Chapter 2 Fluvial geomorphology of Scotland

Fluvial landforms and processes in Scotland

A. Werritty and L.J. McEwen

Introduction

Scottish rivers afford a richer variety of process, form and pattern than other UK rivers because of the greater diversity of environments within which they have evolved. This arises because of deeply dissected relief (particularly to be found in the Scottish Highlands), the juxtaposition of reaches from highland, upland and lowland environments, and the marked rainfall gradient across Scotland from west to east (see (Figure 1.1)a). This chapter will examine the specific geomorphic controls that govern the operation of fluvial processes in Scotland, together with the spatial and temporal patterns of the resulting river systems. It will also examine the conservation issues that arise in the management of Scotland's rivers and explain the rationale and justification adopted for the selection of fluvial geomorphology GCR sites in Scotland.

The general approach to site selection involved two contrasting components. Some sites were selected on the grounds that they represented the best sites of their class within Scotland and as such were 'unique'. Other sites were chosen because they were typical of a widespread and geomorphologically significant class of sites. These two categories of sites taken together afford a benchmark for past and future fluvial studies within Scotland. The majority of the sites chosen have been the subject of intensive localized research. Other potentially noteworthy sites have undergone less detailed analysis and thus provide a set of significant sites for future research. Additional sites, at present devoid of adequate information, are not included in the current list of GCR sites, but are recognized as being significant in terms of this review.

The distinctive geomorphology of Scottish rivers

The assertion that Scotland's rivers afford a richer variety of process, form and pattern than rivers in other parts of the UK clearly requires justification. In terms of the hydrological setting for Scottish rivers, a marked west-east precipitation gradient exists, with Glen Quoich having a mean annual precipitation > 3000 mm but Aberdeenshire, Fife and Berwickshire recording < 700 mm (see (Figure 1.1)a). This, when combined with generally low evapotranspiration losses, results in parts of the NW Highlands recording rainfall : runoff ratios > 75% and the majority of Scotland recording ratios > 50% (see (Figure 1.1)b). This is in marked contrast to East Anglia where rainfall : runoff ratios can be as low as 25% (Ward, 1981). This close relation between rainfall and runoff becomes even clearer when mean annual flood values (standardized for area) are mapped, revealing extreme values for high-rainfall areas in the SW Grampians and NW Highlands and very much lower values in the drier south and east (Figure 2.1). This rainfall-controlled pattern of runoff gives the River Tay by far the largest mean flow for any river in Britain (152 m³s⁻¹), although in terms of drainage area it ranks only third (Ward, 1981).

Given this pattern of runoff, it might be expected that the largest and most powerful rivers would also be located in the NW Highlands and SW Grampians. But this is not the case because of the extremely asymmetric location of the main east-wept watershed in Scotland (Figure 2.2). This asymmetry has arisen because the most active ice streams during the Pleistocene operated from an east-west iceshed located close to the present-day watershed. This in turn was inherited from a watershed that developed during the latter part of the Tertiary (George, 1965; Sissons, 1967). To the west of the present-day watershed, steep and relatively short icestreams produced a series of deeply incised glaciated valleys often with hanging tributary valleys (e.g. Glen Coe). Following deglaciation, 'the relatively short river systems in this part of Scotland have merely occupied courses previously excavated by the major icestreams. In contrast, the largest rivers in the Highlands (e.g. River Tay, River Spey and River Dee) flow eastwards from the main watershed and it is only in the Southern Uplands that the watershed between the River Clyde and River Tweed is more symmetrically located (Figure 2.2).

Another significant legacy of repeated Pleistocene glaciation is the calibre of the bed material, with many of the large rivers of Scotland having gravel beds down to their marine limits (e.g. the Spey, Tay and Tweed). This arises not because such material is continuously transported from source to mouth but because lateral channel migration constantly reworks the Late Quaternary glacial and glaciofluvial deposits adjacent to the valley floors. A further inherited glacial feature is that many of the upper reaches of upland channels are cut directly into bedrock, these occasionally being so confined that they represent the courses of former subglacial meltwater channels (e.g. Corrieshalloch Gorge on the River Droma and Randolph's Leap on the River Findhorn). Such former meltwater channels are a persistent element in the upper reaches of many Highland rivers, producing stepped long profiles in which alluvial reaches occupy a series of basins separated from each other by bedrock sections. The precise pattern of such alternating alluvial and bedrock channels owes much to differential glacial erosion during the Devensian and is exemplified particularly well in the Middle and Lower River Findhorn and the Tay–Turnmel–Garry river system.

