California Rivers and Streams: The Conflict Between Fluvial Process and Land Use / Edition 1 available in Paperback
California Rivers and Streams provides a clear and informative overview of the physical and biological processes that shape California's rivers and watersheds. Jeffrey Mount introduces relevant basic principles of hydrology and geomorphology and applies them to an understanding of the differences in character of the state's many rivers. He then builds on this foundation by evaluating the impact on waterways of different land use practiceslogging, mining, agriculture, flood control, urbanization, and water supply development.Water may be one of California's most valuable resources, but it is far from being one we control. In spite of channels, levees, lines and dams, the state's rivers still frequently flood, with devastating results. Almost all the rivers in California are dammed or diverted; with the booming population, there will be pressure for more intervention.Mount argues that Californians know little about how their rivers work and, more importantly, how and why land-use practices impact rivers. The forceful reconfiguration and redistribution of the rivers has already brought the state to a critical crossroads. California Rivers and Streams forces us to reevaluate our use of the state's rivers and offers a foundation for participating in the heated debates about their future.
|Publisher:||University of California Press|
|Edition description:||First Edition|
|Product dimensions:||6.00(w) x 9.00(h) x 0.90(d)|
About the Author
Jeffrey F. Mount is Professor of Geology at the University of California, Davis. Janice C. Fong is Principal Illustrator in the Department of Geology at the University of California, Davis.
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California Rivers and StreamsThe Conflict Between Fluvial Process and Land Use
By Jeffrey F. Mount
The University of California PressCopyright © 1995 Regents of the University of California
All right reserved.
Chapter OneThe Shape of a River
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Most of the business of a river is conducted through its channel. The day-to-day task of handling discharge, the year-to-year task of eroding, transporting, and depositing sediment, and the long-term adjustments toward some equilibrium are all dependent on processes that occur within or immediately adjacent to a river's main channel. The morphology and behavior of channels has long been considered a sensitive indicator of the "state" of any river as well as a record of processes acting within a watershed. For much of this century geomorphologists have been measuring and analyzing river channels in order to tie them in some predictable way to aspects of the hydrology and geology of their watersheds. For every study conducted, it seems there is a new set of equations (with fudge factors) that quantify these relationships. The scatter in the data is immense, the usefulness of the results suspect. What has emerged from these studies is the recognition that there are broadly applicable principles that govern the response of river channels to change within, or differences between, watersheds.Although it is difficult to predict the precise extent of the change, the nature of change can be estimated. The qualitative associations between river morphology and hydrology are explored in this chapter, with an emphasis on the response of rivers to changes in watershed conditions.
CHANNEL CROSS SECTIONS
A river balances and minimizes its energy expenditures through adjustment of its channel cross section. Along the entire length of a river the shape and size of an infinite number of cross sections are in constant variation, adapting to the discharge and sediment load that is delivered to it by the channel reach that lies immediately upstream. In aggregate these adjustments produce the distinctive channel patterns that record the establishment of dynamic equilibrium within the overall river system.
Bankfull Discharge and Channel Geometry
Rivers construct channel cross sections that are best adapted for the wide range of discharges delivered by their watersheds. It is clear from the all-too-common flooding of some regions in California that rivers do not form channels capable of containing the entire range of flows. This is somewhat counterintuitive, since most of the work that a river does takes place during infrequent runoff events. It seems logical to assume that these highly competent, rare large flows should scour large channels that are capable of accommodating all of the discharge without creating any flooding. Conversely, most of the time it appears that channels are, in fact, overdesigned for their discharge. During the major part of any given year, the flows that move through rivers do not come close to taxing even half the channel capacity. These common flows have such low overall stream power that they are incapable of eroding and transporting much sediment. Thus they exert little influence on the configuration of the channel.
A number of workers have noted that although unusually large discharge events are capable of greatly affecting river channels and river geomorphology, their occurrence is so rare that, when viewed in the long term, their effects are usually masked by intermediate, more frequent discharges. The ability of intermediate flows to erode, transport, and deposit sediment allows them to eventually undo the effects of the larger events and to control the equilibrium configuration of the channels. The intermediate discharge that appears to exert the greatest influence on the shape and size of channel cross sections and thus on the overall geomorphology of the river is generally known as bankfull stage or bankfull discharge. For most rivers, bankfull stage occurs when discharge fills the entire channel cross section without significant inundation of the adjacent floodplain.
