Mountain Rivers Revisited

576 Pages·2010·5.481 MB·English

by  Ellen Wohl(auth.)

Water Resources Monograph 19
MOUNTAIN RIVERS
REVISITED
Ellen Wohl
Washington, DC
Published under the aegis of the AGU Books Board
Kenneth R. Minschwaner, Chair; Gray E. Bebout, Joseph E. Borovsky, Kenneth H. Brink,
Ralf R. Haese, Robert B. Jackson, W. Berry Lyons, Thomas Nicholson, Andrew Nyblade,
Nancy N. Rabalais, A. Surjalal Sharma, and Darrell Strobel, members.
Library of Congress Cataloging-in-Publication Data
Wohl, Ellen E., 1962-
Mountain rivers revisited / Ellen Wohl.
p. cm. — (Water resources monograph ; 19)
Rev. ed. of: Mountain rivers, c2000.
Includes bibliographical references and index.
ISBN 978-0-87590-323-1
1. Rivers. 2. Mountains. I. Wohl, Ellen E., 1962- Mountain rivers. II.
Title. III. Series: Water resources monograph ; 19.
GB1203.2W64 2010
551.48'309143—dc22
2010044835
ISBN 978-0-87590-323-1
ISSN 0170-9600
Copyright 2010 by the American Geophysical Union
2000 Florida Avenue, N.W.
Washington, DC 20009
Cover photographs by Ellen Wohl. (front) Tributary of Tonahutu Creek, Rocky Mountain
National Park, Colorado. (back) Rio Sardinal in Braulio Carillo National Park, Costa Rica.
Figures, tables, and short excerpts may be reprinted in scientific books and journals if the
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Printed in the United States of America.
CONTENTS
Preface ......................................................................................................................................vii
1. Introduction ............................................................................................................................1
1.1. Characteristics of Mountain Rivers ...............................................................................2
1.2. Advances Since the First Edition ...................................................................................3
1.3. Purpose and Organization of This Volume ....................................................................4
1.4. A Mountain River Described and Enumerated ..............................................................5
1.4.1. North St. Vrain Creek, Colorado, USA ...............................................................8
2. Mountain Drainage Basins ...................................................................................................13
2.1. Mountain Rivers and Tectonics ...................................................................................14
2.2. Hillslopes .....................................................................................................................20
2.2.1. Controls on Slope Morphology .........................................................................21
2.2.2. Steady-State Hillslopes ......................................................................................23
2.2.3. Bedrock Weathering and Soils...........................................................................24
2.2.4. Mass Movements ...............................................................................................25
2.2.4.1. Landslides .............................................................................................26
2.2.4.2. Debris flows..........................................................................................28
2.2.5. Diffusive Sediment Transport on Hillslopes .....................................................35
2.2.5.1. Creep ....................................................................................................35
2.2.5.2. Rainsplash and overland flow ..............................................................36
2.2.5.3. Modeling diffusive transport ................................................................38
2.2.6. Modeling Slope Morphology and Sediment Movement ...................................39
2.3. Climate and Hydrology ...............................................................................................40
2.3.1. Generation of Precipitation ................................................................................41
2.3.2. Glacier and Snow Melt ......................................................................................45
2.3.3. Down Slope Pathways of Water ........................................................................46
2.3.4. Modeling Hillslope Hydrology ..........................................................................55
2.3.5. Pressing Hydrologic Needs for Mountain Regions ...........................................57
2.4. Channel Initiation and Development ...........................................................................58
2.4.1. Channel Initiation ..............................................................................................58
2.4.2. Channel Network Development ........................................................................64
2.5. Basin Morphometry and Basin-Scale Patterns ............................................................65
2.5.1. Basin Morphometry and Hydrology ..................................................................67
2.5.2. Hydraulic Geometry ..........................................................................................70
2.5.3. Downstream Fining ...........................................................................................73
2.6. Valley Morphology ......................................................................................................75
2.7. Longitudinal Profiles and Bedrock Channel Incision ..................................................78
2.7.1. Processes of Bedrock Channel Erosion .............................................................81
2.7.2. Models of Bedrock Channel Incision ................................................................84
2.8. Knickpoints and Gorges ..............................................................................................87
2.9. Terraces ........................................................................................................................92
2.10. Alluvial Fans ..............................................................................................................96
2.11. Summary ..................................................................................................................102