Another result of selective glacial erosion is the presence of many lochs within river courses. A striking example of this is the Tay drainage system with four lochs > 0.1 km³ in volume (Murray and Pullar, 1910). These lochs have a two-fold impact in terms of fluvial processes: they serve to dampen flood waves and thus reduce the potential stream power in the lower reaches of major river systems, and they also act as sediment traps (McManus and Duck, 1988). However, not all lochs contained within major river systems owe their origin to glacial scouring; others (e.g. Lochs Insh, Alvie and Morlich within the River Spey drainage system) are former large kettle holes which, on final deglaciation, became incorporated into the present-day drainage system.

Fluvial processes: spatial patterns

A variety of channel types arises from the operation of the major controls analysed above and these in turn generate a number of distinctive spatial patterns (Werritty et al., 1994). In an idealized sequence such as that described by Geikie (1865), a typical Scottish river originating in the Highlands and flowing to the sea will commence as a boulder-bed torrent, often interrupted by bedrock reaches. Such a channel will be relatively stable over short timescales and subject to major episodic adjustment only during rare extreme floods (e.g. the Allt Mor, Glenmore; McEwen and Werritty, 1988). In addition, many upland channels pass through 'alluvial basins' (glaciated valley floors infilled with alluvium) which are often separated from each other by further bedrock reaches; for example Glen Deny (McEwen, 1986) and Abhainn an t-Srath Chuileannaich. Downstream from these initial channel types, the reduction of bed material size and channel slope is often combined with a widening of the valley floor. This typically results in a low-sinuosity wandering gravel river which, depending on local controls, may exhibit both divided and undivided channels of variable sinuosity, for example the Rivers Tulla and Feshie (Bluck, 1976; Werritty and Ferguson, 1980) and Dorback Burn (Werritty, 1984). In sediment transport terms such reaches are bedload channels, but the number of competent transport events in any year will vary greatly according to bed material size and the associated entrainment function (Ferguson and Ashworth, 1991).

The supply of bed material from tributary valleys and the location of undercut terraces adjacent to the valley floor also controls the degree of channel stability (e.g. on the upper River Findhorn and the upper River Dee). In such reaches the channel is overwhelmingly an alluvial channel reworking its floodplain and only occasionally confined by bedrock reaches and gorge-like sections. Moving further downstream, the channel may be interrupted by a loch which acts as a sediment trap; for example the River Balvag and Loch Lubnaig (Duck, 1984). However, the channel gradient downstream of the outfall combined with a renewed sediment supply from banks and tributary streams usually ensures that the river continues to display many of the characteristics noted above.

Truly lowland reaches constitute only a relatively small proportion of the total channel length of an idealized large river system in Scotland. This arises because lowland areas are confined to a narrow coastal fringe around the Highlands and Southern Uplands and become extensive only within Central Scotland. In this central belt, in contrast to their upland counterparts, lowland rivers are generally characterized by sand-sized rather than gravel beds. Low channel gradients plus river training along many reaches mean that these channels are highly stable and it is only rarely that major reworking of the floodplain occurs (e.g. the River Clyde at Carstairs (Brazier et al., 1993) and the lower Endrick Water (Bluck, 1971)). Nevertheless, bedrock controls can still provide dramatic changes in channel type, and gorge-like reaches are locally significant even in the Central Lowlands (e.g. the River Clyde at Falls of Clyde and the River Devon at Rumbling Bridge). The final sections of the major river systems which terminate in the Central Lowlands are the well-known firths or estuaries (e.g. Firth of Tay) which serve as major long-term sediment stores.

