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Introduction

JAN ALEXANDER 1 & SUSAN B. MARRIOTT 2 1School of Environmental Sciences, University of East Anglia, Norwich, NR4 7TJ, UK (e-mail: j.alexander@ uea. ac. uk) 2School of Geography and Environmental Management, University of the West of England, Coldharbour Lane, Bristol, BS16 1QY, UK (e-mail: [email protected])

Floodplains are of major socio-economic and system. Mechanisms for the transfer of fine-grained ecological importance, ranging as they do from sediment from a channel to its floodplain have intensely inhabited and industrialized areas, been studied extensively, both in numerical and through high-productivity agricultural land to sites computer simulation models and by field experi- of extraordinary biodiversity and biological ment. The latter is less well documented because of productivity that have suffered little management the difficulties inherent in collecting data during or other human intervention such as some of the flood events. This information, together with the flooded forests of the Amazon Basin. Natural results of sediment budget studies, has also helped floodplains vary in character depending on their in the study of transport and storage of climatic setting, catchment size and character and, contaminants such as heavy metals, pesticides and as a consequence, discharge character and sediment fertilizers. These contaminants tend to be load. Biological communities are sensitive to these associated with particular sediment grain sizes and variations and the major floodplains of the world can, thus, be stored in floodplains for long periods. may be dominated by plant communities with very Eventually, these may be mobilized and have different evolutionary histories. On more local serious biological consequences. scales, there may be much closer ecological, if not The diverse aspects of floodplains, both in terms taxonomic similarities. Floodplain character has of subject area and geographic location, have been changed through geological time because of the studied by various research communities, often evolution of land plants and animals, and changing with little mutual communication. Some flood- atmospheric chemistry, global climate and sea plains and particular processes, such as channel level. bank accretion, have been studied intensively, Over the relatively recent past (c. 50 ka) human sometimes with duplication of research effort; activity has brought about rapid change through, while other areas and processes have been largely for example, forest clearance, water use and ignored. Temperate-climate floodplains such as channel engineering. This book examines both that of the Rhine-Meuse (Asselman; Sehoor et al.) natural features of floodplains whilst taking into are far better documented and understood than account the human impacts on them. This demands either their high- or low-latitude counterparts. But a multi-disciplinary approach and documents the even in well-studied cases such as the Rhine, the evolution of recent research. response of the floodplains and their biological Floodplain deposits reflect the diversity of communities to management or environmental mechanisms by which sediment is transported and change are difficult to predict. deposited. These include transfer from the channel For millennia, floodplains have been favoured during overbank flow, by slope wash from terraces sites for human habitation, because of the and valley sides on distal parts of a floodplain and combination of water supply, fertile soil, navigable by aeolian processes. Apart from colluvial deposits waterways and flat terrain for building and at the edges of a floodplain, most of the material communication. Many of the world's most densely deposited is generally fine-grained - clay/silt to populated areas are on floodplains: yet other fine sand. Floodplains are sinks for this fine- floodplains remain sparsely populated. Floodplains grained material and account for most of the are managed in many ways and for many purposes; transport loss as sediment moves through the some have been managed for long periods and their

From: MARRIOTT,S. B. & ALEXANDER,J. (eds) 1999. Floodplains: InterdisciplinaryApproaches. Geological Society, London, Special Publications, 163, 1-13. 1-86239-050-9/99/$15.00 9 Geological Societyof London 1999. Downloaded from http://sp.lyellcollection.org/ by guest on September 29, 2021

2 J. ALEXANDER ~ S. B. MARRIOTT natural character has been obscured or destroyed. lies adjacent to a stream; geomorphologically, it Floodplain management takes many forms includ- is a landform composed primarily of unconsolid- ing construction of various flood defence systems, ated depositional material derived from sediment engineering for navigation or water-powered mills, being transported by the related stream; hydro- farmland irrigation, fertilization and drainage, logically, it is perhaps best-defined as a landform seasonal vegetation burning, and wetland subject to periodic flooding by the parent stream. management for wildlife. Additionally, indirect management results from engineering works on Although this definition is vague enough to be floodplains, which affect floodwater paths. applicable to most situations, it may not be Mismanagement can result from poor under- adequate for many purposes, such as delineating standing, education, government procedure or the floodplain for administrative decision making. conflicting interests. There is increasing realization In a similar way wetlands can be defined broadly as in some countries that 'hard engineering' to control areas where water table is at or above the land river and floodplain processes has its limits and surface for long enough each year to promote the now engineers are increasingly looking to reinstate formation of hydric soils and to support the 'natural' systems where appropriate. growth of vegetation much of which is emergent Current research into floodplains addresses a (Cowardin et al. 1979). very broad range of physical, chemical, biological, ecological, economic and social problems using Although in many instances wetlands may be very differing techniques. This book consists of equivalent to floodplains there are a lot of cases contributions on many different aspects of a very where the wetland forms a sub-area of a floodplain broad subject area in an attempt to increase and other cases (coastal wetlands) where it is debat- interdisciplinary study. Here, we introduce the able if they have any correspondence. definitions and importance of floodplains and give Flooding defines natural floodplain environ- an overview referring to the topics covered and the ments. Floods control the morphology, the ecology, authors represented in this volume. and the sediment distribution of a floodplain. Thus Floodplains have been variously defined by a floodplain may be defined as an area of relatively geomorphologists and hydrologists, and may mean low relief, adjacent to a stream that floods at least different things to ecologists, engineers and once in a given period. Many works (cf. Nanson & economists. An individual's concept of a floodplain Croke 1992) appear to consider that the floodwater seems to depend on their training (discipline) and in such a definition should be derived as overflow perspective, particularly in respect of geographical from the parent channel. Others would include location and time-scale considered. floods resulting from local runoff or intense rainfall This volume includes studies on areas with a (cf. Alexander et al.), high groundwater (water- wide variety of characteristics under the name of table rising above the topographic surface) and, floodplains, ranging from small areas of temperate even, storm surge events which periodically intro- farmland in Britain (e.g. Cotton et al.; Dinnin & duce marine floodwater into many lowland and Brayshay; Nicholas & MeLelland; O'Donoghue) coastal areas. to vast areas of tropical and Bangladesh Hydraulic definitions of floodplain area (area (Alexander et al.; Hasan et al.) and includes inundated by floods of a particular return period) marine-influenced wetlands (Crooks). The studies are used widely for channel management, insurance are of modern floodplains and deposits of various rate calculations and as design criteria for major geological ages back to the Ordovician (Liu; engineering projects. There is considerable vari- Wright; McCarthey & Plint). ation in the frequency of inundation used to define a hydraulic floodplain. Wolman & Leopold (1957) What is a floodplain? suggested that an active floodplain is an area subject to annual inundation, but Leopold et al. A floodplain is a functional part of a fluvial system. (1964) found that, on average, rivers flood every Its form is the product of a large number of inter- 1.5 years. Such short retuna periods define areas related processes that change over time in response along most rivers that are very small in comparison to external factors. These allocyclic factors such as to what is regarded commonly as a floodplain. The climate change cause variation in, for example, areal extent becomes nearer to the general concept runoff, biological communities, weathering rate of a floodplain (but not the same) if the criterion of and sediment flux. The floodplain, as interpreted by flood frequency is amended to inundation intervals most of the authors in this volume, can be summed of up to 10 years (Schmudde 1968). Most flood up by the broad terms of a definition given by engineering programmes and many administrative Schmudde (1968) decisions rely on the concept of a design flood of ... as a topographic category, it is quite flat and perhaps 100 or 200 year return period (e.g. Philippi Downloaded from http://sp.lyellcollection.org/ by guest on September 29, 2021

