FSA01 11/30/2004 02:17PM Page i

FLUVIAL SEDIMENTOLOGY VII FSA01 11/30/2004 02:17PM Page ii

Other publications of the International 25 Reefs and Carbonate Platforms in the Pacific Association of Sedimentologists and Indian Oceans Edited by G.F. Camoin and P.J. Davies SPECIAL PUBLICATIONS 1998, 336 pages, 170 illustrations

34 Clay Mineral Cements in Sandstones 24 Tidal Signatures in Modern and Ancient Edited by R.H. Worden and S. Morad 2003, 512 pages, 246 illustrations Edited by B.W. Flemming and A. Bartholomä 1995, 368 pages, 259 illustrations 33 Precambrian Sedimentary Environments A Modern Approach to Ancient Depositional 23 Carbonate Mud-mounds Systems Their Origin and Evolution Edited by W. Altermann and P.L. Corcoran Edited by C.L.V. Monty, D.W.J. Bosence, 2002, 464 pages, 194 illustrations P.H. Bridges and B.R. Pratt 1995, 543 pages, 330 illustrations 32 and Megaflood Processes and Deposits 19 Orbital Forcing and Cyclic Sequences Recent and Ancient Examples Edited by P.L. de Boer and D.G. Smith Edited by I.P. Martini, V.R. Baker and 1994, 571 pages, 320 illustrations G. Garzón 2002, 320 pages, 281 illustrations 16 Aeolian Sediments Ancient and Modern 31 Particulate Gravity Currents Edited by K. Pye and N. Lancaster Edited by W.D. McCaffrey, B.C. Kneller and 1993, 175 pages, 116 illustrations J. Peakall 2001, 320 pages, 222 illustrations 3 The Seaward Margin of Belize Barrier and Atoll Reefs 30 Volcaniclastic Sedimentation in Lacustrine Edited by N.P. James and R.N. Ginsburg Settings 1980, 203 pages, 110 illustrations Edited by J.D.L. White and N.R. Riggs 2001, 312 pages, 155 illustrations 1 Pelagic Sediments on Land and Under the Sea 29 Quartz Cementation in Sandstones Edited by K.J. Hsu and H.C. Jenkyns Edited by R.H. Worden and S. Morad 1975, 448 pages, 200 illustrations 2000, 352 pages, 231 illustrations REPRINT SERIES 28 Fluvial Sedimentology VI Edited by N.D. Smith and J. Rogers 4 Sandstone Diagenesis: Recent and Ancient 1999, 328 pages, 280 illustrations Edited by Stuart D. Burley and Richard H. Worden 27 Palaeoweathering, Palaeosurfaces and 2003, 648 pages, 223 illustrations Related Continental Deposits Edited by M. Thiry and R. Simon 3 Deep-water Turbidite Systems Coinçon Edited by D.A.V. Stow 1999, 408 pages, 238 illustrations 1992, 479 pages, 278 illustrations

26 Carbonate Cementation in Sandstones 2 Calcretes Edited by S. Morad Edited by V.P. Wright and M.E. Tucker 1998, 576 pages, 297 illustrations 1991, 360 pages, 190 illustrations FSA01 11/30/2004 02:17PM Page iii

SPECIAL PUBLICATION NUMBER 35 OF THE INTERNATIONAL ASSOCIATION OF SEDIMENTOLOGISTS

Fluvial Sedimentology VII

EDITED BY

Michael D. Blum, Susan B. Marriott and Suzanne F. Leclair

SERIES EDITOR Ian Jarvis School of Earth Sciences and Geography Centre for Earth and Environmental Science Research Kingston University Penrhyn Road Kingston-upon-Thames KT1 2EE UK FSA01 11/30/2004 02:17PM Page iv

© 2005 International Association of Sedimentologists and published for them by Blackwell Publishing Ltd

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Fluvial sedimentology VII / edited by Michael D. Blum, Susan B. Marriott, Suzanne F. Leclair. p. cm. a (Special publication number 35 of the International Association of Sedimentologists) Includes bibliographical references and index. ISBN 1-4051-2651-5 (pbk. : alk. paper) 1. SedimentologyaCongresses. 2. sedimentsaCongresses. I. Title: Fluvial sedimentology 7. II. Blum, Mike D. III. Marriott, Susan B. IV. Leclair, Suzanne F. V. Series: Special publication . . . of the International Association of Sedimentologists; no. 35. QE471.2.F57 2005 552.5adc22 2004003106

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Contents

vii Preface 135 Braided gravel-bed with a limited width: preliminary results of a hydraulic and forms model study C. Marti and G.R. Bezzola 3 Origin of anastomosis in the upper 145 The morphology and facies of sandy Columbia River, British Columbia, braided rivers: some considerations of Canada D. Abbado, R. Slingerland and scale invariance N.D. Smith G.H. Sambrook Smith, P.J. Ashworth, J.L. Best, J. Woodward and C.J. Simpson 17 Review of Amazonian depositional systems A.W. Archer 159 Application of laser diffraction grain-size analysis to reveal depositional processes in 41 Kinematics, topology and significance of tidally influenced systems dune-related macroturbulence: some P. Siiro, M.E. Räsänen, M.K. Gingras, C.R. observations from the laboratory and field Harris, G. Irion, S.G. Pemberton and A. Ranzi J. Best 181 Sedimentology and patterns of 61 Derivation of annual reach-scale the anabranching Baghmati River in the transfers in the River Coquet, Himalayan foreland basin, India Northumberland, UK R. Sinha, M.R. Gibling, V. Jain and I.C. Fuller, A.R.G. Large, G.L. Heritage, D.J. S.K. Tandon Milan and M.E. Charlton 197 Estimating bedload in sand-bed channels 75 Dune-phase fluvial transport and using bottom tracking from an acoustic model of gravelly sand Doppler profiler M.G. Kleinhans P. Villard, M. Church and R. Kostaschuk

99 Morphology and fluvio-aeolian interaction Experimental and numerical modelling of the tropical latitude, ephemeral braided-river dominated Koigab Fan, 213 The morphological and stratigraphical north-west Namibia effects of base-level change: a review of C.B.E. Krapf, I.G. Stanistreet and experimental studies H. Stollhofen F.G. Ethridge, D. Germanoski, S.A. Schumm and L.J. Wood 121 A qualitative analysis of the distribution of bed-surface elevation and the 243 A mass-balance framework for quantifying characteristics of associated deposits downstream changes in fluvial architecture for subaqueous dunes N. Strong, B. Sheets, T. Hickson and S.F. Leclair and A. Blom C. Paola FSA01 11/30/2004 02:17PM Page vi

vi Contents

Quaternary fluvial systems 425 Fluvial–estuarine transitions in fluvial- dominated successions: examples from the 257 The linkage between alluvial and coeval Lower Pennsylvanian of the Central nearshore marine successions: evidence Appalachian Basin from the Late Quaternary record of the Po S.F. Greb and R.L. Martino River Plain, Italy A. Amorosi and M.L. Colalongo 453 Palaeogeography and fluvial to estuarine architecture of the Dakota Formation 277 Depositional processes in latest Pleistocene (Cretaceous, Albian), eastern Nebraska, and Holocene ephemeral of the USA Main Ethiopian Rift (Ethiopia) R.M. Joeckel, G.A. Ludvigson, B.J. Witzke, M. Benvenuti, S. Carnicelli, G. Ferrari and E.P. Kvale, P.L. Phillips, R.L. Brenner, S.G. M. Sagri Thomas and L.M. Howard

295 Fluvio-deltaic floodbasin deposits 481 Improved understanding of fluvial recording differential subsidence within a architecture using three-dimensional coastal prism (central Rhine–Meuse delta, geological models: a case study of the The Netherlands) Westphalian A Silkstone Rock, Pennine K.M. Cohen, M.J.P. Gouw and J.P. Holten Basin, UK K.J. Keogh, J.H. Rippon, D. Hodgetts, 321 Geomorphology and internal architecture of J.A. Howell and S.S. Flint the ancestral Burdekin River across the Great Barrier Reef shelf, north-east 493 Changing alluvial style in response to Australia changing accommodation rate in a proximal C.R. Fielding, J.D. Trueman, G.R. Dickens foreland basin setting: Upper Cretaceous and M. Page Dunvegan Formation, north-east British Columbia, Canada 349 Quaternary alluvial stratigraphical M.P. Lumsdon-West and A.G. Plint development in a desert setting: a case study from the Luni River basin, 517 A new evaluation of fining upward Thar Desert of western India sequences in a mud-rock dominated M. Jain, S.K. Tandon, A.K. Singhvi, succession of the Lower Old Red Sandstone S. Mishra and S.C. Bhatt of South Wales, UK S.B. Marriott, V.P. Wright and 373 The Middle of the Tiber River, B.P.J. Williams central Italy: Plio-Pleistocene fluvial and coastal sedimentation, extensional 531 Reservoir scale sequence stratigraphy for tectonics and volcanism hydrocarbon production and development: M. Mancini and G.P. Cavinato Tarbat–Ipundu Field, south-west Queensland, Australia Pre-Quaternary fluvial systems R.S. Root, S.C. Lang and D. Harrison

