Chapter 1

An introduction to the fluvial

geomorphology of Britain

K.J. Gregory

From:Gregory, K.J. (Ed.), (1997), Fluvial Geomorphology of Great Britain, Geological Conservation Review Series, No.13, Chapman and Hall, London, 348 pages, illustrations, A4 hardback, ISBN 0 41278 930 2For more information see: http://www.jncc.gov.uk/page-3013

Introduction

3

INTRODUCTION

Scenery in Britain is closely associated withrivers. Because no area is very far from the seathere are no very large rivers, but since the coun-try includes areas with more than 1000 mm ofprecipitation annually (Figure 1.1), there arelarge numbers of rivers and streams. TheThames has the largest drainage basin with anarea of 9950 km2, but this is only 0.14% of thearea of the world’s largest river basin, theAmazon. Although the Thames is the largestBritish river, according to drainage basin sizeand also according to length of the main river(239 km), it is not the largest British riveraccording to mean annual flow. The Tay inScotland has a mean flow 2.26 times greater thanthat of the Thames, and the flows of the Trent,the Ness, the Tweed and the Wye are also greaterthan that of the Thames.

The prominence of rivers and streams in theBritish landscape has been echoed by landscapepainters such as Constable and Turner, and ithas been emphasized in prose and in poetry forexample by Ted Hughes (1983) in his book‘River’. Rivers have also been regarded prag-matically as an integral part of the rural environ-ment and they have played an important role inthe location of sites vital for industry. Rivers inthe British landscape have often been associatedwith leisure, and Isaak Walton’s book the ‘TheCompleat Angler’, first published in 1653, hasrun to 300 reprints.

A vision of rivers and streams in the Britishlandscape as constant and unchanging is perhaps an unfortunate perception because,although British rivers are not subject to violentchanges, they have been affected by significantvariations in the past. Fluvial geomorphology isthe branch of earth science that is particularlyconcerned with rivers and with their presentbehaviour, the effects that they have in contem-porary scenery, and the ways in which they havedeveloped in the past. An understanding ofrivers past and present can provide an indicationof how rivers might change further in the future.The development of fluvial geomorphologyprovides a background to the GeologicalConservation Review (GCR) sites described inthis volume.

In the scientific study of scenery, rivers haveenjoyed a prominent role. Until about 1830 thetraditional view was that one sudden, violent

and extraordinary event – the Noachian flood –had fashioned most of the Earth’s scenery, andthis was the simplest version of diluvialismwhich has recently been analysed in detail byHuggett (1989). This diluvial view was succeed-ed by a more uniformitarian interpretation oflandscape development, to which Charles Lyellwas a particularly significant contributor, andwhich is associated with the notion that the pres-ent is the key to the past (Lyell, 1835). Keypoints in the uniformitarian approach to theshaping of scenery were the facts that rivers aresustained by the precipitation falling over theirdrainage basins and that the basin is the unit forcalculating a water balance. Although theseideas had been established by P. Perrault (1674)for the Seine basin in France, it was only duringthe 19th century that their significance graduallybecame registered. Thus George Greenwood(1857) in his book ‘Rain and Rivers’ suggestedhow rain and rivers shaped the scenery ofBritain and of other parts of the world.

An American geomorphologist, WilliamMorris Davis, at the end of the 19th century andduring the first part of the 20th century, pro-posed an approach to the study of scenery thathad a very significant impact. He suggested thatrivers were the central part of the normal cycleof erosion, that the scenery of an area could beinterpreted in relation to its geological structure,the processes operating, and the stage of ero-sion that had been achieved or the length oftime over it operated, and that the cycle of ero-sion proceeded in stages from youth to maturityand thence to old age. His 1899 paper relatedhis ideas to British river development and thesewere explored in research during the next 50years. Contributions from this research centredon the evolution of river systems, including theearly origins of major eastward-flowing systems(Figure 1.2), the association of stages of riverdevelopment with remnants of older land sur-faces or planation surfaces, and the Quaternarydevelopment of river valleys reconstructed fromthe remnants of former valley floors anddeposits still preserved on valley sides.

Although the importance of understandingriver processes had been acknowledged sinceDavis, there had been few quantitative investiga-tions of the controls upon river and streambehaviour. Although Gilbert (1887) had devel-oped what later came to be recognized as thepotential basis for an approach to geomorpholo-