INTRODUCTION 3

1996) and this defines a floodplain area based on sediments deposited during each successive genetic flood risk that is much nearer to the concept of a floodplain period are likely to be different, the floodplain held by most geomorphologists, concept of genetic-floodplain depositional units geologists and the general public. The delineation may be useful. This concept is similar but not quite of the floodplain area, then, depends on historical equivalent to the suite of facies deposited between records of inundation and discharge, and empirical successive channel avulsions (rapid changes of models of runoff and flood storage. The position of channel position, Kraus & Asian 1993; Smith et al. the margins will change with time as a result of 1989) and may be related to the development of natural autocyclic processes (e.g. channel migra- parasequences (cf. sequence stratigraphy, Emery tion), allocyclic factors (such as climate change to & Myers, 1996). reduced precipitation and runoff) and management For the purposes of this volume, the active (e.g. embankment construction, damming and floodplain is a relatively flat area adjacent to a landuse change). stream that is periodically (over a period of The hydrological definition or delineation of a 100-200 years) inundated by flood water, at least floodplain does not consider the processes that part of which emanates from the channel. This is, formed the landform nor the nature of the material by definition, a genetic floodplain. Modern flood- that makes up the area. A hydraulic floodplain may plains often include inactive as well as active flood be underlain by floodplain deposits (sediment areas with areas that are now infrequently or never deposited in the floodplain environment) or inundated as a result of channel migration or anything else including, for example, glacial avulsion, discharge or channel capacity change, deposits, volcanic material, basement geology or incision (and terracing) or artificial restriction (Fig. reclaimed land. 1). Nanson & Croke (1992) proposed the term The difficulty in defining a floodplain increases genetic floodplain, which relates a landform to the with discharge variability. In areas with erratic contemporary climatic and hydrological conditions discharge (for example in areas of unreliable of the parent stream. They defined a genetic monsoon rainfall, erratic tropical cyclone patterns floodplain as a or glacial areas that experience j6kulhlaups), the definition of a 100-year flood (or 200-year flood) largely horizontally-bedded alluvial landform and delineation of inundation area is difficult - due adjacent to a channel, separated from the channel to often short monitoring periods and the extent of by banks, and built of sediment transported by channel and overbank change that occurs with the present flow-regime. successive large discharge events. If a shorter Although the concept of genetic floodplains is duration return period is used to define the useful, this definition needs improvement to hydraulically active floodplain, then it may be include areas where a lot of the sediment accumu- contained within the flood channel and be of little lation is by in situ organic growth (as in the Norfolk use for prediction of flood risk. In yet more extreme Broads, cf. Boar et al.) or those which have a large cases where flow is ephemeral, any hydraulic wind transported component as in the Kuiseb River, definition of the floodplain becomes problematic, Namibia (Ward & Swart 1997). In addition, many especially in extreme cases where the parent workers would wish to incorporate areas where a channel may be defined poorly and may change significant part of the floodplain is constructed with each discharge event. from channel bar and bank material as a result of channel migration (cf. Howard 1992, Bridge et al. 1998; Panin et al.). Nanson & Croke's (1992) The upstream and downstream limits of definition is concerned with contemporaneous floodplains conditions, with the provision that environmental change will produce a new genetic floodplain, The upstream and downstream limits of floodplains related to the new conditions (see also Bravard & are debatable (Fig. 2). Streams where the bed is the Peiry). Therefore, most modern floodplains that same width as the valley floor (with steep slopes have evolved through Holocene climate and sea rising from the channel), cannot really be said to level changes (e.g. Cotton et al.; Crooks; Panin et have a floodplain or to support riparian vegetation, al.) are composed of more than one genetic although they may overtop their banks and cause floodplain and are, thus, described as polyphase considerable damage. They do not generally store floodplains by Nanson & Croke (1992). This much sediment, except during periods of channel- concept is comparable to that of an alluvial plain, bed aggradation. Central to this problem is the although the latter may be related to more than one criterion for delineating the point when a bank- parent stream. attached bar becomes part of the floodplain. There Given that the character and distribution of is no well-defined threshold between an upland Downloaded from http://sp.lyellcollection.org/ by guest on September 29, 2021

4 J. ALEXANDER ~ S. B. MARRIOTT

Alluvial plain ...... genetic floodplain I engineered or r'- administrative-7 I floodplain r------n active bluff line

face ~ , %il ~ ' /;:"

/ ' 'S '<" ~E":

/ L" f]+" " .

J J / ~- i..