399 Transport modes and grain-size patterns in 557 Recognition of a floodplain within braid fluvial basins delta deposits of the Oligocene Minato P.F. Friend and W.B. Dade Formation, north-east Japan: fine deposits correlated with transgression 409 Gulf of Mexico Basin depositional record of K. Yagishita and O. Takano Cenozoic North American evolution 569 Index W.E. Galloway FSA01 11/30/2004 02:18PM Page vii

Preface

Papers in this IAS Special Publication are derived the beginning of the twenty-first century, and from oral and poster presentations at the 7th included the following: International Conference on Fluvial Sedimento- 1 General topical sessions logy (ICFS), held in Lincoln, Nebraska (USA), • flow, and bedform 6–10 August 2001. The ICFS series was initiated dynamics in Calgary, Alberta in 1977, has been held every • fluvial systems—modern and four years, and has become a staple within the ancient international fluvial sedimentology community. • fluvial systems—modern and The 7th ICFS was attended by 289 professionals ancient and students from 28 countries, who repres- • sequence stratigraphy of alluvial successions ented universities, government institutions and • fluvial systems and economic resources private enterprise. The next meeting in this • river management series, the 8th ICFS, will be held in 2005 at 2 Special interest symposia Delft, The Netherlands, and will be hosted • alluvial architecture by Delft University of Technology (http:// • dryland rivers—process and products www.8thfluvconf.tudelft.nl/). • deposits in mud-dominated rivers The 7th ICFS was hosted by the Department • alluvial and tectonic system interactions of Geosciences of the University of Nebraska- • fluvial system response to climate change Lincoln, and, like its predecessors, operated through time without an umbrella provided by formal affilia- • alluvial responses to accommodation tion with professional scientific organizations. changes Indeed, the 7th ICFS could not have taken place • response of near-coastal fluvial systems to without the generous sponsorship and logistical sea-level change support provided by the University of Nebraska, • fluvial reservoirs the hard work contributed by many University • fluvial–estuarine transitions of Nebraska faculty and students, the efforts of • the Late Quaternary Rhine–Meuse system the many conference field trip leaders, and the A similar topical and geographical diversity is financial assistance provided by the International reflected in the 29 papers included in this Special Association of Sedimentologists, Society for Publication. In the first group, one set of papers Sedimentary Geology, American Association of focuses on flow, sediment transport and bedform Petroleum Geologists, Conoco Inc., ExxonMobil dynamics. J. Best presents a series of laboratory Upstream Research Co., Phillips Petroleum Com- and field observations on dune-related macrotur- pany, Schlumberger Reservoir Technologies and bulence. I. Fuller and others quantify reach-scale STATOIL. sediment transfers in the River Coquet, England. The 7th ICFS included four days of technical M. Kleinhans summarizes results of flume experi- sessions, with 175 oral presentations and 60 ments coupled with empirical data from the Rhine posters, plus nine pre-, post- and mid-conference to discuss dune-phase bedload transport and field trips. Conference themes reflected the topical the importance of sorting processes. S. Leclair and geographical diversity of exciting research and A. Blom present results of flume experiments being conducted by fluvial sedimentologists at designed to decipher controls on the probability FSA01 11/30/2004 02:18PM Page viii

viii Preface

distribution of bed-surface elevations, and the quantification of downstream changes in alluvial structure and texture of the associated deposits, architecture based on mass-balance considerations. under dune-forming conditions. C. Marti and P. The third group includes papers that address the Bezzola present early results of numerical and responses of Quaternary fluvial systems to climate physical modelling of Alpine streams, which are change, active tectonics, and/or sea-level change. designed to assist redevelopment of more natural A. Amorosi and M. Colalongo discuss alluvial and braided patterns in rivers that have been coeval nearshore marine successions of the Po artificially narrowed as a result of river training River Plain, Italy, and consider some implications activities. Last, P. Villard and others discuss the for sequence-stratigraphy models. M. Benvenuti measurement of bedload in sand-bed channels and others describe depositional processes, facies using an acoustic Doppler profiler, and the merits and the latest Pleistocene to modern evolution of this technique relative to traditional mechanical of discontinuous ephemeral streams of the main samplers. Ethiopian rift. K. Cohen and others discuss how The second set within the first group focuses detailed studies of fluvial–deltaic deposits of the on the characteristics of modern fluvial landforms, Rhine–Meuse delta, The Netherlands, record environments and systems. D. Abbado and others differential subsidence. C. Fielding and others describe the role of high floodplain describe the response of the ancestral Burdekin rates in promoting anastomosis along a reach River, north-east Australia, to sea-level fall, as it of the Columbia River in British Columbia. A. cut across the Great Barrier Reef Shelf. M. Jain and Archer provides a very useful and extensive others document the Quaternary stratigraphical review of the state of knowledge on Amazonian development of the Luni River system, in the Thar depositional systems. C. Krapf and others describe Desert of western India, and the influences of the processes, characteristics and importance of tectonic activity and climate change over a variety fluvial–aeolian interactions for the Koigab Fan of time-scales. Finally, M. Mancini and G. Cavinato in north-west Namibia. G. Sambrook Smith and summarize the Plio-Pleistocene evolution of the others discuss the spatial scale invariance of Tiber River system, central Italy, in response to shapes and scour depths in modern braided extensional tectonics, volcanism and climate rivers, as well as the difficulties of applying scale change. invariant concepts to older deposits owing to the The final group of papers addresses a variety importance of temporal evolution and rates of of topics based on studies of pre-Quaternary migration of bar forms. P. Siiro and others use fluvial systems. The first paper by P. Friend and laser diffraction analysis to compare the grain-size W.B. Dade presents a model for transport modes distributions of Cretaceous and the Miocene epi- and grain-size patterns in fluvial basins. W. continental embayment/seaway systems in South Galloway summarizes his conference keynote and North America, so as to unravel the formation address, and in doing so provides an overview of of sand–mud couplets within tidally influenced Cenozoic North American drainage basin evolu- inclined heterolithic strata. Finally, R. Sinha and tion, as recorded in the northern Gulf of Mexico others describe the sedimentological characteris- basin, offshore Texas and Louisiana, USA. S. Greb tics and avulsion history within an anabranching and R. Martino describe fluvial–estuarine trans- reach of the Bahgmati River, India, which flows itions in the Lower Pennsylvanian of the Central from the Himalayan foothills into the rapidly Appalachian Basin, eastern USA. R.M. Joeckel sudsiding foreland. and others present a detailed summary of fluvial A short second group focuses on physical ana- to estuarine architecture and palaeogeography of logue and numerical modelling. F. Ethridge and the Cretaceous Dakota Formation from eastern others provide an overview of a generation of Nebraska, USA. K. Keogh illustrates the utility of experimental studies at Colorado State University thre-dimensional models of alluvial architecture, on the morphological and stratigraphical effects using the Westphalian A Silkstone Rock, Pennine of base-level change. This is followed by N. Strong Basin, UK as a case study. M. Lumsdon and A.G. and others who present a new approach for the Plint discuss how changes in the rate of generation FSA01 11/30/2004 02:18PM Page ix

Preface ix

of accommodation affects alluvial style in the P. Carling, M. Church, I. Cojan, J. Crabaugh, A. Upper Cretaceous Dunvegan Formation, north- Czajka-Kaczka, R. Dalrymple, S. Davies-Vollum, east British Columbia, Canada. S. Marriott and K. Eriksson, K. Farrell, H. Feldman, M. Filgueira- others discuss fining upward sequences in a mud- Rivera, J. Friedmann, P. Friend, W. Galloway, rock dominated succession of the Lower Old Red P. Ghosh, M. Gibling, Steve Greb, M. Guccione, Sandstone of South Wales, UK, and suggest an P. Heller, M. Hicks, J. Holbrook, P. Houben, environment of deposition similar to that of the J. Howell, P. Hudson, J. Isbell, C. James, Channel Country in central Australia, where M. Kleinhans, N. Lancaster, S. Lang, A. Large, bedload often consists of sand-sized mud aggre- J. Laronne, S. Leclair, M. Leeder, G. Lowey, gates. R.S. Root and others use a data set from the D. Maddy, B. Makaske, S. Marriott, A. Mather, Tarbat–Ipundu Field in south-west Queensland, D. May, P. McCarthy, S. McClelland, D. Mohrig, Australia, to demonstrate how sequence strati- G. Nadon, G. Nanson, C. Paola, F. Pazzaglia, graphy can be applied at the hydrocarbon reser- J. Pederson, G. Plint, G. Postma, T. Rittenour, voir scale. Finally, K. Yagishita and O. Takano A. Roy, G. Sambrook Smith, G. Saunders, describe floodplain strata and their sequence- R. Sinha, R. Slingerland, N. Smith, R. Smith, stratigraphy significance within braid delta H. Stollhofen, E. Straffin, J. Swenson, S. Tooth, deposits of the Oligocene Minato Formation, T. Törnqvist, P. Villard, G. Weissmann, A. Wilbers, north-east Japan. J. Woodward, C. Wooldridge, P. Wright, and Such a diversity of papers requires a diversity of V. Zlotnik. We owe them a debt of gratitude for expertise to provide the critical review necessary this valuable service. to bring them to publication form. Indeed, a large number of individuals, conference attendees and MICHAEL BLUM others from the broader community served as Baton Rouge, Louisiana, USA reviewers for one or more papers submitted to this IAS Special Publication. These reviewers are SUSAN MARRIOTT J. Abbott, J. Alexander, M. Allison, A. Amorosi, Bristol, United Kingdom A. Archer, A. Aslan, P. Ashworth, W. Autin, J. Baas, M. Benvenuti, H. Berendsen, S. Bennett, M. Blum, SUZANNE LECLAIR M. Bourke, R. Brenner, J. Bridge, G. Browne, New Orleans, Louisiana, USA FSA01 11/30/2004 02:18PM Page x FSC01 11/30/2004 02:18PM Page 1

