floodplain is called a terrace, and is flooded infrequently, only by the largest floods. Adapted from Cramer, Michelle L. (managing editor), 2012, Stream Habitat Restoration Guidelines, WDFW, Olympia, WA.
In this example, we have interplay between form and process. When the process (erosion) changed, the form (channel shape) began to change. And eventually, a form was reached in which the processes of erosion and deposition were once again balanced. The interesting point about this equilibrium is its guarantee that the form of a river is no accident. In fact the various components of form are related to each other mathematically, and are related to the magnitude of deposition and erosion. By taking measurements of the dimensions such as width, depth, slope and curvature of numerous rivers in stable condition, it is possible to recognize when a river departs from a form that can remain stable. It also becomes evident that engineered modifications which depart from this stable form will be at odds with the natural processes at work, and will ultimately fail. These ideas, and Leopold’s contributions to river science, will be explored further in the next Word from the Wild blog post.
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Paul Bakke is a fluvial geomorphologist with the U. S. Fish and Wildlife Service in Lacey, Washington, working mostly in river restoration.
Luna Leopold’s original speech, Let Rivers Teach Us, can be found at:
http://stream.fs.fed.us/news/streamnt/pdf/SN_7-01.pdf
A biography and comprehensive list of his publications is available at:
http://eps.berkeley.edu/people/lunaleopold/