Bankfull stage or bankfull discharge, [Q.sub.b], usually occurs with a frequency of 1.5 to 2 years for natural, undammed rivers. This does not mean that these flows will occur like clockwork every two years. Rather, over very long periods, bankfull discharge will occur on average every 1.5 to 2 years (see discussion of flood frequency, chap. 14). Apparently, bankfull discharges meet two key criteria for shaping channel cross section: (1) the flows contain sufficient stream power to erode bank materials and to transport and deposit large volumes of sediment; and (2) they occur often enough that their effects are not muted by the weaker, but higher-frequency, smaller-discharge events.
The interaction between bankfull discharge and its channel produces a wide range of channel cross sections. The causes of these variations are numerous but are usually tied directly to interaction between the flow and the bank materials. Within any given channel reach, the cross-sectional profile of the river varies from symmetric to asymmetric (fig. 4.1). This variation is due primarily to the tendency of a river to develop meanders rather than a perfectly straight channel (discussed below). Within meander bends that are tightly curved, the cross-section profile becomes strongly asymmetric. In the relatively straight stretches between meander bends, the profiles are more symmetric. Disruptions in the overall profile shape are typically associated with obstructions or the development of channel bars.
The shape of a channel controls the structure of the flow that travels through it (and vice versa). Bed shear stress, the necessary ingredient for entrainment of sediment, is proportional to the velocity gradient (change in velocity with distance from the bed). In symmetric channels, the highest flow velocities and highest velocity gradients are located near the center of the channel, with the lowest gradients occurring near the margins. In contrast, in asymmetric channels, the velocities and gradients are always located adjacent to the steep-walled cut banks. During bankfull stage, the differences in distribution of bed shear stresses within symmetric and asymmetric channels control the style and magnitude of channel cross-section modification. The concentration of bed shear stress along the cut bank margin of asymmetric channels will cause them to erode the channel wall and expand laterally, whereas the concentration of bed shear stress in the center of symmetric channels will cause them to incise or deepen. To maintain continuity of flow during bankfull discharge (Q = vA), increases in channel cross section associated with erosion must be balanced by either a decrease in the velocity of the flow or by compensating deposition and reduction in cross-sectional area elsewhere in the channel. In symmetric channels, this deposition usually takes place in channel bars or along the margins. In asymmetric channels, deposition is usually restricted to the low-velocity margin of the channel opposite the most intense erosion. The balance between erosion on one side of a channel and deposition on the opposite side is the driving force behind lateral migration of channels (see discussion below).
Where channel reaches inundate their floodplains with a frequency greater than the 1.5 to 2 years typical of bankfull stage, depositional processes, rather than erosional processes, can act to expand the channel capacity. Discharge that is fully confined to a channel maintains high competence. When discharge exceeds channel capacity, there is a dramatic increase in cross-sectional area associated with expansion onto the floodplain (fig. 4.2). The velocity and depth of water flowing outside of the channel declines rapidly with distance away from the channel. The decline in depth and velocity, which produces sharp drops in [R.sub.e] and [F.sub.r] values, produces a rapid loss in stream power and competence. This decline acts to hydraulically segregate and deposit material that was formerly in suspension within the channel. The coarsest sediment (usually fine sand and silt) undergoes rapid deposition immediately adjacent to the channel, while the finest sediment is deposited away from the channel out on the floodplain. This is why Californians are always scraping mud, rather than sand, out of their homes after floods. Multiple flooding constructs a berm or levee adjacent to the active channel. As these levees increase in height the channel capacity increases, allowing the channel to contain larger and larger discharges.
Since a river adjusts to handle its load and discharge, it is logical to assume that the geometry and size of a channel cross section are controlled solely by these factors. However, another important control on channel morphology is the nature of bed and bank materials in which a river establishes itself. Where rivers traverse valleys filled with alluvium, the cohesiveness of the sediment and its resistance to erosion will greatly influence the shape of a channel and, ultimately, the behavior of flow within it. Where bank materials are soft and easily erodible, scour of the banks causes channels to expand laterally, forming cross sections that are wide and shallow. Where channel margins are resistant to erosion, there is a tendency for cross sections to become narrower, with steeper banks. The resistance of alluvial bank materials is dependent on a variety of factors. Finer-grained materials, like clays and muds, tend to be more cohesive and resistant to erosion. In addition, the degree of cementation and consolidation of sediments will dictate their resistance. However, one of the most important factors controlling cross-section geometry in alluvial rivers is the presence and type of bank-stabilizing vegetation. The role of riparian vegetation in shaping channel cross sections is usually underappreciated. The mesh-work of roots from trees and the diverse riparian vegetation that are directly dependent on river water can, in many cases, be as effective in stabilizing riverbanks as the scenic concrete and riprap liners that are the preferred fare of the U.S. Army Corps of Engineers (chap. 15).