Fluvial processes and forms FSC01 11/30/2004 02:18PM Page 2 FSC01 11/30/2004 02:18PM Page 3

Spec. Publs int. Ass. Sediment. (2005), 35, 3–15

Origin of anastomosis in the upper Columbia River, British Columbia, Canada

DIMITRI ABBADO*, RUDY SLINGERLAND*,1 and NORMAN D. SMITH† *Department of Geosciences, Pennsylvania State University, 503 Deike Building, University Park, PA 16802, USA (Email: [email protected]); and †Department of Geosciences, University of Nebraska, 214 Bessey Hall, PO Box 880340, Lincoln NE 68588, USA

ABSTRACT

To understand the origin of anastomosis on the Columbia River between Spillimacheen and Golden, British Columbia, Canada, a geomorphological and sedimentological survey was undertaken during the summer flood of 2000. On the basis of these observations, the study reach can be divided into two sub- reaches: a highly anastomosed section with three to five channels, and a weakly anastomosed section with one to two channels. The highly anastomosed reach occurs immediately downstream from the Spillimacheen and is characterized by a higher channel slope, a higher number of crevasse splays, a larger combined crevasse splay area, a wider valley and a coarser bedload. Higher rates of floodplain aggradation in the highly anastomosed reach are suggested by modern sediment budgets and radiocarbon dates. These geomor- phological and sedimentary associations are consistent with the hypothesis that anastomosis of the Columbia River is maintained by a dynamic equilibrium between the rates of channel creation and channel abandonment. Rising base- level, fine bedload and low bed-slope are not necessary immediate conditions for anastomosis of the Columbia River.

INTRODUCTION described (Smith & Putnam, 1980; Smith & Smith, 1980; Rust, 1981; Smith, 1983, 1986; Anastomosed rivers consist of two or more Nanson et al., 1986; Schumann, 1989; Miller, interconnected, coexisting channels that typic- 1991; Knighton & Nanson, 1993; Smith et al., ally enclose concave-upwards floodbasins. The 1997, 1998; Makaske 1998, 2001), the origin of channels are usually straight or slightly sinuous, anastomosis is still an unresolved matter (Nanson but braided and meandering patterns are also & Huang, 1999; Makaske 1998, 2001). Indeed, known. Thus, anastomosed rivers are different Makaske (1998) argued that understanding the from braided rivers because the latter contain causes of anastomosis ‘is one of the major chal- multiple enclosing convex bars within a lenges in fluvial research’, and Nanson & single channel (Makaske, 2001) whereas anasto- Huang (1999) asserted that anabranching rivers mosis defines a network of anabranched channels. (including anastomosed rivers) ‘remain the last Although the geomorphological characteristics of major category of alluvial systems to be described anastomosed rivers have been recognized and and explained’.

1Corresponding author. FSC01 11/30/2004 02:18PM Page 4

4 D. Abbado, R. Slingerland and N.D. Smith

Three hypotheses exist for the origin of anasto- multiple channels in avulsion belts, so the anasto- mosis. In the first, anastomosis is a consequence mosis should be transitional and short-lived. A of frequent avulsions and slow abandonment of subsequent palaeoenvironmental reconstruction earlier channels (see e.g. Makaske (2001) and refer- of Columbia River deposits (Makaske, 1998) has ences therein). According to this point of view, shown that its anastomosed channels are long- the fluvial system exists in a perpetual transition lived and not the result of delta growth into state consisting of multiple coexisting channels. shallow lakes. Therefore this hypothesis will not Anastomosis is thus not a ‘graded’ state, but rather be considered further here. a by-product of the competition between channel The purpose of this paper is to describe the creation and abandonment. Makaske (2001), for hydraulic and morphological properties of the example, defined an anastomosed system as the anastomosed reach of the upper Columbia River product of a dynamic balance between frequent in British Columbia, Canada, in order to assess the avulsions that create multiple channels and slow origin of its anastomosis. The Columbia River channel abandonment. According to Makaske, the near Golden, British Columbia is an appropriate immediate causes of the frequent avulsions are field site, being one of the best-known examples of a rise in base-level, subsidence (Smith, 1983), and anastomosis (Locking, 1983; Smith, 1983; Makaske, high rates of aggradation, whether of the channel 1998, 2001; Adams, 1999; Machusick, 2000). Fur- belt or within the channel. The immediate cause thermore, hydrological and photographic records of slow abandonment is conjectured by Makaske are available starting from the first half of the 1900s. (2001) to be low power, although few data exist. In the second hypothesis, anabranching and LOCATION AND GEOMORPHOLOGY anastomosed rivers are thought to be an equilibrium OF THE STUDY AREA form where channels are adjusted in geometry and hydraulic friction to just transmit the imposed The study reach is a section of the upper Columbia water and sediment discharges. In cases where River near Golden, British Columbia, Canada gradient cannot easily be increased to carry a (Fig. 1). The Columbia River starts at Columbia larger sediment load, Nanson & Knighton (1996) Lake in southern British Columbia, approximately and Nanson & Huang (1999) proposed that a shift 80 km south-east of the study reach, and flows from single to multiple channels leads to an north-north-west in a 1–2-km-wide valley for a increase in sediment transport rate per unit water distance of 160 km along the Rocky Mountain . Thus, like changes in slope and channel Trench before turning west and south-west. It form, anastomosis is conjectured to be another consists of a single channel between Columbia mechanism whereby a fluvial system can maintain Lake and the town of Radium, an anastomosed grade. Makaske (1998) challenged this idea, how- reach between Radium and a kilometre upstream ever, arguing that the multichannel state of the of Golden, and a braided reach at Golden where it upper Columbia River cannot be taken as a flows across the of a tributary, the response of the system to maximize water and Kicking Horse River. Anastomosis is particularly sediment throughput because, in spite of its evident downstream of Spillimacheen, and this anastomosed morphology, the bulk of its water report concentrates on the 55-km reach between and sediment moves through a single channel. Spillimacheen and Golden. Access is provided by The third hypothesis was put forward by Galay Route 95 along the north-east side of the valley, by et al. (1984) from a study of the Columbia River. bridges at Nicholson, Parson and Spillimacheen, They postulated that ponding behind alluvial fans and by a railway right-of-way. The area lies within led to the formation of large lakes in the upper the Cassiar–Columbia Mountain physiographic Columbia Valley. The lakes gradually were filled region and in the Interior Douglas-Fir biogeocli- by river-dominated ‘bird’s-foot’ deltas of which matic zone (Farley, 1979). Mean annual precipita- the present anastomosed river system is a final tion varies between 40 and 50 cm yr−1, and mean stage. This type is thought to result from con- daily temperature varies from −12 to 15°C in temporaneous filling of shallow lakes and scour of January and July, respectively (Farley, 1979). FSC01 11/30/2004 02:18PM Page 5

Anastomosis of the upper Columbia River, BC 5

forming alluvial fans that narrow the valley and THE COLUMBIA RIVER act as local sediment sources. The two largest

WesternCanada are the Spillimacheen River (drainage Manitoba Moberly British 2 Columbia Aberta basin of 1430 km ) and the Kicking Horse River Goldenon Sask. ls Columbia o River 2 Nich NORTH 95 (drainage basin of 1850 km ), which respectively do McMur define the upstream and downstream limits of the on s le Par a red e Cast at study reach. The Spillimacheen River catchment Harrog

Spillimac heen is a major sediment source for the study reach, Edelweiss co Kicking Horse Bris contributing silt to fine gravel. Golden xor r Lu te Riv er gewa An important observation bearing on the Ed um Canyo di n cholson Ra origin of anastomosis is the number and location Cree Ni k e er eek se Cr Inverm ere ) of channels and their evolution through time. Hor rm m de (799 Win ke KOOTENAY La Vibracores show that anastomosed channel

do ur deposits in the study reach are characteristically ia McM b ) olum C (809m TOWER RANG ke PEAK La 5–15 m thick, narrow, interconnected stringers