An introduction to the geomorphology of Britain

4

gy founded upon analysis of processes, thiscould not be adopted as easily as the approachadvocated by W.M. Davis (1899) and so was notpursued until the mid-20th century. Therefore,since the 1960s, greater attention in research hasbeen accorded to fluvial processes and initiallyto studies of small drainage basins (Gregory,1978). Improved understanding of riverprocesses has also provided a basis for analysingthe impact of human activity on rivers and theirbasins. In addition, it has been possible toimprove understanding of past river systems,and palaeohydrology has been developed as aninvestigation of the hydrological records(Gregory, 1983). Research has therefore provid-ed a basis for understanding river mechanics inthe future against the background of the analysisof contemporary and past river behaviour. Thus,much of the early phase of modern fluvial geo-

morphology was based on using scientific prin-ciples and fundamentals to understand fluvialprocesses and their operation in a system. Muchwork was aimed at identifying regular and sys-tematic variations and was based on an assump-tion of equilibrium between form and process.For a time, there was a phase of intense concen-tration on small-scale and short-term processes,with use of instrumentation being a key compo-nent. Since then, analysis of changes at varioustimescales, including that of human impact, hasgained significance and the link between formsand processes has been re-emphasized. Theevolution of ideas gives a basis for an outline,below, of river processes, an indication of howriver history has been deciphered, and sugges-tions of the types of river system that have nowdeveloped in relation to the pressures that riversystems have to sustain.

Figure 1.1 (a) Annual precipitation for the whole of Britain (after Ward, 1981). (b) The discharge ratio for thewhole of Britain (after Ward, 1981).

River processes

5

RIVER PROCESSES

Rivers and streams act like a conveyor belt,which is critical for the development of scenery.The network of river channels that makes up thefluvial system is really a series of linked convey-or belts because water and sediments do notprogress continuously through the system.Storage of water in lakes or in the deeper poolsof river channels occurs for relatively short peri-ods of time, but sediment and solutes can bestored in or adjacent to river channels for muchlonger periods of time.

River discharge (Q) derives from precipitation(P) and these can be related in a simple

hydrological or water balance equation in which

Q = P – E ± S

where E is evaporation from land and water sur-faces and S represents changes in storage ofground water and soil moisture. The amount ofevaporation can be greater than 50% of the pre-cipitation received in southern and easternEngland, although in the Highland zone it canbe less than 25% of total annual precipitation.The proportion of precipitation that eventuallyreaches the river channel can follow one of twogeneral routes. Delayed flow is water that infil-trates through the soil and rock to reach thegroundwater table and subsequently emergesfrom springs to provide a base flow componentof river discharge. Quick flow is that portion ofprecipitation which either flows over the groundsurface as overland flow, falls directly onto thestream channel, or infiltrates into the soil or intothe rock above the water table and then flowslaterally beneath the ground surface towards theriver or stream channels. Water can follow avariety of pathways: these range from matrixflow, where the water flows through very smallspaces in soils, to flow through soil “pipes’which are often 10 cm in diameter and some-times much larger. Flow can also occur at dif-ferent levels, through the soil as throughflow, orthrough the unsaturated part of the rock asinterflow.

Whenever precipitation falls on a drainagebasin, the amount of water that will flowthrough the basin and the routes that it followswill be determined by the characteristics of thebasin, including the rock type, soil, topographi-cal shape, and land use and vegetation, and alsoby the preceding weather conditions. If theground water levels are high, then a differentpattern of flow routes will be used from the onethat will operate when ground water levels arelow after a dry period. In essence, this is thebasis for a dynamic view of river discharge pro-duction that is now an integral part of hydrologi-cal models of river flow through drainage basins.

The ‘conveyor belt’ also transports solid mate-rial, which is rolled or jumped along the bed ofthe channel as bedload, or carried as sedimentsuspended in the stream flow. Some materialsdissolve in the water and are transported insolution. Such solutes are derived from a great

Figure 1.2 The original drainage pattern for Englandand Wales as initiated by Early Tertiary uplift of thelate Cretaceous sea floor (Brown, 1960), and the ini-tial watersheds and initial drainage of Scotland (afterSissons, 1967).

An introduction to the geomorphology of Britain

6

variety of sources since they can be present inthe precipitation that falls over the basin, theyare obtained from vegetation from the surfacesof leaves for example, and they are derived fromsoils and weathered rock and also from the rocktypes which underlie the drainage basin. At theother extreme, bedload is obtained largely byerosion of the river bed and the banks of theriver channel. The size (calibre) of the bedmaterial available to the river may reflect thelegacy of glaciation, if glacial deposits provide asource of bedload for the river. During down-stream transport of the bedload, considerabledifferences exist in the size of the material,which decreases downstream; in the roundnessof the material, which rapidly increases to a par-ticular level and is then maintained; and in thecomposition of the bed sediments because somesurvive longer than others. Intermediate incharacter is the suspended sediment, which canbe derived from channel bed and banks, but alsofrom slopes, pipes and cultivated areas.