As channels migrate laterally within alluvial valleys, they erode their own floodplain as well as channel deposits left behind by previous lateral migrations. In this way, they usually erode material that is compositionally similar to the load being supplied by the watershed. At the same time, the channel and overbank deposits that are accumulating at present in a river will be the bank materials of the near future as the channel migrates back and forth. For this reason, rivers with high suspended load/bedload ratios tend to have erosionally resistant banks composed of silt and clay. This resistance typically leads to the development of steep-sided, narrow channels that are, in an illustration of the complex circularity of feedback within these systems, ideal for transporting high suspended sediment loads. Rivers that are dominated by bedload tend to have less-resistant banks composed of sand and, to a lesser extent, gravels. These channels are more likely to be broad and shallow as well as highly unstable. The increase in wetted perimeter of these channels makes them ideal for transporting coarse bedload.
In upland regions where rivers are actively cutting into bedrock, the bank materials are an independent influence on channel geometry. The erosional resistance of the bedrock and the regional rates of uplift will typically dictate the channel geometry. Where the channel is fully contained within a resistant bedrock unit, the shape will be narrow and deep, whereas erodible bedrock will typically produce broad, shallow channels. Where incising channels encounter an erosionally resistant layer of bedrock, they will initially expand laterally faster than they incise. When they are fully contained within the underlying resistant bedrock, they will reestablish a narrow, deep channel. In areas where uplift rates are high, steep-walled narrow channel cross sections will typically reflect rapid rates of incision, regardless of bedrock type.
Like a two-year-old child, a river cannot hold still. The innumerable and incessant adjustments that occur in a channel cross section translate to constant change in channel pattern and character. Since most unregulated rivers achieve dynamic equilibrium, this change can occur at various scales, ranging from incremental lateral migration to dramatic channel abandonment and switching. The reasons behind this incessant motion are among the more puzzling aspects of rivers and have given rise to numerous innovative and clever hypotheses and some enjoyable philosophical arguments. Regardless of the causes of this mobility (explored below), it is fundamental to all rivers and is the root of most morphologic classification schemes.
River and stream channel patterns can be grouped into two general classes: single channel and multichannel. The flow in single channel rivers is restricted to a discrete, sinuous channel. The larger rivers of California, like the San Joaquin, Sacramento (fig. 4.3), and Klamath, all occupy one relatively stable main channel surrounded by an extensive floodplain. Multichannel rivers, like the Santa Ana, Santa Clara, and Santa Maria and many of the small rivers and creeks that emerge from dry, steep mountain ranges (fig. 4.4), consist of numerous, unstable channels that bifurcate and join across a relatively broad wash. The differences between single and multichannel rivers reflect contrasting watershed conditions.
Single Channel Rivers
When examining single channel rivers in map view (fig. 4.5), the greatest variation appears to be in the way they snake across the landscape. This snakelike property is termed channel sinuosity. The sinuosity of a river is variably defined but is generally a reflection of the channel length required to cover a given point-to-point or straight-line distance. As shown in figure 4.6, the irregular course of a river usually occupies a portion of a valley, termed the meander belt. A line drawn down the center of this meander belt is referred to as the meander belt axis. In large valleys like the Sacramento, the meander belt axis does not always parallel the valley walls and thus tends to be longer than the valley itself. There are two possible axial measurements within the river channel: the thalweg, which is the deepest portion of the channel, and the channel axis, which is equidistant from the channel walls. Since the channel cross section of most meander bends is asymmetrical, the thalweg rarely coincides with the channel axis. Based on these features a sinuosity index (SI) can be defined for channels where
SI = thalweg length/valley length (4.1)
SI = length of channel axis/length of meander belt axis. (4.2
Excerpted from California Rivers and Streams by Jeffrey F. Mount Copyright © 1995 by Regents of the University of California. Excerpted by permission.
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Table of Contents
Preface Acknowledgments Part I. How Rivers Work 1. Introduction to the Rivers of California: The First 4 Billion Years 2. Water in Motion 3. A River at Work: Sediment Entrainment, Transport, and Deposition 4. The Shape of a River 5. Origins of River Discharge 6. Sediment Supply 7. River Network and Profile 8. Climate and the Rivers of California 9. Tectonics and Geology of California's Rivers Part II. Learning the Lessons: Land Use and the Rivers of California 10. Rivers of California: The Last 200 Years 11. Mining and the Rivers of California 12. Logging California's Watersheds 13. Food Production and the Rivers of California 14. A Primer on Flood Frequency: How Much and How Often? 15. The Urbanization of California's Rivers 16. The Damming of California's Rivers 17. The Future: Changing Climate, Changing Rivers