3. Channel Processes ..............................................................................................................103
3.1. Hydrology ..................................................................................................................103
3.1.1. Discharge Estimation and Flow State ..............................................................103
3.1.2. Paleoflood Indicators .......................................................................................107
3.1.3. Modeling Stream Discharge .............................................................................111
3.1.4. Bankfull Discharge ..........................................................................................112
3.1.5. Floods ..............................................................................................................114
3.1.5.1. Outburst floods ...................................................................................116
3.1.5.2. Geomorphic effects of floods .............................................................119
3.2. The Hyporheic Zone ..................................................................................................123
3.3. River Chemistry .........................................................................................................128
3.3.1. Dissolved Nutrients .........................................................................................133
3.3.2. Organic Matter and Gases ...............................................................................136
3.3.3. Trace Metals and Pollutants .............................................................................138
3.4. Hydraulics ..................................................................................................................141
3.4.1. Resistance Coefficient .....................................................................................141
3.4.2. Resistance Partitioning ....................................................................................149
3.4.3. Velocity and Turbulence ..................................................................................154
3.4.4. Bed Shear Stress ..............................................................................................163
3.4.5. Stream Power ...................................................................................................165
3.5. Sediment Processes ....................................................................................................166
3.5.1. Bed Sediment Characterization .......................................................................166
3.5.1.1. Sampling and measurement ...............................................................166
3.5.1.2. Coarse surface layers ..........................................................................169
3.5.2. Particle Clusters ...............................................................................................171
3.5.3. Sediment Entrainment .....................................................................................172
3.5.4. Measurement of Bedload Transport ................................................................179
3.5.5. Mechanics of Bedload Transport .....................................................................182
3.5.6. Downstream Bedload Transport Patterns, Rates, and Frequency ....................185
3.5.7. Bedload Transport Equations ..........................................................................191
3.5.8. Bedload Yield and Sediment Budgets .............................................................197
3.5.9. Processes of Deposition ...................................................................................205
3.5.10. Suspended Sediment ......................................................................................207
3.6. Bank Stability ............................................................................................................213
3.7. Instream Wood ...........................................................................................................215
3.8. Channel Stability and Downstream Trends ...............................................................223
3.9. Summary ....................................................................................................................226
4. Channel Morphology .........................................................................................................229
4.1. Spatial and Temporal Variability in Channel Morphology ........................................229
4.2. Channel Classification Systems .................................................................................234
4.3. Channel Morphologic Types ......................................................................................236
4.3.1. Step-Pool Channels .........................................................................................236
4.3.2. Plane-Bed Channels .........................................................................................242
4.3.3. Pool-Riffle Channels .......................................................................................243
4.4. Incised Alluvial Channels ..........................................................................................249
4.5. Braided Channels .......................................................................................................250
4.6. Anabranching Channels .............................................................................................255
4.7. Spatial Distribution of Morphologic Types and Network Heterogeneity ..................256
4.8. Summary ....................................................................................................................257
5. Mountain River Biota ........................................................................................................259
5.1. River Ecology ............................................................................................................259
5.1.1. Only Connect ...................................................................................................259
5.1.2. The Physical and Chemical Environment of a River Ecosystem ....................262