Measurements of material transported assolutes and sediment have been necessary toestimate the rates at which river scenery is beingchanged. There are many was of estimatingrates of erosion and the rates deduced necessar-ily vary according to the size of areas investigat-ed, with the highest values obtained from small,easily eroded basins with steep slopes andincomplete vegetation cover. Ranges of valuesobtained for rates of erosion in Britain include:

Accumulation of sediment in reservoirs:equivalent to 10–1000 mm lowering of theland surface per 100 years

Calculation of sediment transported by rivers insolution and suspension:

equivalent to 2–10 000 mm per 100 yearsMeasurement of specific small areas:

in gulleys – 16 000 mm per 100 yearsover slopes – 1400 mm per 100 yearsunvegetated moorland –- 3810 mm per 100yearserosion of cultivated fields – 60–1050 mmper 100 years

Some idea of rates of deposition in Britain hasbeen obtained from measurements of flood-plains in lowland Britain, where increases of1–10 mm in the land surface occur per year.

If erosion was allowed to continue uninter-rupted at the estimated present-day rates indi-

cated above, then it would take several millionyears for the scenery of Britain to be eroded tosiginificantly lower levels. In most rivers inBritain, the greatest amount of load, calculatedby relating the amount of solutes and sedimentstransported to the water discharge available, istransported as solutes. However, althoughsolutes are very significant in the generaldenudation of the landscape, sediments – par-ticularly bedload – are the dominant compo-nents of contemporary channel features.

Sediment and solutes do not continue unin-terrupted on the ‘conveyor belt’ throughout anydrainage basin. It is the interaction betweenstorage and transport of sediment and waterwhich gives rise to characteristic forms ofscenery, or “landforms’, associated with riversand their channels. For example, along thecourse of a river channel there is an alternationbetween shallow areas, called riffles, and deeperpools. The spacing of the pools and riffles isdirectly related to the size of the river channel.Similarly, the time for which sediment is “stored’in fluvial systems varies according to the particu-lar location. Within a river channel, sedimentmay accumulate in bars and these are the tem-porary locations for material that is graduallybeing moved downstream by one storm afteranother. Longer-term storage of sediment canoccur in the floodplain, which is the level areaimmediately adjacent to many middle- andlower-course river channels. The floodplain isbuilt up of fine sediments which are slowlydeposited from floodwaters that have exceededthe capacity of the river channel, and also ofcoarser sediment that has been deposited aschannel bars and incorporated into the flood-plain as the river channel has moved position.Over time, river channels gradually shift by ero-sion of one bank and deposition on the other,and in this way meanders can gradually be trans-lated in a downstream direction. During thistranslation, the gravel bars accumulated in thechannel as point bars systematically becomeincorporated within the floodplain sediments.

The ‘conveyor belt’ in the river basin is madeup of streams and rivers which collectively makethe drainage pattern. This pattern is dynamic inextent because, just as the river channels them-selves have low flows for much of the time butcan fill up to bankfull or greater levels, so alsocan the extent of the drainage network increase.Streams are perennial if they flow all year, inter-

The history of fluvial processes

7

mittent if they flow seasonally when the watertable is high, and ephemeral if they flow onlyduring, or immediately after, rainstorms. Thusthe drainage network in any basin expands andcontracts according to the prevailing climatolo-gical conditions. Along the course of individualstreams and rivers in the drainage network, it ispossible to classify their pattern according to theway in which they would appear from the air.This river channel planform is usually dividedinto two major types, those that are single-thread and composed of a single river channel,which may be either meandering or for shortdistances may be straight; and those which havemore than one channel, are multi-thread, andare described as “braided’ river channels. Thebraided channels occur particularly where basinslopes are steeper, where supplies of sedimentare readily available and where river flows varyquite significantly. Conversely, the single-threadmeandering channels tend to occur where thesediment is finer, where the basin slopes tend tobe lower and where there are no unlimitedinputs of sediment into the fluvial system. Riverchannel patterns are not always simply distin-guished as meandering or braided, and inScotland other classifications are used. Riverchannel patterns are not always free to change,because in some locations they are confined bythe valley sides and sometimes they may bestrictly confined by human activity. For example,the development of the railway network inBritain often involved detailed changes to riverpatterns, or necessitated changing the riverchannel to stop erosion that might otherwisehave jeopardized the railway line.

The size and character of any river channel isrelated to the position in the basin and to thecharacteristics of that basin. Thus there is a sim-ple relationship between the size of river mean-ders and the drainage area, and there is also arelationship between the area of the cross-section of the river channel and its position inthe drainage system. The size of a river channelcross-section reflects the discharges that flowalong it, the sort of sediment into which thechannel is cut, and the local characteristics ofslope and vegetation that also exert an influ-ence. Simple relationships established betweenriver channel dimensions and the discharges ofBritish rivers can be used (Wharton et al., 1988)to estimate river discharge from natural riverchannel dimensions. Thus river channel width

(W in metres) can be used to give an approxi-mate indication of the discharge Q (1.5) in cubicmetres per second, which is the flow that wouldoccur in the river channel at least on averageonce every 1.5 years, using the following equa-tion:

Q (1.5) = 0.217 W1.76

The general relationship that exists between theform of river channels and river landforms withfluvial processes should not lead to the assump-tion that all morphological features are pro-duced by events that occur on average once ortwice each year. It has been realized that rareflood events can have significant impacts inshaping river channels and drainage systems inBritain. Although Britain does not receive theextreme precipitation or flood events that occurin some parts of the world, nevertheless extremeevents are experienced, and the effects of suchfloods at more than 70 locations over the periodsince 1686 have been recorded in the literature.For 39 of those events it has been possible toderive an estimate of the rainfall that producedthe flood, and the large events are plotted inFigure 1.3 in relation to the line of maximumknown falls in the Britain and a higher envelopeline showing the maximum world falls. Thisinventory of flood events was compiled usingthose recorded by Beven and Carling (1989) as abasis, and the envelope curve for rainfall excess-es was derived by Rodda (1970). The signifi-cance of such events is a reminder of the occa-sional impact of large floods: a number of thesites in this volume record aspects of floodevents and are important because they offerresults which are similar to a type of laboratoryexperiment. Subsequently it is important thatwe retain evidence of a particular flood eventand also are able to analyse the rate at which thatevidence is changed and assimilated into the flu-vial system.

THE HISTORY OF FLUVIALPROCESSES

There is no easy separation between what ispresent and what is past as far as the contribu-tion of river activity to scenery is concerned.This is exemplified by flood events, since theimpact of a recent large flood could be thoughtof as a rare event typical for present conditions,

An introduction to the geomorphology of Britain

8

or it could be envisaged as an example of a typeof flood that occurred in the past. Thus theLynmouth flood, that devastated Lynmouth andthe East and West Lyn river channels in 1952,was the sort of event that could happen onaverage once in 500–1000 years under presentclimatic conditions. Furthermore, it is not easyto separate the present from the past because ofthe way in which human activity has slowly andprogressively changed the landscape of Britain.Although the direct effects of human activity maybe obvious, for example in the engineering ofriver channels, other changes are often muchmore difficult to decipher. Thus the riverchannels that today are bordered by woodlandshow how river channel processes aresignificantly affected by trees and organic debris(Gregory and Gurnell, 1988), and thesevegetation influences were much more extensivein the past when many more rivers in Britainwere flowing through woodland basins.

Reconstruction of the way in which scenerywas fashioned can use evidence from

morphology and landforms, from sediments andfrom deductions about the way in whichprocesses operated in the past. However, forpast environmental situations sufficient informa-tion is rarely preserved and Lewin (1980) hasdrawn the analogy with a series of windows intime that allow us to see fragments of the formerlandscape. It is those fragments of morphologyand sediments that can be used to deduce whatprocesses were like in the past and so toreconstruct how changes occurred. Britishscenery still contains many legacies that wereproduced under rather more dramaticconditions than those of the present time. Someof these previous conditions were associatedwith rivers and others were associated withenvironments affected by glaciation anddeglaciation.

It is imperative that we identify and conserveevidence of former environmental conditionsbecause these provide the fundamental basis forinterpreting the history of our scenery, and anumber of the sites in this volume contain vital

Figure 1.3 Rainfall magnitude–duration relationships for the world and the UK. The largest falls (from Rodda,1970) and magnitude–duration relationships of geomorphologically significant UK floods, where shown, aregiven.

The history of fluvial processes

9

evidence of this kind. Particularly dramatic land-forms created under past conditions are largemeandering valleys, now occupied by smallunderfit streams, which were produced whenriver discharges were much larger than those ofthe present time. In some cases river valleys thatreceived water from glacial drainage channels,which were created by meltwater from meltingglacier ice, or draining from lakes impounded byice, were a feature of the Quaternary environ-ments of northern and western Britain. In theheadwaters of drainage basins, particularly onlimestone and sandstone rocks, there are exten-sive networks of dry valleys, and examples areincluded in the Quaternary GCR Blocks. Thesevalleys, without any trace of stream channels atthe present time, were also produced when thehydrological cycle of the past involved greateramounts of surface runoff.

Some features were produced by changes insediment distribution and, for example, alongsome small rivers in upland Britain, there aresequences of deposits laid down as alluvial fanswhich have now been abandoned and dissectedby present streams. Abandoned channels sur-vive along many rivers as remnants that may bedated from the organic deposits which infill theold channels. Many of the river systems ofBritain are still endeavouring to recover fromthe most recent glacial and cold phases, whichproduced vast quantities of sediment. Materialsfrom glacial deposits are still being released intothe fluvial system. In many parts of Britain, suchas Scotland and north-east England, the presentfluvial system clearly records the legacy of recentand of earlier glaciations. We therefore have alandscape today in which river systems are stillrecovering from the impact of different process-es in the past.