PUR on ars E CELL MOUNTAINS P of sand (Smith, 1983) that contain sandy crevasse- ale d stle Ca splay fringes. These facies are stacked vertically,

e at og indicating that the channels occupy the same Harr Sp illi valley location for durations of up to approx- lake mach een een R imately 3000 yr or maybe even longer (Smith, Spillimac h iver 1983; Makaske, 1998). Vertical aggradation rather than lateral accretion is the dominant sedimenta- SOLES tion pattern, a conclusion also supported by the BASIN 0 5km virtual absence of modern oxbow-lake and point- bar deposits. In one cross-valley stratigraphical Fig. 1 Location of the study area. The Columbia River section (Makaske, 1998), at least nine channels flows north-north-west between the Kootenay Range have existed over the past 3000 yr. Of these, of the Rocky Mountains and the Purcell Mountains. six came into existence and three went extinct, Soles Basin, selected as a typical floodplain indicating the long-term existence of the anasto- of the anastomosing reach, lies immediately mosed pattern and the episodic nature of channel downstream of the Spillimacheen River, an important tributary. creation. There is also some indication that the longer lasting channels are wider than 30–50 m (Makaske, 1998), possibly because smaller channels can be occluded by log jams or have their gradient strongly diminished by beaver . Geomorphology and sedimentology of the study The Columbia River sediment load consists reach of 59 to 82% suspended material (Makaske, 1998), In the study reach, the Columbia River consists of or if is also considered, 89% (Locking, multiple, relatively stable channel belts containing 1983). Locking’s sediment budget indicates that at low-sinuosity to straight, low-gradient, sand-bed the end of the anastomosed reach near Nicholson channels. and crevasse splays of the channel (6 km upstream of Golden) the supply of suspended belts bound floodbasins containing shallow wet- load is much less than the transport capacity lands and lakes. The channel belts show little of the river. This decline in is lateral migration over their lifetimes, as indicated evidence of a significant sediment sink in the by the absence of scroll bars on the modern anastomosed reach (Locking, 1983). Permanent floodplain and by the near vertical accretion of sequestration of a portion of the bedload also channel facies as seen in cores (see e.g. Makaske, occurs, with channel and crevasse splay storage 1998, fig. 3.5). The channels are relatively straight, roughly estimated by Smith to be, respectively, although smaller channels are slightly more sinu- 66% (Smith, 1986) and 10–20% (D.G. Smith, as ous. Thirty-seven tributaries enter along the reach, reported in Makaske, 1998). FSC01 11/30/2004 02:18PM Page 6

6 D. Abbado, R. Slingerland and N.D. Smith

Discharge of the Columbia River at Nicholson from 1947 to 2000 800

600 -1 s (a) 3 400 m

200

0 1950 1955 1960 1965 1970 1975 1980 1985 1990 1995 2000 year 400 Average monthly discharge from 1947 to 1999 Average monthly discharge in 2000 300 -1 s 200 (b) 3 m 100

0 1 2 3 4 5 6 7 8 9 10 11 12 month Discharge of the Spillimacheen River at Spillimacheen from 1950 to 2000 (excluding 1985-94 due to lack of data) Fig. 2 Discharge data for the 300 rivers studied. (a) Discharge of 250 the Columbia River at Nicholson 200 1947–2000. (b) Average monthly -1 s discharge of the Columbia over 3 150 (c) m 100 the interval 1947–1999 compared with year 2000 average monthly 50 discharge. (c) Discharge of 0 1950 1955 1960 1965 1970 1975 1980 1985 1990 1995 2000 the Spillimacheen River at year Spillimacheen 1950–2000. 150 (d) Average monthly discharge of Average monthly discharge from 1950 to 1999 the Spillimacheen over the interval Average monthly discharge in 2000 100 1950–1999 compared with year

-1 2000 average monthly discharge. s (d) 3 Year 2000 flood was average in m 50 magnitude but of short duration and delayed owing to cold weather in 0 1 2 3 4 5 6 7 8 9 10 11 12 June (Data from Environment month Canada, 2001).

frequency distribution shows that the maximum Hydrology peak of 351 m3 s−1 registered at the Nicholson gaug- for the Columbia River at Nicholson ing station during the year 2000 occurs on average (1947–present) and the Spillimacheen River near every 1.2 yr (1-yr flood). its mouth (1950–present) indicate that discharges for both are highly seasonal (Fig. 2). Minimum discharge for the Columbia occurs in February LONGITUDINAL VARIATIONS IN (average = 24 m3 s−1), and maximum discharge ANASTOMOSIS AND RELATED occurs in June and July (average = 321 m3 s−1), with FEATURES overbank discharge of 45 days per year on average occurring almost every year (Locking, 1983). Our To better understand the necessary conditions field observations were taken during the year 2000 that give rise to anastomosis, the degree of ana- flood, which was average in magnitude but short stomosis of the Columbia River was correlated with in duration and somewhat delayed owing to cold channel gradient, crevasse splay distribution, weather in June (Fig. 2b & d). The peak flow valley width, alluvial fan area, and channel-bed FSC01 11/30/2004 02:19PM Page 7

Anastomosis of the upper Columbia River, BC 7

grain size. These parameters were measured and crevasse splays are indicated, as well as during the summer of 2000 or were observed on definition sketches of alluvial fan area, splay area aerial photographs taken in 1996 at high stage, and valley width. when the discharge measured at Nicholson was the third highest of the previous 10 yr. Given Degree of anastomosis the hydrological data available from Environ- ment Canada (formerly Water Survey of Canada) To quantify the degree of anastomosis, the number from 1947 to present, the 1996 peak discharge of active channels at each of 29 valley cross- (506 m3 s−1) occurs on average every 3.3 yr (3-yr sections was counted. The number of channels, flood). used here as a measure of anastomosis, varies from For our purposes, a channel belt is defined as one to five with an average near two (Fig. 4a). On active, in contrast with non-active, dry, or aban- the basis of these differences, the study reach can doned, if channels within it contain turbid water be divided into an upper highly anastomosed reach on the 1996 aerial photograph, thereby implying (three to five channels), a weakly anastomosed at least modest through-flow. Main channels reach (one to three channels), a single channel and are defined as those wider than 40 m; narrower a lower braided reach. The braided reach occurs channels are here termed secondary channels. as the Columbia River crosses the alluvial fan of Figure 3 shows a highly anastomosed section of the Kicking Horse River and will not be discussed the study reach where active/non-active channels further here.

inactive channel active channels active crevasse splay inactive crevasse splay

alluvial fan alluvial fan

dth

wi

Fig. 3 Morphological elements of crevasse splay width area the river system. (Top) 1998 aerial crevasse splays photograph of the Columbia River valley valley showing active and inactive channels and crevasse splays. (Bottom) Definition sketch showing how crevasse splay area, alluvial fan area, and valley width were valley border computed. See Fig. 6 for location. FSC01 11/30/2004 02:19PM Page 8

8 D. Abbado, R. Slingerland and N.D. Smith

Highly anastomosed Weakly anastomosed Single channel Braided

Spillimacheen River

Spillimacheen Harrogate Castledale Parson Nicholson

5 km N GOLDEN Kicking Horse River

5 (a)

0 0 10 20 30 40 50 60 70 80 Number of channels

776 S=0.0000902 S=0.000215 774

S=0.0000678 772 (b)

Elevation (m) 770 S=0.000442 ) 2

m 768 5

766 030405060708010 20 10 Avulsion site Number of crevasse splays Crevasse splay area (105 m5)

and 5 (c)

0

crevasse splay area (10 01020 30 4050607080 Number of crevasse splays 10 Avulsion site

5 (d)

0 to valley area (%)

Ratio of splay area 0 1020304050607080 104 Valley width Alluvial fan area 3 103 Fig. 4 Comparison of selected 2 102 (e) 1 10 morphological parameters of the 1 Alluvial fan area (hectares) Columbia River. (a) Number of Valley width (km) 10 20 30 40 50 60 70 80 channels. (b) Elevation. (c) Number 1 y = −0.0074x + 0.8503 0.8 r2 = 0.3609 and area of splays. (d) Splay/valley- 0.6 (f) 0.4 area ratio. (e) Valley width and 0.2 0 10 20 30 40 50 60 70 80 alluvial fan area. (f ) Mean grain

Mean grain size (mm) Along river distance from Spillimacheen bridge (km) size. See text for explanations.

or when the channel flows across the valley. The Longitudinal profile longitudinal profile is divisible into three sections. Absolute water-surface elevations were measured A relatively steep section from Spillimacheen to at 34 points along the Columbia River using a Castledale (S = 0.000215) is conjectured to reflect Leica 500 differential global positioning system steepening of the Columbia River gradient as a (GPS) with a subcentimetre vertical accuracy. The result of sediment input from the Spillimacheen points were measured over the period 13–15 River. A steep section at Golden (S = 0.000442) October 2000, between the bridge at Spillima- arises as the Columbia River crosses the coarse- cheen and the Kicking Horse River and corrected grained alluvial fan of the Kicking Horse River. for a falling water level of 1 cm day−1. The water Between Castledale and Golden is a more gentle elevations (Fig. 4b) are plotted against along- central portion (S = 0.000068) in which the minor channel distance rather than valley distance to fans along the valley show little, if any, effect on avoid anomalies introduced by variable sinuosity channel gradient. FSC01 11/30/2004 02:19PM Page 9