.I/ ~':~'~" terrace -7%-- .- . ,~(f{:{~.~,) / Jz" edge ; .. / '" S:''""" :::~i~ / /

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Fig. 1. Diagrammatic representation of floodplain and alluvial plain. The floodplain has been modified by engineering such that the active floodplain is restricted by artificial levees. It is debatable whether in this situation the genetic floodplain is also confined by the levees or includes the protected land area between the levees and terrace edge.

stream that has a floodplain and one that has not. problematic. Many people include intertidal and The same valley reach may have a floodplain at supratidal flats as part of the floodplain system (e.g. some point in geological time and not at others Crooks), while others do not. In the geological because of changing runoff. For example, during record the limit of marine influence has been Holocene pluvial periods a valley may have been suggested as the delineation point as in the occupied fully by the channel, while in drier distinction between upper and lower delta plain (cf. periods the valley may have been occupied by an Fielding 1985). Such delineation on many modem underfit stream with floodplains yet underlain by alluvial surfaces may be made on the basis of genetically unrelated channel deposits. Although dominant plant communities or soil conditions, but such upland floodplains, and also those within in other modem sites is difficult and even more canyon or gorge settings, are small in area, discon- difficult in ancient deposits. The extent of marine tinuous, persist for relatively short periods, and are influence on channel and overbank environments insignificant in the geological sedimentary record, varies through time on both short- and long-time they are locally of major importance as building scales. We consider that the floodplain should land despite their inherent flood hazard. include all dominantly terrestrial surfaces adjacent The downstream limit of floodplains is defined to a stream that are periodically inundated with variously. The downstream limit is relatively easy non-marine floodwater. This does not exclude areas to define where a stream enters a lake or sea where that are periodically inundated by marine water and tidal range is small. In such sites, the boundary may does not require definition of a boundary based on be defined as a shoreline and the downstream part the limit of marine influence. of the floodplain may be part of a delta plain. Even in these low-gradient coastal areas, the boundary may alternatively be defined as the transition to salt The distinction between channel and flat, sand or mud flat, mangrove swamp or back barrier lagoon. floodplain In settings with a high tidal range, definition of Most definitions of floodplains refer to the area the seaward limit of a floodplain is more adjacent to the main stream and the channel is not Downloaded from http://sp.lyellcollection.org/ by guest on September 29, 2021

INTRODUCTION 5

I" ~.,~*,~,,' ros .d$~ /_ / oj,,,, ,I

jt ~., i~, e,

-qS-- "\ / r:;'S-"/ I - -%-+ / __.-Z? / - ,< -_= / / t'~ marine ( / ~/~__~ (! / influenced \ / / ::,)a'a~176176176176176 "~/ delta plain" -~.... \X /__.t( ----<, ..... /.

Fig. 2. Diagrammatic representation of floodplain limits. (A) with a high tidal range downstream limit of the floodplain passes into tidal flats, whereas (B) with a small tidal range the downstream limit approximates to the shoreline.

included in the floodplain although tributary and proportion of a floodplain deposit (cf. Kraus & distributary streams may be. In contrast, some Gwinn 1997). regard the floodplain as the whole low-lying area Aerial photography and other forms of remote including the channel(s). The latter obviates the sensing demonstrate that large proportions of many problems associated with distinguishing the floodplains are underlain by channel deposits. This channel in ephemeral or erratic discharge systems results from situations where but is less satisfactory for many purposes, such as (a) channel-belt width approaches floodplain width planning regulation. (cf. Bridge) If the main channel(s) is regarded as distinct (b) avulsion may occur relatively frequently from the floodplain area it is necessary to define the despite a low aggradation rate, or boundary between floodplain and channel and (c) where channel belts migrate (cf. Alexander et establish criteria for the point where a channel bar al. 1994). becomes part of the floodplain or when splay deposits change to channel deposits on avulsion (cf. So what determines when a bar surface becomes Prrez-Arlucea & Smith 1999). It has been demon- part of the floodplain? One criterion would be on strated that avulsion deposits (those sediments the basis of the mean shoreline position, which in deposited during an avulsion rather than between perennial channels approximates to the vegetation subsequent events) may make up a significant line (the boundary between lower bar and upper bar Downloaded from http://sp.lyellcollection.org/ by guest on September 29, 2021