Three main reasons explain the differencesbetween the past and present conditions. Themost obvious is the impact of changes of climate.Not only did the Quaternary bring a series ofglaciations that affected much of the northernparts of Britain, but to the south of such glacierice were climatic conditions that resembled thecontemporary Arctic climates of northernCanada or the former Soviet Union. This meantthat the regimes of rivers were typically very sea-sonal, with little or no flows during the wintermonths and very large floods before and imme-diately after the spring thaw, followed by lowerflows for much of the summer. Under such con-ditions, when the ground was frozen so that

infiltration was not possible, extensive networksof valleys were produced, later to become dryvalleys, and larger river discharges characteristi-cally occurred along river valleys compared tothose of the present time. Secondly, there havebeen changes along river valleys instigated bysea-level change. Such sea-level changes haveaffected the levels to which river activity couldwork and so allowed the destruction of originalvalley floors, remnants of which now remain asriver terraces. In some cases the development ofincised meanders occurred after significant low-ering of river levels. A third difference betweenthe present and the past is that the influence ofhuman activity today is very substantial, whereasin the past it was often less significant. Manyrivers and river channels have been modified asa consequence of deliberate changes by humanaction so that streams have been channelized forflood control, for drainage, to prevent erosionand for improvement of navigation. In addition,land use change in the basin has often indirectlyaffected the river channels. Along the Itchen inHampshire, the river channel was apparentlymade navigable as early as the 12th century(Hadfield, 1969). Other major changes havebeen much more difficult to detect, such as theway in which stream networks have beenchanged because seepage areas have beenreplaced by stream channels, ditches or fielddrains (Ovenden and Gregory, 1980). A particu-larly dramatic way in which fluvial systems of thepast were changed was when deforestationbetween 2000 and 4000 years ago releasedquantities of fine sediment which were trans-ferred into the river systems and which are veryevident in the floodplain sediments of majorrivers. In the upper Thames basin it has beenshown how flooding and alluviation were large-ly restricted to the past 3000 years (Robinsonand Lambrick, 1984). Palaeohydrology, men-tioned above as an approach that allows a retro-spective way of analysing environmental change,has been employed to enhance our understand-ing of past fluvial changes. An internationalproject from 1977 to 1987 (Starkel et al., 1990)included a study of the palaeohydrology of theSevern basin (Gregory et al., 1987). Theconclusion of the Severn study showed how fourmajor phases of development could be identi-fied in the past 15 000 years and each of thesephases, associated with particular types of cli-mate and land cover, could be identified in theimpact on the fluvial system (Figure 1.4a).

An introduction to the geomorphology of Britain

10

Years BP Phase Precipitation Temperature Vegetation Hydrology Channel processesPre-15 000 Glacial Snow Low None Summer floods Multithread channels

and fluvioglacial systems15 000–11 000

Earlier LateGlacial

Amelioratingcold winters

Grass sedgesucceeded bybirch woodland

High peak discharges,decreasing with forestspread

Multithread channels

11 000–10 000

Late-glacialZone III

Extreme cold Herb-richgrassland

Run-off reduced butsediment supplygreatly increased

Fluvial deposits andinstability in fluvialsystem

10 0004 000

Flandrian/Holocene

Rising Mixed oak forest,some clearances

Run-off reduced,sometimes lower thanpresent

Fluvial erosion andsingle-threadmeandering channels

4000–present

Flandrian/Holocene

Drier, thenoceanic

Deteriorationthen fluctuating

Deforestation Seasonalityfluctuating, dischargeregulated

Lowland cut-offsFloodplain accretionSedimentation

Figure 1.4 (a) The sequence of environmental and palaeohydrological change in the Severn basin (afterGregory and Lewin, 1987). (b) The sequence of palaeohydrological changes in the Severn basin (after Gregoryand Lewin 1987

Fluvial landscapes and pressures

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FLUVIAL LANDSCAPES AND PRESSURES

The present fluvial system in Britain blendstogether the impact of present processes withthe landforms produced by processes of thepast, and the sites in this volume were selectedfor the GCR as the best examples to exemplifythese two stages.

There is a general sequence of four majortypes of fluvial landscape in Britain (Figure 1.5),which can be thought of as proceeding fromheadwater areas, through gorges, to floodplainsand finally to estuarine tidal areas. Each of thesemajor types has particular characteristics, whichare indicated in Figure 1.5, but there are alsomajor contrasts between the upland and low-land zones and between the glaciated and non-glaciated areas. A major distinction within ariver valley is between bedrock reaches, mainlyin the upstream sections, and alluvial reaches,mainly downstream. The dividing line betweenthe highland and lowland zones, approximatelyfrom the mouth of the River Exe to the mouth ofthe Tees, separates areas to the north and west,which tend to have the most active river systemsand which can be expressed in terms of thestream power (a product of discharge, slope andwater quality that is reflected in the amount ofsediment carried by the water), whereas areas tothe south-east of the line have much lowerstream power values. Contrasts in river charac-teristics can be related to rock types and theirinteraction with relief and water quantity (Figure1.5b). A further contrast is between areas thathave been glaciated, especially in the mostrecent Quaternary glaciation, and those thathave not. In the former there are ample depositsfor rivers to excavate and to modify, whereas inthe latter there has been ample opportunity forthe impact of cold climatic conditions and dif-ferent fluvial regimes (Lewin, 1981).