Anastomosis of the upper Columbia River, BC 9

mean particle diameter using the relationship of Distribution of active crevasse splays Dietrich (1982). The study reach was divided into 29 cross-valley Mean grain size shows considerable scatter swaths, each 2 km wide, in which the numbers (Fig. 4f ), probably owing to variations in texture at of active crevasse splays and their total surface the crest and troughs of dunes and the occasional areas were determined. A crevasse splay was con- introduction of coarse material from tributaries. sidered to be active if turbid water was flowing Nevertheless, the bed material shows a statistic- across its surface in the 1996 aerial photographs. ally significant fining downstream in the study The number of active crevasse splays and total reach from 1.4–2.2 mm upstream to 0.5–1.1 mm crevasse splay areas are both relatively high in the downstream. upper 12 km of the study reach (Fig. 4c). Figure 4d shows the percentage of the valley floor covered Interpretation by active crevasse splays. There are 12 active crevasse splays in the upper 18 km and only six The above data indicate that the study reach of crevasse splays along the remaining reach. The the Columbia River (excluding the braided section area covered by active splays decreases monoton- and single-channel reach) can be divided into two ically with distance, the exception being an active subreaches, a 17-km long, highly anastomosed avulsion site at kilometre 37. At this site, an reach with three to five channels starting immedi- ongoing avulsion blankets the whole floodbasin ately below the confluence with the Spillimacheen with sediment, and small levees have formed since River, and a 38-km long, weakly anastomosed 1960 (Adams, 1999). The study reach therefore reach containing one to three channels. The can be divided into two sections: an upstream highly anastomosed reach is characterized by a reach with a high number of crevasse splays; and a relatively steep channel slope, a higher number of downstream reach with a low number of crevasse crevasse splays, a higher total crevasse splay area, splays. a higher splay-area/valley-area ratio and coarser bed material (Table 1). These are particularly interesting observations because previous studies Valley width have concluded that low gradients and fine grain Valley width is potentially an important parameter sizes are necessary conditions for anastomosis (cf. in determining anastomosis because it defines the Makaske, 2001). Previous studies also conjectured maximum available space in which channel belts that rising base-level is a necessary immediate can form. Variation in valley width is controlled condition for anastomosis (Smith & Smith, 1980), by prograding alluvial fans from side tributaries. but that is not supported by these data either. Measurements from aerial photographs of valley The intensity of crevasse splay activity is inter- width and alluvial fan area (Fig. 4e) show little preted to indicate that alluviation rates are higher correlation with anastomosis. in the upstream, highly anastomosed reach. Testing this interpretation with actual measured aggrada- tion rates is difficult, however. The spatially Bed material grain size averaged sedimentation rate during the 1982 flood Bed material was sampled during high stage on cycle for the entire reach from Spillimacheen to 24 June and 6 July 2000 from the mouth of the Nicholson was 3.7 mm yr−1 (Locking, 1983). This Spillimacheen River to 5 km upstream of the town probably is an overestimation of the long-term of Nicholson. Twenty-five samples were collected average because it is based on the 1982 flood, along the main using a bucket sampler which was well above average. A detailed sedi- (height 15 cm, diameter 10 cm) with three re- ment budget and geomorphological study of a plicates each to capture cross-channel variability. floodbasin in the highly anastomosed reach Mean grain size was computed using a self- (Fig. 1, Soles Basin) during the year 2000 flood constructed rapid sediment analyser to obtain a (Abbado, 2001) shows that it is being actively mean fall velocity that was then converted to filled at a rate of 2.2 mm yr−1 by a combination of FSC01 11/30/2004 02:19PM Page 10

10 D. Abbado, R. Slingerland and N.D. Smith

Table 1 Comparison between the upper and lower anastomosed reaches of the Columbia River in the study area.

Area of Number of crevasse Splay area/ Valley Degree of Number of Slope crevasse splays valley area width Grain size Reach anastomosis channels (cm km−1) splays* (m2 km−1) (%) (km) (mm)

Upper High 3–5 21.5 10 c. 60 000 3.3 1.4–2.2 0.5–1.1 Lower Low 1–3 6.8 1 c. 500† 0.035 0.7–1.8 0.3–0.6

*Average number of crevasse splays per 10 km wide transverse swath. †Excluding avulsion site.

short-lived crevasse splays, intrafloodbasin chan- Here, the question arises whether the Columbia nels and settling of grains in temporary lakes. This channels are adjusted to maximize sediment estimate was obtained by simultaneously measur- transport rate, as suggested by Nanson & Huang ing the sediment flux into and out of the basin (1999) and Huang & Nanson (2000). In traditional through crevasses and over levee tops during the equilibrium channel theory, a river adjusts its slope, 2000 flood. This must be considered a minimum geometry and roughness to convey the water sup- because the flood of 2000 was shorter in duration plied and sediment discharge. Nanson & Knighton than the average flood (Fig. 2b) and only suspended (1996) and Nanson & Huang (1999) suggested that load was measured. In contrast, 16 km further a river might also change its number of channels down the study reach, an average aggradation rate to yield the same effect. Based on field observa- of 1.7 mm yr−1 was obtained using a radiocarbon tions, they asserted that a reduction in total top- date of 4500 cal. yr BP from Scirpus lacustris width causes a multichannel network to convey nuts buried 7.9 m in a floodbasin (Makaske, 1998). more sediment per unit of total stream power, or, Although the data are inconclusive, they are at holding slope constant, per unit of discharge, than least consistent with the conjecture that aggrada- a single channel. Thus, if an original channel is tion rates are higher upstream in the more ana- 100 m wide and, say, 3 m deep, three channels, stomosed reach. Also consistent is the relatively each 25 m wide and carrying the same discharge steep slope observable in the longitudinal water at the same slope, will carry more bedload because profile, which can be interpreted as a wedge of a reduced width/depth ratio (W/D) is more con- sediments prograding downstream as alluviation ducive to water flow and sediment discharge. occurs. Finally, as Robinson & Slingerland (1998) The hypothesis to be tested here is that the and Paola (2000) have argued, the downstream- highly anastomosed reach of the Columbia River fining itself is suggestive of preferential aggrada- is adjusted in channel number and channel width/ tion in the upstream reach. Although upstream depth ratios to carry more sediment than a single bed-armouring could produce a similar down- channel, all other factors such as Manning’s n and stream fining trend, it does not adequately explain cross-sectional shape being equal. To test the the present data, because at the time of sampling hypothesis an abstracted Columbia channel net- the pavement appeared to be broken and the bed work was considered (Fig. 5) in which cumulative was in general motion. top-width, depth and bed-slope are kept constant at 120 m, 3 m and 10−4, respectively, consistent with values observed in the upstream portion SEDIMENT TRANSPORT MODELLING of the study reach (Fig. 6 & Table 2). As Table 2 shows, width/depth ratios (defined as top-width The observations presented so far do not discrimi- divided by the hydraulic depth; see footnote in nate between the two hypotheses for the origin Table 2) range from 45 to 8. In addition, the dis- of anastomosis in the Columbia because both tribution of channel widths is bimodal with the hypotheses predict that the degree of anastomosis minimum occurring between 40 and 50 m. This will be correlated with excess sediment supply. minimum was used to separate the channels into FSC01 11/30/2004 02:19PM Page 11

Anastomosis of the upper Columbia River, BC 11

two groups: main channels and secondary channels. Based on these data from the Columbia River, the abstracted model contains a single channel W/D~40 of W/D ≅ 20, which progressively bifurcates into second-order channels of W/D ≅ 20 and third-order channels of W/D ≅ 10. Interestingly, the observed W/D~20 width/depth ratios of main channels decrease

W/D~20 Fig. 5 (left) Generic model of an anastomosing river W/D~10 W/D~10 with width/depth (W/D) ratios typical of the Columbia. The W/D ratio progressively decreases with increasing number of channels.

Table 2 Width/depth ratios of cross-sections in the study reach.