6 J. ALEXANDER ~ S. B. MARRIOTT deposits cf. Bridge et al. 1998) or the marginal zone downstream that are used for agricultural (National Rivers Authority 1992). This is unsatis- production. factory in many (most?) cases as the criterion for Near coasts, floodwater may be augmented by delineating floodplain boundary and totally storm surge, driving saline floodwater inland. inadequate in ephemeral systems and those with Saline water in channel settings can poison fish, very variable discharge. Better distinction may be while on farmland it can damage standing crops made by defining areas of dominantly riparian and with effects lasting for some years. dominantly floodplain plant communities (cf. The beneficial aspects of flooding are less Fielding et al. 1997) or possibly by defining obvious to many people, and particularly to those hydraulic bankfull level, for example, on mean whose dwellings are at risk of flood inundation. annual flood. However defined, channel migration Flooding promotes exchange of materials and results in channel bars becoming part of the organisms between habitats and plays a key role in floodplain. This is addressed in part in a case determining the level of biological productivity and example in the Brahmaputra-Jamuna River by diversity (Petts 1996). Flooding is particularly Hasan et al. beneficial for soil fertility. One classical illustration of this is in the Nile Valley agricultural land which before the construction of the High Aswan dam in Flooding 1964 was supplied with nutrient rich sediment and Flooding represents a major human hazard. water with each annual flood. In other areas, such Hazards such as drowning or structural damage by as the Kakadu National Park, Australia, flood currents are fairly obvious, whilst others are more distribution influences forest fire risk and fire type. insidious. Flooding also brings many benefits, particularly for ecological variability and, locally, Flood risk soil fertility. The predominant human desire to reduce or prevent flooding may not be the best The risk of flooding may be predicted from management strategy in the long term, and in empirical data and catchment monitoring. To assess several areas this practice is being reversed (cf. flood risk fully requires full documentation of Andrews; Schoor et al.; Adams & Perrow). hydrological characteristics, including records of Many of the world's floodplains support the life peak discharges, flood stage and duration, flood- cycles of the 60 or so species of the mosquito genus wave velocity, rate of rise to the flood crests, water Anopheles that transfers the malaria parasite velocities, sedimentation and degradation patterns plasmodium between humans. Over two billion in the channel, flood channels and flood basins and people are exposed to malaria and around 250 the effects of floods on water quality. It is million suffer from it at any one time. Floodwater impossible to obtain such data for all points in may form standing pools that persist for long every catchment, and in many cases there are few enough to allow mosquito reproduction and or no such records, consequently, hydrological consequent increases in malaria, dengue fever and modelling based on available data has to be used. bilharzia. Many parasites, diseases and pests For floodplain management it is important to become more widespread during or following understand flood flow behaviour, but it is generally floods (e.g. liver fluke, leptospira; causing for difficult to measure the nature of flow and sediment example Weil's disease in people). Any infectious transport during flood events. This difficulty is pathogen (such as the bacteria Brucella and compounded where the floodplain covers large Salmonella) that is shed in urine or faeces in high areas. Investigation of the nature of flow in a numbers and survives for weeks or months, can be channel bounded on one or both sides by relatively spread in floodwater from one area to another (one shallow flow on the floodplains (the problem of farm to another) and deposited on pasture land to compound channel flow) has been studied infect new herds and flocks. theoretically and using experimental approaches An insidious hazard of flooding is its effect on (Sellin et al. 1993; James 1985). Most experimental pollutant transport and redistribution. Flooding can studies have used fixed-bed models to study directly cause pollution incidents by, for example, flooding problems (e.g. Loveless et al. 1999) but as flooding waste sites or preventing free drainage of bed, bank and floodplain changes are common sewage. Floodwater may remobilize previously during flooding, research is needed with mobile- deposited pollutants, an example of this is the bed experiments (cf. Smith 1998; Ashworth et al. redistribution of radiocaesum originating from the 1997). The study of flow patterns in natural floods 1986 Chernobyl accident (van der Perk et al.). The is more difficult. A small case study of the radiocaesum is readily absorbed by soil particles, hydrodynamics of a floodplain recirculation zone particularly illites, and these are seasonally eroded is described in this volume by Nicholas & and redeposited on the banks and floodplains McLelland. Downloaded from http://sp.lyellcollection.org/ by guest on September 29, 2021

INTRODUCTION 7

Increased population pressure on floodplains Interception of runoff and extraction from rivers results directly in increased risk to human life and locally make major impacts on channel and increased economic loss in any individual flood floodplain behaviour downstream. A striking event. In many areas, land use change and example of this is increased water usage from rivers associated increased rates of surface runoff have flowing into the Kruger National Park in South increased the risk of flooding because of reduced Africa (Heritage et al. 1997). For example, the lag times and more peaked flood hydrographs. increasing extraction from the Letaba River has Change in catchment or channel management resulted in that river becoming ephemeral in the often leads to changes in flood character and Park and in Mozambique. Predicted increased intensity, either intentionally or unintentionally. demand in the adjacent Sabie catchment suggests One example of this is the change in flood size that rivers in this catchment will change in a similar distribution along the lower of NE way, with consequent changes to sediment move- Australia that resulted from the construction of the ment and ecology (Heritage et al. 1997). The pro- Burdekin Falls Dam (Alexander et al.). The long- blems of water usage from rivers are increasingly term response of the Burdekin system to the complicated where rivers flow across adminis- changed flood distribution is unknown and may trative boundaries when water extraction and have serious connotations for channel stability. damming in one country affects others. This is Another, now classic example is the High Aswan illustrated by Conway's (in press) discussion of Dam that was built in 1964 for water storage, water usage along the Nile. predominately (90%) to supply year-round irriga- Floodplain sediments form important aquifers in tion. The dam catches all of the annual flood such many areas. Given anticipated human population that the Nile downstream of the dam is now growth and threats in many parts of the world water effectively a canal (Said 1993). resources in floodplain sediments are of great The nature of floodplains changes where the importance both as primary water sources and as flood's size distribution and character change. For water storage sites. One example of this is the example there is natural variation in the area of groundwater storage in the North China Plain inland wetlands along the Nile River system that (Zhao et al.), and another is the proposed use of results from changing inflows due to upstream aquifers in palaeochannels of the Ken River in changes in rainfall distribution; notable area India to alleviate acute water shortages (Gautam reductions occurred during the 1960s (Conway in 1994). press; Conway & Hulme 1993; Howell et al. 1988). Floodplain waterways are important for fisher- Similar changes can be caused by managed changes ies, navigation and recreation. There is economic in discharge pattems. interest in maintaining navigation routes and also Many fluvial systems are being managed to considerable environmental impact of such traffic reduce flood risk (both predictability and scale). on bank erosion, ecology and so channel behaviour. This is the aim of the floodplain restoration along the Cosumnes River, California, USA (Andrews). In this example, re-establishing cottonwood forest Sedimentation and floodplain deposits and riparian oak woodland is reversing the changes caused by earlier land clearance. The author has Floodplains are natural sites of sediment storage used an understanding of floodplain ecology to and also act temporarily, semi-permanently or improve management. Similarly, the establishment possibly permanently as pollution traps. Sediment of alluvial forest species in European floodplains is accumulates in floodplains by: channel bank underpinned by information on, for example, the accretion (point bar growth and translation, braid timing of seed dispersal, germination ecology and bar bank attachment, lateral bar expansion), the responses of tree seedlings to water level sediment fall out from overbank flow, authigenic fluctuations (van Splunder et al. 1995, 1996). mineral precipitation (associated with flood and groundwater evaporation), organic growth, aeolian dust and sand input, and more rarely, volcanic or marine sedimentary input. Of these, channel bank Water resources on floodplains accretion has been documented best because it is Floodplain water bodies (rivers, tributary and often relatively rapid, dynamic and studied distributary streams, lakes and ponds) are major relatively easily: it is easier to measure a change in water resources for domestic, agricultural and channel position than a vertical change in field industrial use. Water is often extracted from elevation. This predominance of channel studies floodplain deposits (alluvial aquifers) and also may also partly relate to the greater ease in intercepted (precipitation) before entering the observation and also in teaching a visually dynamic fluvial system. subject. Vertical accretion has been measured in a Downloaded from http://sp.lyellcollection.org/ by guest on September 29, 2021