A further contrast between the south and eastof Britain and the north and west is the degreeof human activity which has affected the presentfluvial system and which continues to exert pres-sure upon it. Direct changes are very well exem-plified by the distribution of channelization inEngland and Wales (Brookes et al., 1983) whichis illustrated in Figure 1.6. This shows thatbetween 1930 and 1980 the extent of directmodification of river channels by engineeringworks or for channel maintenance was very sub-stantial, and such channelization has a density

20 times greater than the density of channeliza-tion of rivers in the USA, where the concernsabout channelization effects have been muchgreater. Similar direct influences of human activ-ity on the river channel can be seen at the sitesof dams and reservoirs, where water power hasbeen generated, where gravel or river depositshave been extracted, or where river diversionshave been engineered. In addition, there havebeen indirect changes arising as a consequenceof these direct pressures. After direct modifica-tion of stretches of the river channel, for exam-ple for flood prevention or for water supply byriver regulation through dam construction, ithas been shown that considerable lengths of theriver channel downstream have been modifiedas a consequence of the effect on the river flow.Thus downstream of channelization works,flows can be increased and so may induce ero-sion, whereas downstream of dams a decrease offlows can lead to accretion of sediments, such asalong the Derbyshire Derwent, where Petts(1977) identified changes of the channel of theRiver Derwent following impoundment of reser-voirs in the headwaters of the basin. In addition,changes of drainage areas, for example from for-est to farmland or from farmland to urban areas,can significantly change the hydrology of thedrainage basins, and the different flows thatresult can produce significant changes in theriver channels downstream. Most dramatic arethe changes downstream of urban areas, and ithas been found that the increased floodflowsfrom urban areas can produce stream channeldimensions downstream that are twice thedimensions expected (Gregory, 1976). A rangeof pressures on the drainage basin, includingboth spatial ones associated with land-usechanges and point pressures (for example wherebridges are constructed across rivers or whereoutfalls of storm water drains augment the flowand sediment of the river channel) can inducesubstantial amounts of river channel adjustmentdownstream. A series of associated questionsrelate to what causes the river channel change,how much will occur, when it will take place andwhere it will be located (Gregory, 1987). Someanswers to these questions are known but oth-ers require more detailed investigation.Therefore, it is imperative that there are sitesavailable for further research where natural land-scape features from the present and past havenot been obliterated so that the extent to whichchanges have occurred can be ascertained. The

An introduction to the geomorphology of Britain

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Figure 1.5 (a) An idealized section showing the relationship between river profile, channel materials and chan-nel pattern (after Smith and Lyle, 1979). (b) Map showing ‘surveyed’ rivers in Great Britain (after Smith andLyle, 1979).

FLuvial landscapes and pressures

13

extent of detrimental modification partlyaccounts for the relatively sparse number of sitesin central and south-east England.

Some sites are relatively static in their charac-teristics and need to be conserved as represen-

tatives of features inherited from past types oflandscape-forming conditions. Other sites arestill dynamic, undergoing active processes thatcontinue to change the landforms. It is veryimportant that the latter sites are allowed to

Figure 1.6 A map of England and Wales showing rivers channelized, 1930–1980 (after Brooks et al., 1983).

An introduction to the geomorphology of Britain

14

remain dynamic so that processes can be studiedand the evolution, rate and nature of change canbe elucidated. Some of these changes willinclude responses to human activity and also toclimatic change. So, for example, by under-standing river adjustments scientists will be bet-ter placed to predict impacts of global warmingas well as to understand the origin of the fea-tures within the sites themselves.

Investigation of the sensitivity of sites to arange of natural and human-induced pressuresis required, but sensitivity has been interpretedin several ways (Downs and Gregory, 1992).Sensitivity is relevant to the management of riverchannels because it can help to indicate whichchannels are adjusting and which may be sus-ceptible to adjustment in the future. Four typesof interpretation of sensitivity have been used indifferent studies and these relate to ratios,thresholds, recovery times and sensitivity analy-sis (Downs and Gregory, 1995). In most investi-gations it is feasible to relate proximity to thresh-olds to the imbalance of forces, when sensitivitymay be defined in terms of the relationship ofdisturbing forces to a specific threshold condi-tion.

Awareness of the value of an understanding ofgeomorphology and of river channel change hasincreasingly influenced management methodsand restoration schemes. In the USA anapproach of working with, rather than against,the river was advocated (Winkley, 1972), and asimilarly sympathetic approach to river manage-ment has been developed in Britain, whereBrookes (1992) showed how the recovery andrestoration of British river channels could beeffected using a knowledge of river channelbehaviour. Techniques necessary for the holisticappraisal of river projects have been collectedtogether in a manual (Gardiner, 1991).