Hydraulic Cumulative Cross-section Line Top-width, W depth, D Number of top-width location number* Date (m) (m)† W/D channels‡ (m)§ Source

Main channels¶ 1 May 2000 125.7 2.86 44 1 125.7 This study 4 July 2000 125 2.9 43 2 143 This study 5 May 2000 88.9 2.87 31 3 120 Filgueira-Rivera** 6 May 2000 88 2.88 31 3 120 Filgueira-Rivera** 8 July 2000 90 2.95 31 3 180 This study 11 May 2000 55 2.82 20 4 188 Filgueira-Rivera** 12 June 1988 67.6 2.6 26 3 150 Adams, 1999 17 June 1988 84.5 2.94 29 4 160 Adams, 1999 20 July 1994 57.5 4.44 13 4 100 Makaske, 1998 22 May 2000 141 3.12 45 1 141 Filgueira-Rivera** 23 May 2000 101.6 3.34 30 2 130 Filgueira-Rivera** Secondary 2 May 2000 19 1.96 10 2 143 Filgueira-Rivera** channels 3 May 2000 18.7 2.23 8 2 143 This study 7 May 2000 23.2 2.07 11 3 120 Filgueira-Rivera** 9 May 2000 13.7 2.25 6 3 180 Filgueira-Rivera** 10 May 2000 38 2.95 13 4 172 Filgueira-Rivera** 13 June 1988 30.4 1.58 19 5 Unknown Adams, 1999 14 May 2000 38.9 2.9 13 5 Unknown Adams, 1999 15 May 2000 31 3.64 9 5 Unknown Filgueira-Rivera** 16 May 2000 35 3.49 10 4 Unknown Filgueira-Rivera** 18 June 1988 20.3 1.25 16 4 Unknown Adams, 1999 19 July 1994 22.5 1.84 12 4 100 Makaske, 1998 21 July 1994 22.5 2.1 11 4 100 Makaske, 1998 24 May 2000 21 2.27 9 3 Unknown Filgueira-Rivera**

*See Fig. 6 for locations. †D = hydraulic depth, i.e. channel cross-sectional area divided by its top-width. ‡Number of channels equals the sum of main plus secondary channels along a cross-valley transect passing through the particular channel cross-section. §This is the summed top-width of all channels in a valley-wide transect passing through this location. ¶ Main channels are defined here as having a top width greater than 50 m; all other channels are called secondary channels. **Personal communication. FSC01 11/30/2004 02:19PM Page 12

12 D. Abbado, R. Slingerland and N.D. Smith

Location of Fig. 3 Flow

Fig. 6 Aerial photograph collage of the highly anastomosed reach of the Columbia River. Measured cross- sections are indicated by a white line; W/D ratio in parentheses.

50 Q = VA (1) Main Channels (r2 = 0.77) 45 Secondary Channels (r2 = 0.32) 3 −1 −1 40 where Q (m s ) is water discharge, V (m s ) 2 35 is average velocity, and A (m ) is channel cross- 30 sectional area. Velocity, V, is expressed by the 25 Chézy formula: 20 = √ W/D of Channels 15 V C RS (2) 10 where R (m) is hydraulic radius, S is channel 5 slope and C (m1/2/s) is the Chézy constant. Values 0 0 1 2 3 4 5 for R and C are given by: Number of Channels along a Valley Cross-Section R = A/(2D + W) (3) Fig. 7 Width/depth ratios of channels as a function of number of channels passing flow through any valley C = (1/n)R1/6 (4) cross-section. Main channels are defined as those possessing widths greater than 50 m. in which D (m) is the water depth, W (m) is the channel width and n is the Manning constant. The system of equations (1)–(4) yields the following fifth-order polynomial in D: with an increasing number of channels along k3W 5D 5 − 4D 2 − 4WD − W 2 = 0 (5) any valley cross-section (Fig. 7), which could be interpreted as consistent with the Nanson–Huang where k =√S/(nQ). For fixed Q, n, S and width, conjecture. In contrast, the ratio for secondary equation (5) can be solved for D, thereby yielding channels increases. An additional important a specific width/depth ratio for that combina- characteristic of secondary channels is that their tion of values. In the following computations thalwegs sit at higher elevations compared with Q = 270 m3 s−1, n = 0.026, and S = 0.0001, consist- the main channels and thus they are active only ent with typical values for the Columbia River in during high stage. the study area. Sediment transport through this abstracted sys- Once the flow hydraulics are known, bedload tem is calculated under uniform and steady flow and suspended load sediment transport rates are conditions assuming channels of rectangular calculated by two methods: (i) the Bagnold (1977) cross-sectional shape in which: bedload formula coupled with the Rouse (1937) FSC01 11/30/2004 02:19PM Page 13

Anastomosis of the upper Columbia River, BC 13

Table 3 Theoretical sediment transport magnitudes as a function of sediment transport formula and number of channels: Q ≈ 270 m3 s−1.

Water Total Total suspended Number of Width Depth velocity bedload flux load flux Total sediment Formula channels (m) (m) W/D (m s−1)(m3 s−1)(m3 s−1) load (m3 s−1)

Bagnold and Rouse 1 120 2.94 40.8 0.78 0.0061 0.0161 0.0222 2 60 3.00 20 0.77 0.0058 0.0156 0.0214 4 30 3.11 9.6 0.74 0.0051 0.0147 0.0198 Van Rijn 1 120 2.94 40.8 0.78 0.0036 0.0336 0.0372 2 60 3.00 20 0.77 0.0033 0.0310 0.0343 4 30 3.11 9.6 0.74 0.0028 0.0264 0.0292

suspended sediment formulation, and (ii) the photographs and channel bed elevations were Van Rijn functions for bedload and suspended load obtained by GPS and cross-section surveys. Water (Van Rijn, 1984a,b). These methods were selected depths and flow velocities at two valley cross- because they are appropriate for the grain sizes sections, one where the Columbia River consists and slopes observed in the Columbia River, and of a single channel and one where it consists of because they compute both bedload and suspended three channels, were used to recompute total load. sediment fluxes through the two cross-sections. Solutions of the above system of equations for Predicted sediment fluxes through the reach with total sediment transport rate indicate that total three channels were 25 times less than through the sediment load is reduced as the flow is divided single channel, thus supporting the conclusions into additional channels (Table 3). Total transport reached from the idealized model. rate decreases by approximately 11% and 21% for the Bagnold–Rouse and Van Rijn formulas, respectively. In particular, bedload transport DISCUSSION rate, which is more important because it controls in-channel alluviation, decreases by 16% and 22%, The multiple channels of the Columbia do not respectively. Water velocities decrease by 2% appear to be adjusted in width, depth and number moving from one to two channels, and 5% moving to increase water velocity and sediment transport from two to four channels. rates over that of a single channel. These results In order to generalize these conclusions, it are interpreted to mean that the Nanson & Huang is possible to argue that bed roughness should (1999) hypothesis does not apply to the particular be greater in the smaller channels because of case of the Columbia River. This is not to say increased vegetation and because bedform heights that the Nanson–Huang conjecture is everywhere there are a greater proportion of the flow depth. invalidated. In cases where the cumulative top- This, however, would only further reduce the width of multiple channels is reduced relative total sediment load in the multichannel reaches. to a single channel, sediment transport rates It could be argued that the idealized model does will be increased. In the Columbia, however, the not capture the greater sinuosity and slightly observed width/depth ratios and cumulative higher bed elevations of the secondary channels. top-widths do not effect increases in sediment To address these concerns the steady-state flow transport rates as the number of channels is field through the actual Columbia River network increased. in the highly anastomosed reach was computed Rather, it would appear that anastomosis of using FESWMS, a two-dimensional, finite ele- the Columbia River is a consequence of frequent ment code for non-uniform free-surface flows. avulsions (i.e. crevassing) and slow abandonment Channel geometries were traced from aerial of earlier channels. High sediment flux from the FSC01 11/30/2004 02:19PM Page 14

14 D. Abbado, R. Slingerland and N.D. Smith

Spillimacheen River has overloaded the Columbia, causing high in-channel alluviation rates. These CONCLUSIONS high alluviation rates increase the probability of levee overtopping as well as levee crevassing and The anastomosed reach of the Columbia River can crevasse splay formation. Increasing crevassing, be divided into highly anastomosed and weakly in turn, creates numerous new channels through anastomosed subreaches. The highly anastomosed floodbasins. The new channels, flowing generally reach occurs immediately downstream of the con- cross-valley, are usually super-elevated compared fluence with the high-sediment-load Spillimacheen with the main channel. For this reason they are River. The highly anastomosed reach is character- mainly active during high stage, and are slowly ized by a higher channel gradient, a greater number abandoned because of low flow velocities. Thus, of crevasse splays, a greater crevasse splay area, long-lasting channels and complete avulsions of greater splay-area to valley-area ratio, and coarser the main channel are tied to gradient advantages. channel-bed grain size. Circumstantial evidence The narrow valley means that cross-valley gradi- indicates that aggradation rates are higher in the ent advantage rarely occurs and the main down- highly anastomosed reach as well. A rising base- valley channels remain active for thousands of level downstream does not seem to be a necessary years. In contrast, secondary channels on average immediate condition for anastomosis. are shorter lived. The number of channels active Calculations using Bagnold, Rouse, and Van at any time is proportional to the rate of creation Rijn sediment transport formulae show a decrease of new channels and to their average lifespan, in sediment flux with increasing number of chan- and inversely proportional to their rate of aban- nels, given typical Columbia channel geometries, donment. If the rate magnitudes are comparable bed-slope, and grain size. This is contrary to the and relatively constant through time, then the predictions of Nanson & Huang (1999), leading number of active channels at any instant is also us to conclude that anastomosis of the Columbia relatively constant, the exact number being fixed River is maintained by a dynamic equilibrium by the channel lifespan. It is in this sense that between the rates of channel creation and channel anastomosis of the Columbia River is a dynamic abandonment. equilibrium pattern. It still remains for the reduction of width/depth ratio of main channels as the number of channels ACKNOWLEDGEMENTS in a valley cross-section increases to be explained (Fig. 7). This probably reflects the fact that it is the This research was funded by the National Science main channels of the Columbia that transport Foundation under contract EAR 9811860 awarded most of the bedload. The bed elevations of the sec- to Rudy Slingerland and Norman D. Smith. For ondary channels are generally higher than the bed assistance in the field we thank Ron and Jan van of the main channel, so more water than bedload Vugt, Irvine Heintz, Matt Machusick, and espe- is siphoned off by secondary channels. The main cially Manuel Filgueira-Rivera who also shared channel must adjust to carry its bedload with less his cross-section data with us. The manuscript discharge, and does so by decreasing its width/ was greatly improved by the insightful comments depth ratio by an amount greater than would arise of reviewers G.C. Nanson and B. Makaske, to from the reduction in water discharge alone. In whom we are grateful. this restricted sense the Columbia main channels are behaving as postulated by Nanson & Huang (1999). REFERENCES This model of anastomosis is consistent with the correspondence between degree of anasto- Abbado, D. (2001) The origin of anastomosis in the Upper Columbia River, British Columbia, Canada. mosis and high slope. As shown by the sediment Unpublished MSc Thesis, Department of Geosciences, routing model, anastomosis induces a decrease in The Pennsylvania State University, 86 pp. sediment transport rates, which is manifested by Adams, P.N. (1999) The origin and characteristics of differential deposition. natural levees. Unpublished MSc Thesis, Department FSC01 11/30/2004 02:19PM Page 15