Such is the sequence of channel types in an idealized large Scottish river system originating in the Highlands or Southern Uplands and terminating at the coast, but other types of channel exist alongside this idealized model. Three types which are especially important are 'integrated', 'discordant' and 'progressive' channel systems. The integrated system typically occurs in upland areas and comprises a sediment source, a transportational reach and a depositional sink all within a small area of only a few square kilometres (cf. Schumm, 1977, fig. 1.1). The sediment sources are often areas of deeply dissected till, while the depositional sinks take the form of major alluvial fans. In these integrated drainage systems, the sediment source and the depositional sink are end members of a sequence in which channel morphology and processes rapidly change downstream. There may be abrupt changes in control within this marked downstream change. The concept of an 'integrated' system is therefore scale dependent.

When considering river systems at a larger scale, both 'discordant' and 'progressive' systems are important categories. The former type of channel system occurs when the hydraulic and sediment controls change abruptly at the point at which the river emerges from an upland into a lowland environment and only the transportational reach (using Schumm's terminology) undergoes change. In this situation, the channel is transformed from a steep, low-sinuosity, gravel-bed channel with a large width : depth ratio into a gently sloping, sinuous fine-grained channel which is stable and has a small width : depth ratio. Locally, the channel capacity is reduced, resulting in a greater incidence of over-bank floods. Such sudden changes in downstream hydraulic geometry are likely to be quite common in the Scottish uplands.

In contrast to systems characterized by abrupt changes, there are those where change is more progressive. Occasionally, the transition from a coarse-grained active upland channel to a fine-grained stable lowland type of channel occurs within a few kilometres in response to a local base-level control, even within upland environments (e.g. the Abhainn an t-Srath Chuileannaich). In this situation, the downstream reduction in channel slope is associated with decreases in bed material size, bedload transport and width : depth ratios together with a transition from braided to meandering channel patterns. The Allt Dubhaig, the River Derry and the River Coe all display this response in varying degrees to an imposed base-level control (Ferguson and Ashworth, 1991; McEwen, 1986, 1994b). Where there is a stepped sequence of alluvial basins, there may be a series of these transitional reaches down the same river (e.g. the Lui catchment or Clunie Water, Deeside; McEwen, 1986).

The glacial legacy and recent climatic history of Scotland (see the section on geomorphology of Scottish rivers, above) exercise a strong control on the pattern of downstream fining. Scottish rivers (e.g. the upper River Dee, River Feshie and River Findhorn) do not display classic progressive downstream fining throughout their length. Instead, there is local fining, often within the context of glacially eroded basins that have subsequently been infilled by alluvium and which display a local base-level control (e.g. lower Glen Derry and the Allt Dubhaig). The triggers that induce a local coarsening of bed material downstream are lateral inputs derived either from glacigenic sediments (on or adjacent to the valley floor), or from alluvial fans at the mouth of steep tributary valleys. It is rare that downstream fining develops more than a few kilometres before one or other of these disturbing controls exerts an influence (Werritty and Inglis, 1985). For similar reasons, the long profiles of Scotland's larger rivers (e.g. the River Tay) do not conform to the familiar concave up model, but instead comprise a series of concave segments, often linked by steeper units where the river flows through a bedrock-controlled section.

In terms of specific types of channel patterns, a rich variety has been identified for Scottish rivers. The traditional distinction between 'braided' and 'meandering' channels (Leopold and Wolman, 1957) is inappropriate for many rivers in Scotland, since a given reach often exhibits both types of pattern simultaneously; or, if studied over a number of years, reveals an alternating development of divided and undivided channels of varying sinuosity (Werritty and Ferguson, 1980). Bluck (1976), in his general model of low-sinuosity Scottish rivers, has produced a series of transitional channel types from an initially braided to a fully developed meandering planform (Figure 2.3). Robertson-Rintoul (1986a) has gone even further and argued that each of these patterns represents the end members from a continuum of channel types which can be characterized solely in terms of total sinuosity. A channel typology specifically designed for upland Scottish rivers has been developed by McEwen (1994a) taking note of these distinctive features, and this typology forms the basis for site selection described later in this review.