8 J. ALEXANDER ~ S. B. MARRIOTT variety of ways and has become more achievable Primary successions of this type are a major mode with modern geochemical/isotopic and geodetic of natural forest regeneration in many areas, such as methods (cf. Asselman; Walling). Channel the tropical lowlands of the Amazon (Kalliola et al. avulsion is demonstrably important in floodplain 1992) and bottomland areas of the SE Coastal Plain construction, but because of its relatively of USA (Shankman, 1993). In such settings infrequent occurrence it has only very recently pioneers of longer-term ecological successions become the topic of significant research (e.g. P6rez- colonize the newly emerged bar surfaces and later Arlucea & Smith, 1999). stages occur on older surfaces, ensuring habitat spatial heterogeneity. Channelization (stablizing channels) prevents large-scale natural disturbance and thus may cause more homogenous forests with Bank accretion deposits less ecological habitat variation. There have been numerous studies of channel bar growth and bank accretion resulting in floodplain construction (cf. Bridge 1993; Bridge et al. 1995, Overbank or flood basin deposition 1998 and others) and studies of floodplains composed of ancient bar deposits (cf. Alexander et Overbank deposits are those sediments deposited or al. 1994; Panin et al.; and others). This results in a formed on or beyond the river bank (outside the belt of dominantly channel deposits (channel-belt active channel environment) normally resulting in deposits) and the ratio between the channel-belt vertical accretion of the floodplain surfaces. Of width and floodplain width varies greatly (cf. predominant importance in most cases is sediment Bridge). Processes of channel migration and bar deposited from floodwater, although other sedi- and bank accretion lead to deposition of relatively ments, such as peat or wind blown dust, may coarse grade sediment above a basal erosion predominate in some sites. surface. The channel sediment body is commonly Overbank, flood-derived sediment is generally much larger (particularly wider) than the channel finer grained than the contemporary channel responsible for its formation, although, in most thalweg deposits but may be coarse-grained sand cases, the top of the coarse sediment body will not (cf. Burdekin flood deposits, Alexander et al.) and represent the level of the top of the channel banks. locally, gravel (cf. Shaw & Kellerhals 1977). Rates of channel migration have been estimated Sediment distribution (size, composition, and from historical records (e.g. Lecce 1997; Mack & thickness) resulting from individual events is Leeder 1998), radiocarbon dating (e.g. Bridge et al. generally poorly documented. However, Marriott 1995) and dendrochronology (e.g. Everitte 1968; (1992, 1996) has recently compared grain-size Kalicki & Krapiec 1995; Burckhardt & Todd 1998) distributions derived from James' (1985) model for but the complete three-dimensional depositional overbank sedimentation with the distribution history of any bar (let alone channel belt) has not obtained from sediment collected following the been recorded. 1990 flooding on the River Severn (UK) floodplain In-channel processes are largely omitted from and found similar patterns. this volume except where relevant directly to Because of the difficulties in examining floodplain interaction. This does not suggest that sedimentation in or deposits of individual flood in-channel processes are unimportant but rather is events, many workers have attempted to measure or an attempt to redress an imbalance with abundant model sediment aggradation over longer time documentation of channel processes and products. periods (years to thousands of years (Bottril et al.). Howard's (1992) model of meander migration and Most of these approaches require dating and this floodplain aggradation suggests that meander-belt has been achieved by various methods including width is generally very slow to increase beyond palynological studies (e.g. pollen and spore about two or three times the size of the largest stratigraphy, Dinnin & Brayshay), archaeological meander (which itself is controlled largely by material, radiocarbon dating (e.g. Panin et al.), discharge; Schumm 1968) and so, in the absence of tephra chronology and use of other radionuclides channel-belt migration or repeat avulsion, wide from, for example, resulting from nuclear weapon floodplains must be composed largely of overbank testing and the 1986 Chernobyl accident (cf. van deposits. der Perk et al.; Walling). Channel migration and bank accretion are This rapidly increasing but still sparse data set on important in maintaining ecological habitat vari- floodplain aggradation generally tends to confirm ability. Bank accretion forms sites for primary that the rate of aggradation varies with surface age, vegetation successions, which unlike clearance elevation and distance from the main (supplying) sites do not have residual vegetation or soil seed channel (cf. Pizzuto 1987; Walling et al. 1992; banks and so primary plant successions develop. Mertes 1994). Downloaded from http://sp.lyellcollection.org/ by guest on September 29, 2021

INTRODUCTION 9

Abandoned channels Abandoned channel sediment bodies consist of active-channel, syn-abandonment and post- Channels abandoned by avulsion, reach or meander abandonment deposits in varying proportions. The cut off, river capture or plan-form change make up deposits can be classified on the basis of channel- a significant proportion of many floodplains, for fill mechanism into active-channel deposits (bar example see the abandoned channels of the River and bank accretion and bed aggradation); Eden terraces, UK (Cotton et al.) or the lower abandonment fill associated with waning flow Burdekin River (Alexander et al.) (Fig. 3). Before (including products of bank collapse); post- channel abandonment, in-channel processes build abandonment misfit channel deposits; alluvial sediment bodies by active bar and bank migration overbank deposits (sediment brought in by flood and bed aggradation, but the abandonment of the water); organic deposits (such as peat); lacustrine channel inevitably leaves a topographic depres- deposits; and other deposits including sion - processes of bank and bar accretion and bed (i) marine sediments following transgression, aggradation cannot lead to complete filling of a (ii) aeolian dust or sand, river channel. Decreasing channel discharge will (iii) glacial sediments, or lead to sedimentation within the channel, but such (vi) volcanic ash or lava. discharge reduction will be accompanied by declining sediment load capacity and later stages of Although active channel processes are responsible channel fill, therefore, will be finer grained. It for the development of extensive coarse alluvial would be unusual for a stream channel to fill sediment bodies, the channel depression is usually completely with sand during gradual discharge filled by post abandonment processes and such reduction. Many channel abandonment processes deposits may be preserved more commonly in the are relatively rapid, for example, resulting from geological record. abrupt avulsion of a stream to a new course. This Abandoned channel deposits are often of major will leave the abandoned reach with either reduced socio-economic importance for a variety of flow (misfit stream), standing water or dry reasons. Coarse grained sediment often forms depressions. significant shallow aquifers that are exploited