5.2. Aquatic Communities ................................................................................................264
5.3. Riparian Communities ...............................................................................................269
5.3.1. Basic Characteristics .......................................................................................269
5.3.2. Interactions Between Riparian Vegetation and Channel Processes .................272
5.4. Conceptual Models ....................................................................................................277
5.5. Biological Stream Classifications ..............................................................................283
5.6. Mountain River Ecosystems ......................................................................................284
5.7. Case Studies of Human Impacts to Mountain River Ecosystems .............................286
5.7.1. The Columbia River Basin ..............................................................................286
5.7.2. The Danube River Basin..................................................................................288
5.8. Summary ....................................................................................................................292
6. Mountain Rivers and Humans ...........................................................................................295
6.1. Types of Impact .........................................................................................................296
6.1.1. Indirect Impacts ...............................................................................................297
6.1.2. Direct Impacts .................................................................................................313
6.2. Contemporary Status of Mountain Rivers .................................................................327
6.3. Hazards ......................................................................................................................328
6.3.1. Debris Flows ....................................................................................................329
6.3.2. Landslides ........................................................................................................334
6.3.3. Floods ..............................................................................................................336
6.4. River Management ....................................................................................................339
6.4.1. Restoration and Rehabilitation ........................................................................339
6.4.2. Instream and Channel Maintenance Flows and the Natural Flow Regime .....345
6.5. Summary ....................................................................................................................346
7. Field Data, Experiments, Computers, Brains .....................................................................349
Glossary .................................................................................................................................353
References ..............................................................................................................................357
Index ......................................................................................................................................565
PREFACE
I wrote the first edition of this book, published in 2000, in response to a need
expressedbyoneofmyPh.D.studentsatthetime,DavidMerritt,whowalkedintomy
officeoneafternoonforasummaryreferenceonmountainrivers.WhenIrealizedthat
suchareferencedidnotexist,Isetouttocreateone.Theinclusionoftopicsreflected
myownbeliefthatriversneedtobeexaminednotsolelyasphysicalsystemsbutalso
asriverecosystemswithchemicalandbiologicalcomponentsthatexistinthecontext
of pervasive and long duration human alteration of the environment. As research on
topics related tomountainrivers grew dramatically during thepastdecade,Idecided
that it was time to write a second edition, and I reorganized the book to reflect my
understandingofevolvingknowledge.
As with the first edition, this second edition is aimed primarily at an audience
already familiar with the basics of river process and form, although the reader with
littleknowledgeofrelatedtopics,suchasriverchemistry,hyporheiczones,orriparian
andaquaticecology,canalsogainaquickintroductoryoverviewofthosetopicsfrom
thisvolume.Advancedundergraduates,graduatestudents,andprofessionalscientists
and engineers who possess some general knowledge of river systems will find this
volume of use, both for its own sake and to help them build on their existing
knowledgeofmountainriverstobetterunderstandtheuniqueaspectsoftheserivers.
Youcanreadthebookstraightthrough,becauseeachsectionbuildsuponthesections
that precede it, or use the book as a spot reference to provide a synthesis of current
knowledgeonspecifictopics.
Thefirsteditionbenefitedsubstantiallyfromdiscussionswith,andcriticalreviews
by, Paul Carling (University of Southampton, England), Dan Cenderelli (U.S. Forest
Service),AlanCovich(UniversityofGeorgia),JanetCurran(U.S.GeologicalSurvey),
Jim Finley (Telesto Solutions, Inc.), David Merritt (U.S. Forest Service), and LeRoy
Poff (Colorado State University) and AGU reviews by John Costa (U.S. Geological
Survey),AvijitGupta(UniversityofLeeds,England),andMalcolmNewson(Univer-
sity of Newcastle upon Tyne, England). Much of that material is still in this edition,
and I thank each of these individuals for their efforts. The second edition has also
benefited from discussions with Gordon Grant (U.S. Forest Service), Bob Hilton
MountainRiversRevisited
WaterResourcesMonograph19
Copyright2010bytheAmericanGeophysicalUnion.
10.1029/2010WM001038
vii
viii Preface
(Durham University), Neils Hovius (Cambridge University), Mark Macklin (Univer-
sity of Aberystwyth), and Grant Meyer (University of New Mexico) and reviews by
Jim O’Connor (U.S. Geological Survey) and an anonymous reviewer, as well as the
enhanced energy and concentration provided by Whole Foods’ organic French roast
coffee.
As with the first edition, I would like to dedicate this second edition to my
graduatestudents.Theycontinuetochallenge,engage,andsurprisemeandtoprovide
muchofthepleasurethatcomesfromworkinginfluvialgeomorphology.