Fluvial processes: temporal patterns

Different Scottish fluvial environments are characterized by varying rates of temporal adjustment in response to flow resistance, entrainment thresholds and local stream power. Flow resistance and entrainment thresholds, both dependent upon bed material size, are largely inherited from local patterns of deglaciation (see the section on geomorphology of Scottish rivers, above). Stream power, on the other hand, is controlled by channel gradient and runoff, with Highland Scotland registering some of the highest values in Britain (> 100 Wm^-2, Ferguson, 1981). These two sets of controls on channel type should be regarded as continua rather than discrete states. However, for reasons of clarity in discussing rates and modes of temporal adjustment, only the four end members are presented in detail below.

A: High-energy: high-threshold environments

These fluvial environments are characterized by episodic bursts of intense activity interspersed with periods of relative quiescence and are represented in Scotland by alluvial fans and mountain torrents. Such sites are reworked by extreme floods typically produced by low-frequency, high-intensity rainfall (see Ferguson, 1981; McEwen and Werritty, 1988; Acreman, 1991). The seasonality of such floods and associated high rates of geomorphic activity varies across Scotland (see McEwen, 1986; Black, 1992). It is clear, however, that the landforms produced during such rare events tend to persist in the landscape since, under normal flow conditions, the river is not competent to undertake major geomorphic work and re-establish former equilibrium conditions.

B: High-energy: low-threshold environments

This combination results in the highest rates of channel adjustment since, under these circumstances, floodplains and low-angle alluvial fans can rapidly be reworked, resulting in frequent channel migration (Ferguson and Werritty, 1983; Werritty and Brazier, 1991b). Detailed chronologies of deposits related to earlier fluvial activity are not usually evident since they are progressively destroyed (Brazier and Ballantyne, 1989).

C: Low-energy: high-threshold environments

Such environments, which are very common across Scotland, result in stable channels which display little change even throughout the whole of the Holocene. Very often this arises because bed material inherited from deglaciation is very coarse, and the present-day river is not competent to transport this material except under exceptionally rare floods. The channel will thus remain relatively unmodified by present-day processes. Such channels characterize the middle reaches of many Scottish river systems (McEwen, 1986).

D: Low-energy: low-threshold environments

This final class is characterized by sites which record, slow progressive change in response to the crossing of intrinsic thresholds. An example of such a site would be a lowland meandering stream where progressive sedimentation eventually leads to the breaching of meander necks. The adjacent floodplain typically displays oxbow lakes and ridges and swales as evidence of former channel alignments (Brazier et al., 1993). These palaeoforms are often well-preserved on account of the low rates of channel migration.

The extent of geomorphic interest in sites within each of these four classes depends on the specific interests of the geomorphologist, the timescale under consideration and the spatial scale of the study. Thus far, categories A, B and D have provided the most profitable sites for fluvial research in Scotland.

It is important to note that the fluvial controls and responses at any given site will have undergone change throughout the Holocene. Such changes in channel regime may arise from both climatic and land-use changes as well as from the incidence of rare, random high-magnitude events and may be active over a variety of timescales. In Scotland, climatic and land-use changes frequently take place side by side, thus making the relative attribution and interpretation of cause and effect difficult (Ballantyne, 1991a,b). In terms of climatic change, the major adjustments of the fluvial system have been in response to changing discharges and sediment supply following deglaciation of the Late Devensian (c. 13 000 BP) and the Loch Lomond Stadial (c. 11 000–10 000 BP) ice sheets. As a result many Scottish rivers have experienced a paraglacial phase (Church and Ryder, 1972) which has generated intermittent sediment stores within the river's long profile. Such stores locally determine both the calibre and the volume of sediment which can be reworked by present-day fluvial processes. The resulting pattern of sediment transport is often highly episodic and has been likened to the movement of a 'jerky conveyor belt' (Newson, 1989). The net result may be akin to the passage of large-scale sediment waves documented by Church (1983) on the Bella Coola River in British Columbia. Subsequent adjustment to these changes in regime is often evident through phases of aggradation and incision, resulting in staircases of terraces, some of the best examples being those along the Rivers Findhorn and Feshie. The specific impact of the glacial legacy varies according to differences in the regional and local glacial history; thus that for the Cairngorms differs from that for the Southern Uplands (Sissons, 1976).