CONCLUSION

Changes exercised by human activity on riverchannels do not necessarily lead to a deteriorat-ing river environment. Indeed, the recent trendto work with the river rather than against it hasmeant that greater understanding of river behav-iour is being sought so that it can be utilized indevising management strategies that are as con-sistent as possible with natural river activity.This contrasts with earlier strategies which tend-ed to replace natural rivers and their channels byvery different concrete structures. To ensure

that we have the best possible input to geomor-phological approaches which work with theriver, it is essential to recognize that the rivers oftoday have emerged from environmental sys-tems of the past, that rivers themselves have a“memory’ (Newson, 1987) and are dynamic andwill continue to change. Indeed, it has beensuggested that the timescales used for the man-agement of a river, which are usually employedby an engineer, are necessarily much shorterthan those necessary to the understanding usedby the geomorphologist. In order to work withnature, it is essential that we have a range of sitesin which natural processes are operating and inwhich naturally derived features are present. Asconsiderable progress is now being madetowards the restoration and enhancement ofengineered river channels utilizing geomorpho-logical research and understanding (Brookes,1990; Gardiner, 1991; Sear and Newson, 1991),it is vital that we perceive the present river sys-tem as part of an evolving sequence which has apast, a memory, a prospect and a future. It is tothis time continuum that the fluvial GCR sitesare especially valuable. However, it is not onlythe direct and immediate benefit to river man-agement that justifies the selection of sites forconservation. It is important that the best exam-ples of fluvial landforms and of operation ofprocesses are conserved, that spectacular andunique fluvial features in our natural heritageare protected, and that a range of sites with pres-ent or potential value for research is available.

In selecting the sites described in this volume,the three GCR components of internationalimportance, presence of exceptional featuresand representativeness have been kept in mind.The full rationale of the GCR and the detailedcriteria and guidelines used in site selection aregiven elsewhere (Crowther and Wimbledon,1988; Allen et al., 1989; Gordon and Campbell,1992; Gordon and Sutherland, 1993; Gordon,1994; Wimbledon et al., 1995; Ellis et al., 1996).However, the selection of fluvial geomorphologysites for the GCR necessarily reflects the sitesthat have been investigated in earlier research;and there is also some imbalance in distributionof the sites not only reflecting the research thathas been undertaken but also related to theextent of the influence of human activity.Nonetheless, it has been possible to group thesites described in this volume (Figure 1.7 andTable 1.1) into five major categories. The fluviallandforms category (A) consists of those which

Concllusion

15

Figure 1.7 A map of Great Britain showing the classification of GCR fluvial geomorphology sites. See also Table 1.1

An introduction to the geomorphology of Britain

16

Site name A B C D E1 Corrieshalloch Gorge 42 Falls of Clyde 4,53 River Findhorn at Randolph’s Leap 4 14 Falls of Dochart 55 The Grey Mare’s Tail 56 River Clyde meanders 1 27 Strathglass meanders 1 1 18 Abhainn an t-Srath Chuileannnaich 1 1,2 29 Endrick Water 1 2

10 Derry Burn 1 1,2,4 111 River Balvag delta 5 112 The Lower River Spey 1 4 113 Glen Feshie 1 1,3 2,4,514 The Allt Dubhaig 4 4 415 Dorback Burn 1 2,416 Glen Coe: river and slope forms 4 1,317 Luibeg Burn 4 118 Allt Mor (River Nairn) 2,4 1 1,519 Allt Coire Gabhail 320 Allt Mor (River Druie) 4 1,4 5 421 Quoich Water alluvial fan 1 5 222 Allt a'Choire 4 4 523 Allt Coire Chailein fan 4 5 124 Eas na Broige debris cone 3 525 Oldhamstocks Burn 1,226 Findhorn Terraces 127 North Esk and West Water palaeochannels 3 128 Glen Roy, Glen Spean and Glen Gloy 1 529 Afon Llugwy between Swallow Falls and Betws y Coed 3,4,530 Afon Rhaeadr at Pistyll Rhaeadr 531 Afon Cynfal at Rhaeadr y Cwm and Rhaeadr Cynfal 3,4,532 Afon Teymyn at Ffrwd Fawr 3,4,533 Afon Glaslyn at Aberglaslyn 4 134 Afon Teifi at Cenarth 4 135 River Dee at Llangollen 2,336 River Wye at Lancaut 2,337 Afon Hepste 638 Afon Mellte downstream of Ystradfellte 639 Afon Dyfi between Dinas Mawddwy and Mallwyd 1,340 Afon Rheidol 141 Afon Vyrnwy 1 142 Afon Ystwyth 4 143 Upper Elan upstream of Graig Coch Reservoir at Bodtalog 9 144 Upper Ruver Severn between Dolwen and Penstrowed 1,4 445 River Severn between Welshpool and the confluence of the

Vyrnwy and Severn 146 River Dee, Holt to Worthenbury 147 Afon Teifi at Cors Caron 148 Maesnant, Pumlumon (Plynlinon) 749 Black Mountain scarp 350 Carlingill Valley, Howgill Fells 1,3 3 5 4,551 Langdale and Bowderdale Valleys, Howgill Fells 1 5 4,552 Langden Brook, Bowland Fells 4,5 453 River Dane, near Swettenham 1 1 6

A Langstrathdale* 4,5 3 4B Wasdale* 4 3 4,5C Fan Deltas at Buttermere and Crummock Mere 5

Table 1.1 Key to the sites shown on Figure 1.7, including classifications used in Table 1.2.