Fig. 3. Photograph of an abandoned channel of the Lower Burdekin River at Kalamia sugar mill near Ayr (Alexander et al.). The abandoned channel is about 500 m across. Downloaded from http://sp.lyellcollection.org/ by guest on September 29, 2021

10 J. ALEXANDER ~ S. B. MARRIOTT easily for both water production and storage (cf. derived from the Chernobyl 1986 accident is taken Zhao et al.). Young abandoned channel reaches into the food chain depends on the geochemical may have pronounced topography and standing characteristics of the soils (van tier Perk). Another water which, depending on its situation, may be is the sporadic and difficult-to-predict occurrence beneficial (for example, water supply or recreation) of acid sulphate soils and consequent hazards to or may be detrimental (for example, as mosquito aquatic life where some sediments are sulphidic breeding areas). (Dent 1986). Abandoned channel deposits are exploited for Organic-rich floodplain deposits have been aggregates (as in the Thames Valley, UK). Such exploited extensively for peat. Such exploitation bulk extraction may create large expanses of water may radically change floodplain form and channel such as the Cotswold Water Parks in the Thames behaviour. For example, digging and dredging in Valley, UK, or be used for landfill sites. Differential Norfolk and Suffolk removed about 25.5 million entrainment, transport and selective deposition in m 3 of peat between about 900-1350 AD and alluvial channels causes local concentration of flooding of the workings created over 1.057 x heavy minerals and consequently economic 10 7 m 3 of interconnected waterways with a mean accumulations of, for example, diamonds and gold, depth of 2.4 m forming a complex of shallow lakes are often found in palaeochannels (e.g. Jones 1991; known as the Norfolk Broads (Smith 1960; Mazzucchelli 1996). Lambert et al. 1965; George 1992). Much larger areas have been exploited in the Low Countries (Pons 1992). In coastal areas the grade of Floodplains as records of environmental agricultural land is influenced by the nature of change sediment deposited. Sandy areas like the Wash (UK), and large parts of the Netherlands have some The wide variety of deposits and features preserved of Europe's highest quality agricultural land. in many floodplains represent a record of environ- Where mud dominates the quality is much poorer. mental change. The ecology and sedimentology of floodplains are environmentally sensitive, so that the sediment and fossil record reflect the sequence Floodplain deposits in the rock record of past change. These records are being studied Alluvial deposits are volumetrically important in increasingly to elucidate both natural and the rock record and have considerable local anthropogenic change (cf. Dinnin & Brayshay). economic significance as hydrocarbon and water The morphology of abandoned channels and reservoirs, and as hosts for mineral deposits such as character of their deposits can be used to interpret gold, diamonds and coal. In addition, they preserve changing hydrological conditions (cf. Panin et al.) evidence of changing terrestrial conditions which may relate to changing catchment runoff. (colonization of the land by plants, evolution, climate and sea level change). Socio-economic importance of floodplain In areas of good exposure, fluvial channel and overbank deposits can be examined together. deposits Overbank deposits tend to be finer grained than in- Floodplain deposits are important in various ways channel deposits because, during normal con- especially as substrate for agricultural production, ditions, in-channel flow velocities are greater than inherent value for mineral and water extraction, overbank velocities. Despite this, coarser grained surface for construction, urban and industrial sediments may be found within floodplain deposits development and storage of contaminants. and fine-grained sediment often fills channels Floodplains provide much more than simply land (particularly abandonment, or post-abandonment for agriculture. Much of the history of human deposits). Channel deposits normally rest on a development has been influenced by floodplain basal erosion surface and often contain complex processes and the wealth of natural resources found hierarchies of sedimentary structures between in floodplain deposits. bounding surfaces which can be interpreted as the Floodplain deposits form soil of enormous products of growth and amalgamation of bars (e.g. spacial variability. This causes problems resulting Allen 1983; Alexander et al. 1994; Bridge et al. from variation in drainage and nutrient supply and 1995, 1998). In most cases it is practical to divide consequently, crop growth over the area of a single alluvial deposits into channel-belt and non-channel- field. Another example is the quality of the crop belt deposits and assume that the bulk of the former grown on differing floodplain deposits (e.g. the will be relatively coarse-grained and the latter quality of thatching reed, Boar et al.). A more relatively fine-grained. This conceptual division is insidious example of the importance of deposit particularly prevalent in the hydrocarbon industry character is that the extent to which radiocaesum where exploration for and production of oil and gas Downloaded from http://sp.lyellcollection.org/ by guest on September 29, 2021