Over a shorter timespan, research has focused on the geomorphic impact of enhanced flood frequency during the Little Ice Age (c. 1430–1850). Increased rates of channel and slope activity associated with this climatic change have been identified in some regions (Innes, 1983, 1985; McEwen, 1989a, 1994b; Brazier and Ballantyne, 1989), but further work is required to establish a more general regional picture.

Land-use changes can also cause major changes in fluvial controls that are identifiable over different timescales. The development of a vegetation cover in the Early Holocene reduced sediment supply from slopes and many fans and debris cones became stabilized (Brazier et al., 1988). By contrast, within the past 200–300 years biotic land-use changes (such as deforestation in smaller catchments (Steven and Carlisle, 1959) and acidic deposition on vulnerable upland mosses (Battarbee, 1984)) may have resulted in increased runoff rates and a new phase of slope instability. In recent decades the Forestry Commission has undertaken downslope ploughing prior to planting conifers and this has locally generated more flashy runoff (Werritty et al., 1993) and accelerated soil erosion (Battarbee et al., 1985). Within individual catchments, such land-use changes may lead to either a new phase of activity in previously stable or stabilizing environments, or increased activity in those sites which are already active (e.g. debris cones and alluvial fans; Brazier, 1987; Innes, 1982).

River management and conservation in Scotland

The geomorphological features of Scotland's rivers may be subject to a variety of potentially damaging human activities, the cumulative impacts of which are difficult to judge. These activities can affect the catchment, river corridor and the channel, and include mineral extraction, afforestation/deforestation, land drainage, regulation and flood control (Werritty et al., 1994). The task of balancing the livelihoods of land-owners and floodplain occupants against the statutory obligation to conserve scientifically important reaches of rivers placed upon Scottish Natural Heritage (SNH), is further complicated by uncertainty in terms of the nature and impact of climate change. There is growing evidence of increased winter precipitation over the past two decades, with winter runoff in northern Scotland being 18.3% higher in 1979–89 as compared with 1969–78 (Arnell et al., 1990). Smith and Bennett (1994) reached a similar conclusion, with the River Tay registering a 40% increase in annual runoff between 1970 and 1989. There has also been a striking clustering of large floods since the late 1980s, with those on the Rivers Spey in 1990 and Tay in 1991 and 1993 being especially noteworthy (Black and Anderson, 1994). This may herald a period of increased flooding with significant implications in terms of both geomorphic activity and river management.

The traditional response of the statutory conservation agencies, as reflected in this volume, has been to adopt a site-based approach to the conservation of scientifically important river landforms and processes through the SSSI system (all GCR sites being proposed for such designation). Such sites are accorded protection under planning legislation and under the 1981 Wildlife and Countryside Act. In practical terms, the conservation of relict landforms and active or dynamic river environments requires different approaches (Werritty et al., 1994). Not all relict landforms are vulnerable to all 'potentially damaging operations', their vulnerability being jointly determined by the sensitivity of the landform in question and the type of development being proposed (see Werritty and Brazier, 1991a). However, if damaged or destroyed, they will not reform since the processes that formed them are no longer active.

Conservation management of dynamic fluvial environments is more complex. The key strategy here is to ensure the survival of the character and behaviour of dynamic process environments, but not necessarily the preservation of ephemeral individual landforms. The concepts of geomorphological 'sensitivity', 'responsiveness' and 'robustness' are proving to be valuable tools in developing this strategy (Schumm, 1991; Werritty and Brazier, 1994). These concepts identify the range of potential responses to externally imposed threats. Such threats include the effects of both climatic and land-use changes operating either directly on the channel or within the wider catchment area. The nature of the active geomorphic system and its limiting thresholds (Figure 2.4) can then determine whether the system absorbs the externally imposed change (robust) or is sensitive to it (responsive). If the system is responsive, then the initial process regime may cross an extrinsic threshold into a new process regime in which a very different assemblage of landforms is likely to develop. Such changes could be exemplified by an aggrading reach of a river trenching as a result of sediment depletion, or a once braided river becoming increasingly sinuous on account of changes in sediment supply or river flow control.