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of Geosciences, The Pennsylvania State University, Nanson, G.C., Rust, B.R. and Taylor, G. (1986) Coexistent 95 pp. mud braids and anastomosing channels in an arid- Bagnold, R.A. (1977) transport by natural zone river: Cooper Creek, Central Australia. Geology, rivers. Water Resour. Res., 13, 303–312. 14, 175–178. Dietrich, W.E. (1982) Settling velocity of natural par- Paola, C. (2000) Quantitative models of sedimentary ticles. Water Resour. Res., 18, 1615–1626. basin filling: Sedimentology, 47 (Suppl. 1), 121–178. Farley, A.L. (1979) An Atlas of British Columbia: People, Rouse, H. (1937) Modern conceptions of mechanics Environment, and Resource Use. University of British of fluid turbulence. Trans. Am. Soc. Civ. Eng., 102, Columbia Press, Vancouver, 135 pp. 436–505. Galay, V.J., Tutt, D.B. and Kellerhals, R. (1984) The Robinson, R.A.J. and Slingerland, R.L. (1998) Origin meandering channels of the upper of fluvial grain-size trends in a foreland basin: the Columbia River. In: River Meandering (Ed. C.M. Pocono Formation of the Central Appalachian Basin: Elliot), pp. 113–125. Proceedings of the Conference J. Sediment. Res., A68, 473–486. River ’83, New Orleans, Louisiana, October 24–26, Rust, B.R. (1981) Sedimentation in an arid-zone ana- 1983, American Society of Civil Engineers, New York. stomosing fluvial system: Cooper’s Creek, Central Huang, H.Q. and Nanson, G.C. (2000) Hydraulic geo- Australia. J. Sediment. Petrol., 51, 745–755. metry and maximum flow efficiency as products of the Schumann, R.R. (1989) Morphology of Red Creek, principle of least action. Earth Surf. Process. Landf., Wyoming, an arid-region anastomosing channel 25, 1–16. system. Earth Surf. Process. Landf., 14, 277–288. Knighton, A.D. and Nanson, G.C. (1993) Anastomosis Smith, D.G. (1983) Anastomosed fluvial deposits: and the continuum of the . Earth Surf. modern examples from Western Canada. In: Modern Process. Landf., 18, 613–625. and Ancient Fluvial Systems (Eds J. Collinson and Locking, T. (1983) Hydrology and sediment transport J. Lewin). Spec. Publs Int. Ass. Sediment., 6, 155– in an anastomosing reach of the Upper Columbia 168. River, B.C. Unpublished MSc Thesis, Department of Smith, D.G. (1986) Anastomosing river deposits, sedi- Geography, University of Calgary, Canada, 107 pp. mentation rates and basin subsidence, Magdalena Machusick, M.D. (2000) The effect of floodbasin type River, northwestern Colombia, South America. on crevasse splay form. Unpublished BSc. Thesis, Sediment. Geol., 46, 177–196. Department of Geosciences, The Pennsylvania State Smith D.G. and Putnam, P.E. (1980) Anastomosed river University, 55 pp. deposits: modern and ancient examples in Alberta, Makaske, B. (1998) Anastomosing Rivers: Forms, Pro- Canada. Can. J. Earth Sci., 17, 1396–1406. cesses and Sediments. Nederlandse Geografische Studies Smith, D.G. and Smith, N.D. (1980) Sedimentation in Vol. 249, Koninklijk Nederlands Aardrijkskundig anastomosing river systems: examples from alluvial Genootschap/Faculteit Ruimtelijke Wetenschappen, valleys near Banff, Alberta. J. Sediment. Petrol., 50, Universiteit Utrecht, Utrecht, 287 pp. 157–164. Makaske, B. (2001) Anastomosing rivers: a review of Smith, N.D., McCarty, T.S., Ellery, W.N., Merry, C.L. their classification, origin and sedimentary products. and Ruther, H. (1997) Avulsion and anastomosis in Earth Sci. Rev., 53, 149–196. the panhandle region of the Okavango Fan, Botswana. Miller, J.R. (1991) Development of anastomosing Geomorphology, 20, 49–65. channels in south-central Indiana. Geomorphology, 4, Smith, N.D., Slingerland, R.L., Perez-Arlucea, M. and 221–229. Morozova, G.S. (1998) The 1870s avulsion of the Nanson, G.C. and Huang, H.Q. (1999) Anabranching Saskatchewan River. Can. J. Earth Sci., 35, 453–466. rivers: divided efficiency leading to fluvial diversity. Van Rijn, L.C. (1984a) Sediment transport, part I. Bed In: Varieties of Fluvial Form (Eds A. Miller and A. load transport. J. Hydraul. Eng., 110, 1431–1456. Gupta), pp. 477–494. Wiley, New York. Van Rijn, L.C. (1984b) Sediment transport, part II. Nanson, G.C. and Knighton A.D. (1996) Anabranching Suspended load transport. J. Hydraul. Eng., 110, rivers: their cause, character and classification. Earth 1613–1641. Surf. Process. Landf., 21, 217–239. FSC01 11/30/2004 02:19PM Page 16 FSC02 11/30/2004 02:19PM Page 17

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Review of Amazonian depositional systems

ALLEN W. ARCHER Department of Geology, Kansas State University, Manhattan, KS 66506, USA (Email: [email protected])

ABSTRACT

Many types of depositional system exist within the Amazon River basin and surrounding areas. These areas provide a number of valuable analogues for large-scale and tropical palaeoriver systems. Only rarely, however, are such important analogues invoked because of a lack of published information. Herein, major components are summarized among the fluvial, estuarine and coastal depositional environments of the Amazon River basin. Of particular importance in the Amazon system are the recurring depositional cyclicities that affect sedimentation. In the upper reaches of the system, yearly water-level fluctuations are related to seasonal variations in rainfall. In some areas these fluctuations exceed 10 m. The resultant flooding of vast areas of rainforest greatly affects sedimentation. In the lower reaches of the system, the dominant water-level fluctuations are related to tides. Tidal ranges are as high as 6 m at the mouth of the Amazon and tidal influences extend more than 800 km up-river. The result is a vast area of tidally influenced, freshwater environments, which are a poorly documented, but important, depositional setting. There is a variety of river types within the Amazonian system, and the depositional environments along each are greatly affected by their differences in sediment load. Some rivers, which include the tributaries that drain the Andes, are rich in suspended sediment and have alluvial valleys containing features typical of meandering rivers, such as levees, floodplain lakes and nutrient-rich floodplains. Other rivers, with drainages entirely within extensive areas of rainforest, drain deeply weathered terrains with highly leached soils. These rivers lack suspended sediment, but have small amounts of bedload, and do not produce alluvial valleys or levees. From a sequence stratigraphy perspective, the differences among rivers have resulted in considerable variation of response to glacioeustatic-induced sea-level fluctuations and the ensuing inland changes in base level. During low stands, the Amazon deeply incised its valley for thousands of kilometres inland from the present position of the mouth. During transgression, rivers with a high sediment load vertically aggraded and kept pace with base-level rise. At the same time, the valleys of sediment-poor rivers flooded and were transformed into river-mouth lakes. These flooded valleys are not restricted to coast-proximal settings, but can be found at distances more than 1000 km inland from the modern coast. Sediment supply has also differentially affected coastal settings along the Atlantic north and south of the Amazon mouth. The plume of turbid Amazonian water moves north-westward along the coast because of predominant winds and currents. The longshore drift of this sediment-rich plume has resulted in a low- slope, mud-dominated, prograding coast. South of the Amazon mouth, however, the lack of sediment influx has resulted in a complexly embayed erosional coast- line. This southern coast consists of sea cliffs and headlands comprised of Mesozoic and younger rocks, which separate numerous small-scale macrotidal . FSC02 11/30/2004 02:19PM Page 18