INTRODUCTION 11 require some understanding of coarse deposit (sand References and gravel) distribution. ALEXANDER,J., BRIDGE, J. S., GAWTHORPE,R. L., LEEDER, This distinction between channel-belt and over- M. R. ~ COLLIER, R. E. L. 1994. Holocene meander bank deposits facilitates modelling of deposit belt evolution in an extensional basin, SW Montana, accumulation. Such alluvial architectural simu- USA. Journal of Sedimentary Research, 64B, lation is illustrated by Bridge. It allows consider- 542-559. ation of allocyclic factors such as tectonic tilting or ALLEN, J. R. L. 1983. Studies in fluviatile sedimentation: sea level change. Studies of alluvial architecture bars, bar-complexes and sandstone sheets (low- have largely concentrated on the distribution of sinuosity braided streams) in the Brownstones (L. Devonian), Welsh Borders. Sedimentary Geology, 33, channel-belt deposits (notably hydrocarbon 237-293. reservoir-quality sandstones) and largely ignored ASHWORTH, P., BEST, J. PEAKALL & LORSONG, J. 1997. other facies, even though in most fluvial settings Aggradation controls on braided alluvial architecture: active channels occupy a small percentage of Ashburton River, Canterbury Plains, New Zealand. 6 th floodplain area. International conference on Fluvial Sedimentology Although the coarse-grained sediment of channel abstract volume, International Association of belt deposits are the major reservoir rocks in Sedimentologists. alluvial deposits, the sand- and gravel-grade BRIDGE, J. S. 1993. The interaction between channel sediments in overbank deposits should not be geometry, water flow, sediment transport and deposition in braided rivers. In: BEST, J. L. & BRISTOW, ignored. Although they contribute little to the net C. S. (eds) Braided Rivers, Geological Society, reservoir volume, they may increase hydraulic London, Special Publications, 75, 13-71. connection between channel-belt bodies (either , COLLIER, R. E. L. & ALEXANDER,J. 1998. Large- during production or possibly during hydrocarbon scale structure of Calamus River deposits (braid bars, migration). point bars and channel fills) revealed using ground- Studying subsurface alluvial deposits is con- penetrating radar. Sedimentology, 45, 977-986. siderably more difficult than either examining --., ALEXANDER,J., COLLIER, R. E. L., GAWTHORPE,R. exposures or studying modem fluvial processes. In L. & JARWS, J. 1995. Point-bar deposits documented in the context of oil exploration/production there are 3-D using ground-penetrating radar and coring, and their genetic interpretation, South Esk River, Scotland. often problems in correlating fluvial sandstone Sedimentology, 42, 839-852. bodies between boreholes and in predicting the BURCKHARDT, J. C. & TODD, B. L. 1998. Riparian forest location of channel-belt deposits from boreholes effect on lateral stream channel migration in the glacial penetrating overbank deposits. Recent advances in till plains. Journal of the American Water Resources the study of overbank deposits, promise to be useful Association, 34, 179-184. for understanding sedimentary facies distribution CONWAY, O. in press. Water Resources Development on and, also, recognizing palaeogeographic and the Upper Blue Nile: Some Environmental and environmental change, e.g. in the interpretation of Hydropolitical Considerations. In: The proceedings of palaeoclimate from the micromorphology of Ethiopia and Eritrea since 1991: Development Experience and Prospects, Natural Resources Institute. palaeosols (cf. McCarthy & Plint). & HULME, M. 1993. Recent fluctuations in precipitation and runoff over the Nile subbasins and their impacts on Main Nile discharge. Climatic Conclusion Change, 25, 127-151. COWARDIN,L. M., CARTER, V., GOLET, E C. 8,: LA ROE, E. Floodplains are a big and complex subject. We now T. 1979. Classification of wetlands and deepwater know enough about them to know that we know habitats in the United States. US Department of Interior very little. In view of their socio-economic and Fish and Wildlife Service Report FWS/OBS-79/3 I. ecological importance it is imperative that we DENT, D. L. 1986. Acid sulphate soils: a baseline for continue to undertake research into the character of research and development. International Institute for Reclamation and Improvement, Publication, 39, floodplains so that they can be managed and Wageningen, Netherlands, 1-203. developed in a sustainable manner and the deposits EMERY, D. & MYERS, K. J. 1996. Sequence Stratigraphy, can be exploited to best advantage. Blackwell Science, London. EVERITTE, B. L. 1968. Use of the cottonwood in an Writing this introduction, organizing the floodplains investigation of the recent history of a floodplain. conference, and editing this volume have all been thought American Journal of Science, 266, 417-439. provoking. Discussions with many people have helped to FIELDING, C. R. 1985. Coal depositional models and the stimulate our ideas. Thank you to all participants at every distinction between alluvial and delta plain stage. Particular thanks to Rosie Cullington and Phil environments. Sedimentary Geology, 42, 41~48. Judge for their help in preparation of this paper. This , ALEXANDER,J. & NEWMAN-SUTHERLAND,E. 1997. paper has been greatly improved by constructive Preservation in situ vegetation in fluvial channel comments on earlier versions by R. B. Boar, D. Conway, deposits - data from the modern Burdekin River of S. Crooks, D. L. Dent and M. S. Stoker. Downloaded from http://sp.lyellcollection.org/ by guest on September 29, 2021

] 2 J. ALEXANDER • S. B. MARRIOTT

north , Australia. PaIaeogeography, a flood deposit, River Severn, UK. Earth Surface Palaeoclimatology, Palaeoecology, 135, 123-144. Processes and Landforms, 17, 687-697.