Site typology adopted for Scottish sites

In selecting a suitably flexible typology to encompass the range of Scottish river channels and to form the basis for GCR site selection, it has been necessary to include sites which display either typical or unique characteristics (see McEwen, 1994a). It is also important that sites representative of lowland and upland areas should be included, thus displaying the full range of fluvial controls. The classification of channels is not designed to be one of mutually exclusive categories; in fact, the most interesting sites frequently fall into several categories. Further details on the form of the typology and the adopted classification for each site are shown in (Table 2.1) and (Table 2.2), the location of each site being shown in (Figure 2.5). It is not appropriate to include the full range of sites on both tables; sites have been included on the basis of applicable site selection criteria. Sites which have not finally been included in the current GCR list but which still rank as significant examples of their kind are included in the discussion below. These may constitute potential Regionally Important Geomorphological Sites (RIGS).

The major subdivision which can be made initially is that between bedrock and alluvial channels (Table 2.1). Little systematic work has been done on the bedrock gorges in Scotland cut by meltwater during at least one phase of deglacia-tion. The most impressive upland slot gorge is that at Corrieshalloch in Wester Ross, which is exceptional in both its length and depth (Kirk et al., 1966). Randolph's Leap on the River Findhorn is also particularly interesting on account of its detailed morphology and its relation to a series of extreme floods. Other significant sites include the Black Gorge near Evanton and the Linn of Dee near Braemar. The Falls of Clyde are unique in that they form a major slot gorge in the middle reaches of a large Scottish river. Not all bedrock sections, however, fall into this category; some being associated with a broadening of the channel perimeter rather than channel confinement, for example the Falls of Dochart. The waterfall category also reveals many potential sites, but few with detailed analysis of the geomorphic controls. The Grey Mare's Tail in Moffatdale is thus selected as a classic example of a hanging valley. The Falls of Glomach in Wester Ross is also significant, principally in terms of its height, but it is not included in the current list of sites in the GCR.

Alluvial channels are subdivided into divided channels and sinuous channels using the classification of Kellerhals, Church and Bray (1976), thus avoiding some of the problems associated with traditional definitions of channel patterns (see the section on fluvial processes, above). The selected sites range from the straight River Balvag and irregularly meandering River Glass to the highly tortuous Derry Burn, Endrick Water and middle River Clyde. The degree of planform regularity of each river is variable and is determined by different local controls (i.e. sediment size, slope, lateral confinement and degree of entrenchment). Meandering is demonstrated at contrasting spatial scales.

Divided channels range from those with little or no localized subdivision through typical wandering gravel-bed rivers, such as Dorback Burn, to the unique highly divided planform associated with the River Feshie and its alluvial fan at the confluence with the River Spey. At the highly divided end of the continuum, the lower River Spey is an unusual fluvial environment with an extensive, actively braided section right down to the river's mouth (Lewin and Weir, 1977). In terms of the complexity of channel subdivision, this site, together with the River Feshie, represents the closest analogue to a sandur-type environment extant in Britain.

Additional characteristics that make Scottish rivers geomorphologically interesting are also included in the GCR site selection (Table 2.2). Sites which display classic fluvial sedimentary facies include the Endrick Water near Drymen, where Bluck (1971) has undertaken a major study on fluvial sedimentation associated with fine-grained, point bar deposits. The Allt Coire Chailein, in contrast, has developed within a coarse-grained environment yielding diagonal and finger bars (Bluck, 1987). Another potential site on a coarse-grained river, the Tulla Water, however, was subject to river training during the mid-1980s. Prior to the engineering works, the site had sustained a long-term geomorphic study (Bluck, 1976). A fluvial site probably unique in Scotland is the Allt Coire Gabhail in Glen Coe, where a landslide has blocked a hanging valley, thereby affording an interesting opportunity to assess long-term sedimentation rates.

Another category of sites is that in which rivers display rapid downstream changes in sediment size, slope and channel confinement. As discussed above (see the section on fluvial processes), these can be subdivided into 'discordant', 'progressive' and 'integrated' systems. Channels associated with abrupt discontinuities in these controls where streams emerge from upland to lowland environments are termed 'discordant' channel systems. At present only one such site, the Allt Mor on the upper River Nairn, has been studied. This is a particularly good example of the category; within a few kilometres, the channel is transformed from a highly active, coarse-grained, wide and shallow channel to a stable, fine-grained, narrow sinuous channel.