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Fig. 1 Comparison of total discharge of the Mississippi, Congo, and Amazon rivers together with relative contribution for the major Amazonian tributaries (adapted from Gibbs, 1967b). Only lower reaches of Mississippi and Congo systems are represented.

and the average slope to the mouth is only about INTRODUCTION 2cmkm−1. The Amazon drainage basin is situated between Amazon drainage basin two ancient crystalline massifs, the Guiana Shield The Amazon River system comprises the largest to the north and the Brazilian Shield to the south, river system on Earth. As described by Sioli with the western margin delineated by the Andes. (1984), the large size of the Amazonian drainage The basin is elongate and is approximately 3000 km basin, which exceeds 7 × 106 km2, and the equat- in length in an east–west direction, about 300-km orial and tropical climate result in tremendous wide in the eastern part and 600- to 800-km wide discharge and a high density of tributaries. The in the western part. This basin contains a thick Amazon complex comprises the world’s largest sequence of Palaeozoic, Mesozoic, and Cenozoic freshwater system and contains approximately rocks ( Junk & Furch, 1985). It is the largest sedi- 20% of the global supply of freshwater (Oltman, mentary basin in the world and is formed upon a 1967). The relative discharge of the Amazon Tertiary plateau consisting of a subdued area of (Fig. 1) has been estimated to be five times that hills and ridges, which are mostly less than 60 m of the Congo River and twelve times that of the above sea level (Irion, 1984). The elevation of Mississippi River (Gibbs, 1967a). much of the basin lies below 100 m (Fig. 2). The slope of the Amazon drainage basin, east of the Andes Mountains, is exceedingly low. At Sequence stratigraphy considerations Iquitos in Peru (Fig. 2), which is a straight-line distance of 2975 km from the Amazon mouth, the From a sequence stratigraphy standpoint, the elevation of the river during low-water stage is Amazon depositional system, as defined by the about 114 m above sea level. Following the actual drainage basin and adjacent areas of the coast and course of the river the distance from the mouth continental shelf, is exceedingly complex. During to Iquitos is 3717 km, making this the longest the last glaciation and lowering of sea level, the navigable inland waterway in the world. At Amazon deeply incised its valley to a position Manaus in Brazil, which is a straight-line dis- approximately 2000 km upstream from the current tance of about 1200 km from the mouth, the mouth (Tricart, 1977). At that time, sea level was elevation above sea level is only about 15 m approximately 100 m lower and sediment was FSC02 11/30/2004 02:19PM Page 19

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Fig. 2 Relative topography and bathymetry of Amazon Basin and surrounding areas. Much of the basin is less than 100 m above modern sea level. The dashed ‘zero line’ is used to measure the extent of the inland penetration of tides and comparison of yearly fluvial flux. Locations of major cities, referred to in the text, are shown along the main channel of the Amazonian system. Topography/bathymetry data are from Smith & Sandwell (1997).

carried through what is now a shelfal submarine influenced the subsequent development of the and deposited in a deep-sea fan termed the Amazon River system. Amazon cone (Milliman et al., 1975; Damuth, The Solimões and Amazon basins are Palaeozoic 1977). The glacial-age sediments of this fan are intracratonic basins bounded by Precambrian dominated by arkosic sands (25 to 60% feldspar), shields. The Purús Arch, a basement feature which suggest the presence of significantly drier located west of Manaus (Fig. 2), separates these climates on the Brazilian and Guiana shields two basins. The Foz do Amazonas Basin, also during sea-level low stand (Damuth & Fairbridge, known as the Marajó Basin, is a Mesozoic feature 1970). Milliman et al. (1975), however, have ques- and is separated from the Amazon Basin by the tioned such interpretations of dramatic climatic Gurupá Arch, which is east of the mouth of change. the Rio Xingu. This arch is a structural high in the Portions of the modern coast exhibit both basement. The Foz do Amazonas Basin relates to submergent and emergent conditions. In general, the Late Jurassic to Early Cretaceous reactivation the coast south of the Amazon mouth is emergent of basement structures, caused by the rifting and and is characterized by rocky headlines and subsequent break-up of Gondwanaland. embayments. In contrast, the coast to the north of The upper Amazon, to the west of Manaus, the mouth, because of the tremendous amounts probably drained to the west prior to the uplift of suspended mud, is a depositional coast. Thus of the Andes. During the Miocene, the area to there are significant differences between these two the east was a vast brackish basin, which changed coastal areas. slowly to a freshwater environment (Beurlen, 1970). Miocene deposition of coarse colluvial material within the basin does not agree with present- Geological evolution of the Amazon day depositional conditions related to the dense drainage basin forest cover. This suggests the former existence of Prior to the Cenozoic Era, neither the Andes a drier, more seasonal climate and would have Mountains nor the Atlantic Ocean existed and the occurred when the river profile was graded to a geological development of these features greatly lower sea level. FSC02 11/30/2004 02:19PM Page 20

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stretches of shoreline catastrophically slumping Types of Amazonian rivers into the river. These phenomena, termed terras The rivers and tributaries within the Amazon caidas (fallen land), can be quite dramatic and the system exhibit significant differences owing to resultant waves have been known to overwhelm variations in sediment types and supply, vegeta- small boats (Sioli, 1975). tion, and relief and geology of the drainage basins. include relict , levees, Wallace (1853) provided a useful and widely ac- back swamps and floodplain lakes. The floodplain cepted river classification, subsequently popular- associated with rivers is termed várzea ized by Sioli (1950), based upon optical properties. (Pires & Prance, 1985). Owing to high suspended The rivers include whitewater, which are light loads, rapid rates of vertical accretion have kept coloured and very turbid owing to high concentra- pace with base-level rise. Bedload of the Rio tions of suspended sediments, blackwater, which Amazonas consists primarily of fine-grained sand. lack inorganic suspended sediment but have a In some places there are small amounts of gravel, significant concentration of humic compounds, particularly near bedrock outcrops. Unlike many and clearwater, which lack suspended materials river systems, however, the average grain size or humic compounds. and degree of sorting within the mainstem bed sediments is essentially constant along the 3300 km reach between Iquitos and Belém (Nordin Whitewater rivers et al., 1980, 1981). This river type includes the Rio Amazonas and its tributaries, primarily the Rios Japurá, Putumayo, Blackwater rivers Napo, Marañón, Ucayali, and Madeira, which drain the Andes. The Madeira is the most eastern The term blackwater is somewhat misleading of this series of whitewater rivers. The headwaters because the water within such rivers is actually consist of readily weathered volcanic and sedi- relatively clear. For the Rio Negro, Sioli (1967) mentary rocks, yielding very high sediment loads compared the river water to slightly contaminated (Dunne et al., 1998). The light colours in these distilled water and stated that the river water has rivers, which are commonly yellowish or brownish the appearance of very weak tea. The low levels of and not actually white, relate to high concentra- suspended sediment, lack of dissolved nutrients tions of dissolved ions and suspended sediment. and acidic conditions prevent the growth of float- Gibbs (1967a,b) estimated that these tributaries ing vegetation. comprise only about 12% of the Amazon drainage Sandy podzolic soils give rise to the blackwater basin, but supply 86% of the dissolved ions and rivers; the colours come from humic acids leached 82% of the suspended sediment. More recent from leaf litter (Leenheer, 1980; Jordan, 1985). estimates suggest that 90 to 95% of the suspended The high concentration of organics creates acidic sediment is derived from the Andes (Meade et al., concentrations and the pH of blackwater rivers 1985; Meade, 1994). is commonly about 4. Although blackwater rivers Whitewater rivers are characterized by mean- lack suspended sediments, they can have signific- dering channel patterns and the ant amounts of sand-rich bedload. One result of produces an abundance of floating and partially the low pH is that these bedload sands are almost submerged trees and other types of terrestrial exclusively quartzose because less stable mineral plant debris. The water is relatively rich in nutri- grains are readily dissolved (Leenheer & Santos, ents and supports the growth of floating herbace- 1980). ous vegetation (as both floating meadows and The combined effects of lack of alluviation islands). Runoff draining the rainforest does not and extensive plant stabilization result in the have typical blackwater colours because organics floodplains of blackwater rivers not having levees, are adsorbed on to clay particles where they are backswamps and floodplain lakes. Thus, riverbanks subsequently decomposed (Jordan, 1985). Erosion are more stable than in whitewater systems and and scour along whitewater rivers, such as the Rio the influx of plant debris into the river is minimal. Amazonas, can undercut banks and result in long Areas of flooded forest are termed igapó (Pires &