GAUTAM 1994. Paleochannels of Ken River as potential -- 1996. Analysis and modelling of overbank deposits. groundwater reservoirs and geomorphic evolution of In: ANDERSON, M. G., WALLING, D. E. & BATES, 19. D. adjoining area, Banda and Hamirpur Districts, UE (eds) Floodplain Processes. John Wiley, Chichester, Indian-Minerals, 48, 181-184. 63-94. GEORGE, M. 1992. The Land Use, Ecology and MAZZUCCHELLI, R. H. 1996. The application of soil Conservation of Broadland, Package Publishing geochemistry to gold exploration in the Black Flag Limited, Chichester. area, Yilgarn Block, Western Australia. Journal of HERITAGE, G., BIRKHEAD, A. & VAN COLLER, A. 1997. Geochemical Exploration, 57/1-3, 175-185. Catchment influences on the geomorphology and biota MERTES, L. A. K. 1994. Rates of floodplain sedimentation of the Sabie and Letaba rivers in the Kruger National on the central Amazon River. Geology, 22, 171-174. Park. Field guide 3, 6th International Conference on NANSON, G. C. & CROKE, J. C. 1992. A genetic Fluvial Sedimentology, University of Capetown classification of floodplains. Geomorphology, 4, International Association of Sedimentologists. 459-486. HOWARD, A. D. 1992. Modelling channel migration and NATIONAL RIVERS AUTHORITY 1992. River Corridor floodplain development in meandering streams. In: Surveys: Methods and Procedures. Conservation CARLING, E A. & PETTS, G. E. (eds) Lowland Technical Handbook No. 1, NRA Bristol. Floodplain Rivers, John Wiley and Sons, Chichester, Ports, L. J. 1992. Holocene peat formation in the lower 1-42. parts of the Netherlands. In: VERHOEVEN,J. T. A. (ed.) HOWELL, E E, LocK, M. & COBB, S. 1988. The Jonglei Fens and bogs in the Netherlands, Kluwer, Germany, Canal: Impact and Opportunity. Cambridge University 7-79. Press, Cambridge. PI~REZ-ARLUCEA, M. & SMITH, N. D. 1999. Depositional JAMES, C. S. 1985. Sediment transfer to overbank patterns following the 1870s avulsion of the sections. Journal of Hydraulic Research, 23, 435-452. Saskatchewan River (Cumberland Marshes, JONES, R. 1991. River diversions at Aredor Mine, Guinea, Saskatchewan, Canada). Journal of Sedimentary West Africa. Institution of Mining and Metallurgy - Research, 69, 62-73. Section A. 100, Al15-AI20. PETrS, G. E. 1996. Sustaining the ecological integrity of KALIC~, T. & KRAPmC, M. 1995. Problems of dating large floodplain rivers. In: ANDERSON, M. G., WALLING, alluvium using buried subfossil tree trunks - lessons D. E. & BATES, P. D. (eds) Floodplain Processes, John from the black-oaks of the Vistula Valley central Wiley, Chichester, 540-551. Europe. Holocene, 5, 243-250. PHILIPPI, N. S. 1996. Floodplain Management - Ecologic KALLIOLA, R., SALt, J., PUHAKKA, M. & RAJASILTA, M. and Economic Perspectives. Academic Press, 1992. New site formation and colonizing vegetation in California. primary succession on the western Amazon PlZZUTO, J. E. 1987. Sediment diffusion during overbank floodplains. Journal of Ecology, 79, 877-901. flows. Sedimentology, 34, 301-317. KRAUS, M. J. & ASLAN, A. 1993. Eocene hydromorphic SAID, R. 1993. The River Nile. Geology, Hydrology and paleosols: significance for interpreting ancient Utilization. Elsevier Science, Oxford. floodplain processes. Journal of Sedimentary SCHMUDDE, T. H. 1968. Floodplains. In: FAIRBRIDGE, R. Petrology, 63, 453-463. W. (ed.) The Encylopedia of GeomorphoIogy. Reinhold

-- & GW~NN, B. 1997. Facies and facies architecture of Book Corporation, New York, 359-362. paleogene deposits, Willwood Formation, Bighorn SCHUMM, S. A. 1968. Speculations concerning palaeo- Basin, Wyoming, USA. Sedimentary Geology, 114, hydraulic controls of terrestrial sedimentation. 33-54. Geological Society of America Bulletin, 79, LAMBERT, J. M., JENNINGS, J. N. & SMITH, C. T. 1965. The 1573-1588. origin of the Broads. In: ELLIS, E. A. (ed.) The Broads, SELLIN, R. H. J. ENVINE, D. A. & WILLETTS, B. B. 1993. Collins, London, 37-65. Behaviour of meandering two stage channels, Journal LECCE, S. A. 1997. Spatial patterns of historical overbank of Water and Maritime Engineering, 101, 99-111. sedimentation and floodplain evolution: Blue River, SHANKMAN, D. 1993. Channel Migration and vegetation Wisconsin. Geomorphology, 18, 265-277. patterns in the southeastern coastal plain. Conservation LEOPOLD, L. B., WOLMAN, M. G. & MILLER, J. P. 1964. Biology, 7, 176-183. Fluvial Processes in Geomorphology, W. H. Freeman, SHAW, J. ~; KELLERHALS, R. 1977. Palaeohydraulic San Francisco. interpetation of antidune bedforrns with application to LOVELESS, J. H., SELLIN, R. J. H., BRYANT, T. antidunes in gravel. Journal of Sedimentary Petrology, WORMLEATON, P. R. CATMUR, S. & HEY, R. D. 1999. 47, 257-266. Experiments with meandering mobile bed channels SMITH, C. E. 1998. Modelling in high sinuosity meanders having overbank flow. IAHR Symposium on river in a small flume. Geomorphology, 25, 19-30. coastal and esturine morphodynamic, Conference SMITH, C. T. 1960. Historical evidence. In. MURRAY, J. volume. (ed.) The Making of the Broads, Royal Geographical MACK, G. H. & LEEDER, M. R. 1998. Channel shifting of Society, London, Research Series, 3, 63-111. the Rio Grande, Southern Rio Grande rift: implications SMITH, N. D., CROSS, T. A. DUFFICY, J. P. & CLOUGH,S. R. for alluvial stratigraphic models. Sedimentary Geology, 1989. Anatomy of an avulsion, Sedimentology, 36, 177, 207-219. 1-23. MARRIOTT, S. B. 1992. Textural analysis and modelling of VAN SPLUNDER, I., CooPs, H., VOESENEK, L. A. C. J. & Downloaded from http://sp.lyellcollection.org/ by guest on September 29, 2021

INTRODUCTION 13

BLOM, C. W. E W. 1995. Establishment of alluvial Lowland Floodplain Rivers, John Wiley and Sons, forest species in floodplains: the role of dispersal Chichester, 165-184. timing, germination characteristics and water-level WARD, J. D. & SWART, R. 1997. Flash flood fluvial fluctuations. Acta Botanica Neerlandica, 44, 269-278. systems of the Central Namib Desert. Field Excursion --, VOESENEK, L. A. C. J., CooPs, H., DE VRmS, X. J. Guidebook 10, 6th International Conference on Fluvial A. & BLOM, C. W. E W. 1996. Morphological Sedimentology, University of Capetown, International responses of seedlings of four species of Salicaceae to Association of Sedimentologists. drought. Canadian Journal of Botany, 74, 1988-1995. WOLMAN, M. G. & LEOPOLD, L. B. 1957. River flood WALLING, D. E., QUINE, T. A. & HE, Q. 1992. plains: some observations on their formation. US Investigation of contemporary rates of floodplain Geological Survey Professional Paper, 282-C, sedimentation. In: CARLrN6, R A. & PETTS, G. E. (eds) 87-109.