Channels in which the transition is triggered by local base-level control often display a more gradational change, with highly divided active channels being transformed into active and then inactive sinuous channels in a 'progressive' system. The Mt Dubhaig displays all of these channel patterns prior to its inflow into Loch Garry. By contrast the Derry Burn and the River Coe exhibit part of this sequence only, prior to flowing into a bedrock gorge and Loch Achtriochtan respectively.

A downstream change of channel form at a rather different scale is provided by examples of 'integrated' channel systems with the sequence of headwater sediment source (deeply incised till units), transportational reach and depositional sinks (alluvial fans) occurring within a few kilometres. Excellent examples of this type can be found in Allt a' Choire on the River Findhorn and Allt Coire Chailein within the River Orchy river system.

Sites that are at the interface between different types of geomorphic activity have also been the object of geomorphic research. The interface between slope and fluvial systems is frequently characterized by gullies, debris cones, alluvial fans and mountain torrents, where sediment is fed directly into a river system. Actual sediment inputs may be either progressive, through continuous weathering processes, or episodic with major inputs from point sources during high flows. The Oldhamstocks gullies near Dunbar which are deeply entrenched into an easily eroded conglomerate provide, albeit on a smaller scale, many of the characteristics of classic semi-arid badlands. Such gullying is locally common in weathered bedrock in the Southern Uplands and in till in the Highlands (e.g. Allt a' Choire). The Feshie cones, supplied from active gullies and undercut by the present River Feshie, afford excellent examples of debris cone development while the Eas na Broige cone, a fluvially modified debris cone in Glen Etive, is a good example where both sediment supply and the dominant geomorphic regime have changed during the Holocene (Brazier et al., 1988). The col-luvial landforms in Glen Coe are also remarkable in terms of both their range and scale. The River Feshie/River Spey and Quoich Water/River Dee confluences are exceptional examples of active, low-angle alluvial fans. Mountain torrents which display a close coupling between slope and fluvial systems are represented by the Allt Mor/River Nairn, Allt Mor/Glenmore and the Allt Coire Chailein.

A neglected area of fluvial research within Scotland is the interface between fluvial and lacus-trine environments. Two contrasting sites have been selected for the GCR. The first is a low-energy environment represented by the River Balvag with its excellent examples of levees, spit and strathlochans at the mouth of Loch Lubnaig (Duck, 1984). The second is a high-energy environment where the River Coe periodically transports its gravel bedload into Loch Achtriochtan (McEwen, 1994b).

Sites that reveal the geomorphic impact of floods have been periodically studied in Scotland although rarely in a systematic fashion. Analysis has involved assessment of both the immediate geomorphic impact of floods and their long-term persistence as landforming agents (Wolman and Gerson, 1978; Newson, 1989). The importance of rare floods of high recurrence interval as landforming agents within environments with high thresholds has been analysed within highly divided gravel-bed rivers such as Glen Feshie (Werritty and Ferguson, 1980); mountain torrents, for example, the Allt Mor, Glenmore (McEwen and Werritty, 1988) and the Luibeg Burn (Baird and Lewis, 1957; McEwen, 1986); low-angle alluvial fans such as the Quoich fan in upper Deeside (McEwen, 1986); and for small headwater catchments in the Southern Uplands (Acreman, 1991).

Sites that display either interesting types or high rates of fluvial adjustment over historical time represent another important category. In terms of sinuous channels, the River Clyde meanders provide, on a large scale, an excellent example of an actively meandering alluvial environment with evidence of major fluvial adjustment over the past 100–200 years (Brazier et al., 1993). Textbook examples of meander cutoffs are present and can be compared with other notable sites on a smaller scale such as the Endrick Water, where the progressive reduction of a meander neck has been monitored (Gluck, 1971) and the Abhainn an t-Srath Chuileannaich, Faster Ross, where there are superb examples of entrenched meander cutoffs. Other sites considered for site selection, but ultimately excluded, are the abandoned meanders on the River Oykell near Rosehall and the Ettrick Water meander cutoffs upstream of Selkirk, both potential sites for future studies of rates of former channel infill.

