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River Rules: the Nature of Streams

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River Rules: the Nature of Streams River Rules: The Nature of Streams "The more rivers are studied, the more wonderful their place in the system of nature is found to be. They wash along in every part of their course some share of the waste of the land on the way to the sea. Mountains may tower aloft . and the streams and rivers bear off their waste until they are worn away." -From Physical Geography by William Morris Davis, professor of physical geography, Harvard University. Published by Ginn & Company, The Athenaeum Press, 1899. By Martha S. Mitchell A hundred years ago, college students opened intricately lithographed geography textbooks and learned about rivers. A basic understanding of river behavior in different climate zones, geologic settings, and watershed zones came with the territory of higher education. But several things caused rivers to fall out of a common curriculum in the 20th century. Advances in science and technology associated with World Wars I and II drew rivers into the disciplines of engineering and hydrology, where they could be studied quantitatively. By the mid-1950s, the descriptive phase of the late-blooming discipline of geomorphology was drawing to a close because, worldwide, nearly all geomorphic forms and processes had been "discovered" and described. By the latter part of the 20th century, the field had moved into a quantitative mode that was beyond the grasp of intelligent lay people. The maturation of the Clean Water and the Endangered Species Acts-with their regulatory reach to stormwater, wetlands, natural resource management, instream habitats, and most recently nonpoint sources of water pollution- has fueled a revival of interest in rivers. The country is moving away from its midcentury mindset, wherein we reveled in our postwar technological ability to master the forces of nature. We've looked over our shoulders at a half century or more of harnessing rivers and have learned a few things. A great popular interest has sprung up in stream restoration. In some areas, stormwater utility groups are reinventing themselves as the watershed people and many state and federal agencies have come to regard streams as the ultimate expression of the health of ecosystems at the landscape scale. Managers face thorny questions when it comes to "fixing" riverbanks. Public safety, facilities, and infrastructure might need to be protected, but protecting a streambank from erosion in one location might create new problems both upstream and downstream. A stream may be changing because of a change in discharge characteristics, sediment supply, or other reasons, and it can be difficult to determine what channel shape and runoff events to expect under current and future conditions. No matter what the issue, decisions about river treatments are difficult and complex. They are made more so by increasing scrutiny of projects with respect to their impacts on fish passage and instream habitats and increasing common knowledge about the dynamics and functions of river systems. Well-intentioned decision-makers need theoretical and practical frameworks within which to consider whether riverbank restoration or protection projects might be successful. They need background to help them become more savvy consumers of professional services for river projects. And they need to know what kinds of projects are likely to be successful and when to walk away from potential disasters. A 1969 photorevision (in purple) of this 1949 topo clearly shows the dynamic changes of the alluvial Sacramento River. Do No Harm The first rule of stream restoration or rehabilitation is: Do no harm. Is the project even necessary? Not all bank erosion is bad, and not all "problems" need to be "fixed." An interdisciplinary analysis of alternatives, costs, and risks can assist with decision-making at the "go" or "no-go" stage. Don't Go It Alone No single discipline has the whole package of technical or analytical skills to go it alone where river design is concerned. By its very nature, this field is interdisciplinary. Depending on the cultural and physical setting of a site and the natural resource issues involved, project teams may consist of people with expertise ranging from invertebrate taxonomy to transportation engineering. The insights of people with professional depth in the biological, physical, and cultural attributes of the site and its contributing watershed need to inform the project at the outset. To Go Forward, First Take a Big Step Back Before considering what might be done to fix a problem at the site, it is imperative to understand the hydrology; the dynamics of sediment production, transport, and deposition in the system; and the morphology of the stream before present-day influences. Historic mapping, original survey notes, records of gravel mining, gauge data, land-use mapping for the watershed, old aerial photos, and interviews with long-time residents can all contribute essential pieces to the puzzle of the stream's original condition. Government offices at all levels can be gold mines of historic information. Local universities might have master's theses that shine light on the system, and musty documents in local government repositories can hold the only surviving records of former river projects and, incidentally, goals and beliefs about river systems. An undisturbed reference stream-one with a similar watershed size, geology, aspect, elevation range, and vegetation-could provide important clues. For many streams, a background search might disclose that runoff quantity and frequency have increased as a result of development or land-use activities in the watershed. Or perhaps a water-diversion or gravel-mining operation has upset the water/sediment balance in the stream. Maybe a few curves were taken out of the river a long time ago, and the channel responded to an increased gradient by downcutting. Perhaps vegetation or sediments have been removed from the channel to improve flood flow, increasing the stream's erosive energy. A headcut might be making its way upstream. Whatever the cause of streambank or channel change at the problem site, it is important to understand both the source of the change and the response of the channel. The Present Is the Key to the Past In 1785, a Scots doctor and farmer, James Hutton, published a paper explaining how streams can cut deep gorges. The principle of uniformitarianism grew from Hutton's observation and became codified nearly two centuries later as the first fundamental concept of geomorphology. The principle states that the same physical processes and laws that operate today operated throughout geologic time, although not necessarily always with the same intensity. Considering the late discovery of this phenomenon relative to the long history of Western civilization's engineering and scientific achievements, our knowledge of river behavior is still young. Notwithstanding, Hutton's principle is as good a place as any to begin to understand rivers. One way to internalize this principle is to stand on the bank of a river where there is a problem and understand the problem as the expression of several variables, of which one or more might be changing while the others are responding. Ah, yes, of course, we might observe. Given the unique circumstances of gradient, discharge, sediment size, and load, this river is behaving exactly as all rivers throughout the history of time would behave under the same conditions. In other words, river behavior is predictable. Modern fluvial morphologists explain that the following major variables work independently to define channel shape: stream discharge at bankfull stage, the amount and particle size of sediments, and the slope of the stream. When one factor changes, the others will adjust. Viewed from this perspective, we are more likely to look for the source of riverbank erosion when we are considering potential fixes for streambank erosion. This realization is revolutionizing the way we treat rivers. The Principle of Least Work Most people are fascinated by big runoff events. Big events remind us of the incredible energy of moving water. They refresh institutional resolves to rely on interdisciplinary decision-making about such actions as land-use planning and natural resource and range management-actions that can have far-reaching impacts on streams, streambanks, and floodplains. Big events make us humble. They reinforce our appreciation of the complex interface between nature and culture. But despite the dramatic results of big events, current wisdom in fluvial morphology is that channel shape is not controlled by catastrophic events but by bankfull flow. This is a cornerstone The top of the active channel bar indicates concept for those involved with streambank restoration, for it bankfull. changes long-held cultural expectations about the river stage to which streambank treatments should be designed. Bankfull flow is the dominant high flow conveyed by the channel- the one that occurs about once each year. Bankfull discharge moves the most sediment and water for the least amount of energy. If we consider an alluvial stream on a topo map, it is easy to see how this is so. We see a sinuous channel with length greater than the length of the valley that contains it. We can take a piece of string and lay it out on the meanders of the river, then measure the length of the string. If we measure valley length over the same horizontal distance, it will be much shorter. Therefore, the gradient of the valley is steeper than the gradient of the river. Bankfull can readily be identified as the top of the active channel bar and the But during bankfull flow, the wetted perimeter of the channel location trim line in the background. increases in depth and width, and as a consequence, the channel at bankfull flow is less sinuous and therefore steeper. The differences in the slope of the water surface in pools and riffles at low water balance out during bankfull flow.
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