EllenWohl
ColoradoStateUniversity
1
1. INTRODUCTION
Riversshapemanyoftheworld’slandscapes.Intheprocessoftransportingwater,
sediment, and dissolved chemicals from uplands, rivers redistribute mass across the
Earth’ssurface.Riverssetthepaceatwhichweatheringanderosionlowerlandscapes,
andcontrolthegradientofadjacenthillslopes.Fundamentally,riversorganizeterres-
trial landscapes into drainage basins. As the rivers incise or aggrade in response to
changes in baselevel, they create valleys that influence local climate; provide travel
corridors for animals and humans; and support aquatic and riparian ecosystems that
containsomeoftheEarth’shighestlevelsofbiodiversity.
Scientistshavesystematicallystudiedriversformorethantwocenturies.Among
the questions asked have been: How do rivers interact with other variables such as
climate, lithology and tectonics that influence landscapes? What governs the spatial
distribution of river channels? What factors control the yield of water and sediment
fromhillslopestorivers?Howdointeractions between water andsediment influence
channelgeometrythroughtimeandspace?
Thisvolumesummarizescontemporaryunderstandingoftheseandotheraspectsof
rivers,inthecontextofriversdrainingmountainousenvironments.Althoughthestudy
ofriversiswell-established,investigatorstypicallyfocusedonthelowlandriversalong
whichmostpeopleliveuntilthefinaldecadesofthe20thcentury.Asubstantialincrease
intheamountofresearchdirectedtowardmountainriversduringthefirstdecadeofthe
21stcenturysupports theneed forthis secondedition of MountainRivers, which was
originallypublishedin2000.Increasedattentiontoriversinmountainousregionsresults
fromseveraltrendswithinscienceandthegreatersociety.Amongtheseisthefocuson
numerically simulating landscape evolution over long timespans, which requires that
modelersquantitativelyparameterizeratesofriverincisionandratesofcrustalupliftin
mountainousregions.Anotherfactordrivingincreasedinvestigationofmountainrivers
isattemptstomaintainorrestoreriversasecologicalrefugesandascriticalcomponents
ofwatersupplyinmountainousregions,whichtendtobelessdenselypopulatedthan
adjacent lowlands. Finally, mountain rivers with steep, coarse-grained, poorly-sorted
beds,andlimitedsedimentsupplyaretypicallypoorlydescribedbyempiricalequations
for hydraulics and sediment dynamics developed for rivers with lower gradients,
makingthestudyofmountainriversanintellectualandmanagementchallenge.
MountainRiversRevisited
WaterResourcesMonograph19
Copyright2010bytheAmericanGeophysicalUnion.
10.1029/2010WM001039
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2 Mountain Rivers Revisited
1.1. Characteristics of Mountain Rivers
In this volume I define a mountain river as being located within a mountainous
regionandamountainousregionashavingameanelevationabovesealevel≥1000m
[Viviroli et al., 2003]. Each of the continentsincludes at least onemajor mountainous
region (Figure 1.1). (Selected images appear in print. All images are available on the
CD-ROM that accompanies the book.) Mountains cover 52% of Asia, 36% of North
America,25%ofEurope,22%ofSouthAmerica,17%ofAustralia,and3%ofAfrica,
aswellassubstantialareasofislandsincludingJapan,NewGuinea,andNewZealand
[Bridges,1990].Mountainriversarethuswidespread.Becauseofthesteeptopography
ofmountainous regions,mountainriverstypically have a gradient≥ 0.002m/m along
the majority of the channel length [Jarrett, 1992], although substantial longitudinal
variability of channel geometry is common in mountainous regions as a result of
longitudinalvariationsinrockresistance,glacialhistory,andhillslopestability.Lower
gradientreachesofchanneltypicallyoccurupstreamofglacialendmoraines,massive
landslide deposits, or beaver dams, for example, but these reaches create relatively
shortinterruptionsbetweenthesteeperchannelsegmentsup-anddownstream.