Channel adjustment within divided channels over historical time has again been studied at a variety of scales. The changing channel patterns on the Quoich fan after the catastrophic flooding of August 1829 have been reconstructed (McEwen, 1986), while the River Feshie's planform has been monitored between 1977 and 1989 by Werritty and Ferguson. The River Spey below Fochabers has dis played periods of channel expansion over historical time both in response to extreme flooding and to late 19th century deforestation of the floodplain (Lewin and Weir, 1977). It provides the best example of large scale divided channel adjustment within the lower reaches of a river in Britain, and displays a large numbers of palaeochannels of differing ages.

Extending the timescale, there are important sites that reflect Lateglacial and Holocene fluvial adjustment and assist in the reconstruction of Late Quaternary alluvial chronologies. Within upland environments, the suite of terraces in Glen Roy is exceptional, with over 20 levels locally identified. This arose from the complex pattern of deglaciation and drainage of the ice-dammed lake sequence in Glen Roy, which yielded a unique Lateglacial history (Gordon, 1993b). The Findhorn terraces near Ballachrochin represent one of the most extensive 'staircases' of terraces in Scotland, with up to 13 levels being identified, the upper levels cut in out-wash while the lowest ones are entirely fluvial (Auton, 1990). Glen Feshie also displays an exceptional sequence of five major terrace levels dated by stratigraphic methods at c. 13 000 to 90 BP (Robertson-Rintoul, 1986b). Within a rather different lowland environment, the River North Esk and West Water palaeosandur provide the best documented suite of low-level terraces and palaeochannels of Lateglacial age (between two and four levels have been identified by Maizels, 1983a,b).

Alluvial fan environments can also reflect Lateglacial and Holocene fluvial adjustment. Thus the Glen Roy fans (for example that in Glen Turret) are exceptional in their degree of incision, terrace formation and display of sedimentary facies, again assisting in the interpretation of Glen Roy's Lateglacial history (Sissons and Cornish, 1983; Peacock, 1986). Another site at which there is considerable potential for further research is the Coire Allt Chailein fan with its three distinct units and groups of palaeochannels across the fan surface. The Allt Lorgaidh fan in Glen Feshie is a site which has substantially added to our knowledge of Late Holocene fluvial activity in the Central Grampians (Robertson-Rintoul, 1986b). By contrast, the Eas na Broige fluvially modified debris cone, in Glen Etive within the SW Highlands, provides insight into periods of enhanced slope activity with two major episodes identified; the first being debris flows after the Loch Lomond Stadial and the second being fluvial reworking in the late 14th century in response to land-use changes (Brazier et al., 1988).

Within the overall list of sites, three areas stand out in that they provide composite sites with an exceptional range of fluvial landforms within a relatively confined area. The first is Glen Roy, with its unique glacial legacy providing superb suites of terraces and textbook alluvial fans related to the rising and falling sequence of lake levels. The second site is Glen Feshie, with its even wider range of fluvial landforms. The two major divided reaches provide some of the best examples of their kind in Scotland, as does the low-angle alluvial fan at the Spey confluence. The tributary alluvial fans and terrace sequences also assist in the reconstruction of fluvial adjustment during the Lateglacial in this part of the Cairngorms, while the debris cones provide insight into periods of increased slope activity during the Little Ice Age. Finally, Glen Coe provides perhaps the best suite of currently active slope processes in Scotland, with steep gullies, debris cones, fluvially modified cones and well-developed alluvial fans. In addition, the River Coe provides a good example of channel adjustment in response to base level control and sediment transfer within a fluvio-lacustrine environment.

Summary

This review has demonstrated the variety of Scottish fluvial landforms worthy of SSSI status identified by the GCR. They should be viewed as a benchmark in terms of both geomorphological research and conservation interest in Scotland in 1997. Also, it has shown that in the past, research has tended not to be systematic in its coverage of scientifically interesting river sites. One of the benefits of the site selection process has been to locate gaps in previous research and identify themes and sites for future investigation. It should be emphasized that the current list of sites should not be viewed as static but rather as mirroring current and predicted future research on Scotland's diverse river systems. Undoubtedly, further potential sites could be added in due course. Finally, it should be noted that geomorphological conservation is moving away from site-specific appraisal to a more holistic assessment and management of valued landscapes.

References