Conclusion

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dominantly provide a landscape feature that isassociated with a particular process or a particu-lar stage in fluvial landform development. A sec-ond group of sites (B) is dominantly associatedwith a particular aspect of contemporary fluvialprocesses which vary across the breadth of GreatBritain and contrast particularly between thenorth and the south. A particular aspect of thefluvial landscape concerns river channel patternand floodplain features (C), which are classifiedas a separate category. As indicated above, inBritain we have considerable evidence ofchannel change which has arisen during theQuaternary, and a category of six types isincluded (D) to indicate these features. Finally,some sites are dominated by the impact ofhuman activity on the fluvial landscape or uponthe river system (E). Although it was convenientto recognize five major groups, it should beemphasized that many of the sites include morethan one type and this is reflected in the key pro-

vided for each of the sites. The balance betweenthe sites shows that 52% are associated with flu-vial landforms, 42% with fluvial processes, 40%with river channel pattern and floodplain, and34% with channel change while 6% are dominat-ed by the impact of human activity. The selec-tion therefore reflects a combination of valuedlandforms, of sites where processes and featurescan be seen at their best, and of sites where sig-nificant scientific work has been carried out.This reductionist approach to the sites, however,belies their potential contribution because theyshould be seen as a range that individually andcollectively provides clues to the present andpast fluvial system in Britain, and therefore givesus one basis for wise stewardship of the fluvialenvironment in the future. Once this selectionhas been made, then, to facilitate stewardship itis necessary to assess the sensitivity of sitesaccording to liability to adjustment in relation tonatural processes, and to their vulnerability to

Site name A B C D E54 Black Burn 1 4 155 Garrigill, River South Tyne 5 156 River Nent, Blagill 5 1 2 157 The Islands (Alston Shingles), River South Tyne 1 558 Blackett Bridge, River West Allen 559 River Tyne at Low Prudoe 1 160 Harthope Burn 261 Shaw Beck Gill 162 Beckford 463 River Severn at Montford 364 River Axe at Axminster and Whitford 8 265 River Exe at Brampford Speke 566 River Ter at Lyons Hall 667 River Derwent at Hathersgate 1 268 Highland Water 7,869 River Lyn 3 170 River Itchen near Knightcote 371 River Cherwell at Trafford House 372 Ashmoor Common 173 River Severn, Buildwas 1 674 Alport Valley 475 Bleaklow 376 Lydford Gorge 477 Mimmshall Brook at Water End 778 Aysgarth 579 Dovedale 1 680 River Culm at Rewe 4,5 681 River Lugg 382 Wilden 1 6*Potential GCR sites.

Table 1.1 Continued

An introduction to the geomorphology of Britain

18

human activity by direct or indirect means. Apreliminary analysis of the sites (Figure 1.7,Table 1.2) involved the development of auniqueness index similar to that originallydevised by Leopold (1969) and this was appliedto the classification of 30 categories (Downs andGregory, 1995). It is important to analyse notonly the extent to which a site is unique but alsoto know its susceptibility to change. The sitesdescribed in this volume represent a selectionmade at a particular time, and it is important toremember that they are not all unique represen-tatives and that some are more liable to changethan others. Just as the management of riverchannels is now seen within the context of adynamically changing river system, so these siteshave to be visualized in a context of change overtime.

Classification Number ofsites

representingthe category

A. Fluvial Landforms1 Terrace 142 Incised Meanders 33 River Capture/rejuvination 94 Mountain torrent/slot gorge 175 Waterfall 96 Karstic site 27 Soil pipe/swallow hole 2B. Fluvial processes1 Process event-flood 192 Accelerated erosion 23 Debris flow/cones 84 Sediment movement 75 Floodplain sedimentation 56 Discharge control on capacity 17 Vegetation influence 18 Bank erosion 29 Response to confinement variation 1C. River channel pattern and floodplain1 Meander 142 Wandering 33 Outwash sandur 14 Braided 105 Alluvial fans 12D. Channel change1 Palaeochannel 182 Planform change 73 Underfit stream 34 Palaeofans/sediments 65 Palaeoterraces 56 Palaeoconditions 5E. Human1 Mining 42 Reservoir 13 River management 1

Table 1.2 Numbers of GCR sites representing thecategories shown on Figure 1.7.