As with lowland rivers, mountain rivers exhibit great variability in hydrologic
regime; channel planform; channel gradient, grain size, and bedforms; sediment dy-
namics; and aquatic and riparian biota, both within individual mountain ranges and
among diverse mountainous regions. Mountain rivers, as defined here, include first-
order channels less than a meter wide fed by snowmelt draining an alpine meadow
(Figure 1.2); wider rivers cutting steep-walled valleys that dense tropical rain forest
vegetationcannotstabilizeagainstperiodiclandslides(Figure1.3);ephemeralchannels
incised into bedrock in arid mountains (Figure 1.4); boreal rivers with cutbanks
exposing permafrost (Figure 1.5); and big, powerful rivers like the Indus that carry
thousands of kilograms of sediment down to the adjacent lowlands each year (Figure
1.6).Perhapstheonlyconsistentcharacteristicofmountainriversistheirtypicallysteep
gradients,althoughsteepgradientstendtocorrelatewithothercharacteristics,including
& erosionallyresistantandhydraulicallyroughchannelboundariesassociatedwith
bedrock and coarse clasts;
& highly turbulent flow with numerous longitudinal transitions between sub- and
supercritical flow;
& limited supply of sediment of fine gravel and smaller size;
& bedloadmovementthatishighlyvariableinspaceandtime,withhigherthresholds
for initiation of motion than many lowland rivers;
& strongly seasonal discharge regime associated with glacial melt, snowmelt, or sea-
sonal rainfall;
& substantialspatial variability indischargeasaresultofspatial variability inpre-
cipitation and runoff caused by differences in elevation, basin orientation, and
land cover;
& large longitudinal variations in channel geometry associated with variations in
tectonics, lithology, glacial history, and sediment supply;
Wohl 3
& insomecases,lessertemporalvariationsinchannelgeometrythanlowlandrivers
because only infrequent floods or debris flows can exceed boundary resistance
sufficiently to cause substantial channel change;
& relatively narrow valley bottoms with limited development of floodplains and
lateral movements by rivers;
& in the absence of wide valley bottoms and the associated buffering of stream
channelsfromhillslopeprocesses,mountainrivershavethepotential fororders-
of-magnitude increase in water and sediment yield over a period of a few years
following watershed-scale disturbances such as wildfire or timber harvest; and
& longitudinal zonation of aquatic and riparian biota influenced by river charac-
teristics and by elevation as it relates to temperature and precipitation.
Mountainriverstendexhibithighdegreesofconnectivity.Landscapeconnectivity
[Brierley et al., 2006] is high because individual landforms such as hillslopes and
streamchannelsarecloselycoupledwithinadrainagebasin.Hydrologicalconnectivity
[Bracken and Croke, 2007] is high because water moves rapidly from one landform
to another and through the entire drainage basin relative to lowland watersheds with
extensivegroundwaterstorage.Sedimentconnectivity[Fryirsetal.,2007]ishighbecause
limitedstoragemeansthatsedimentmovesrelativelyrapidlyfromproductionsiteson
hillslopesthroughthedrainagebasin.Increasingresearchemphasisondifferentforms
ofconnectivityreflectsadesiretomovebeyondsmallspatialandshorttemporalscales
of investigation in order to focus on emergent properties that evolve from the self-
organizationinherentinrivercatchments[Phillips,2003;McDonnelletal.,2007;Reid
etal.,2007b;AliandRoy,2009].
1.2. Advances Since the First Edition
Writing the second edition proved to be a much more time-consuming and
expansiveprocessthanIhadinitiallyexpected,butthisreflectsthedynamicnatureof
contemporarystudiesofgeomorphologyandmountainrivers.Manyareasofinvesti-
gationhaveexpandeddramaticallysincethelate1990sandthevolumeofassociated
literaturehasgrowncorrespondingly.Dramaticincreasesintheamountofresearchin
topics such as: the interactions of tectonics, topography, and climate [Willett et al.,
2006]; hillslope hydrology and modeling [Franks et al., 2005]; debris flows and
associatedhazards[JakobandHungr,2005];soildevelopmentandhillslopeprocesses
[Heimsath et al., 2001; Roering, 2004]; hydraulics of steep channels [Ferguson,
2007]; braided river process and form [Sambrook Smith et al., 2006]; diverse types
of numerical models and associated predictions [Wilcock and Iverson, 2003; Tucker
and Hancock, 2010]; geochronology [Madsen and Murray, 2009]; and instrumenta-
tion[Jonesetal.,2007]havemadeitchallengingtokeeptrackofandsynthesizethe
literature. As a result, I have introduced several new sections to the second edition,
substantially expanded other areas, and altered the organization of the volume to
reflect changing research emphases within the community.