Geomorphological variability among microtidal estuaries from the wave-dominated South African coast

Geomorphology 40 Ž2001. 99–122 www.elsevier.comrlocatergeomorph

Geomorphological variability among microtidal estuaries from the wave-dominated South African coast J.A.G. Cooper Coastal Studies Research Group, School of EnÕironmental Studies, UniÕersity of Ulster, Coleraine, Co. Londonderry, BT52 1SA, Northern Ireland, UK Received 23 June 2000; received in revised form 17 November 2000; accepted 30 November 2000

Abstract Over 300 independent river outlets exist around the microtidal, wave-dominated South African coast. Most of these are located in drowned river valley settings and have acquired their present morphology during the Holocene marine transgression. A variety of geomorphological forms are identified in these estuaries that result from variation in antecedent topography, fluvial sediment supply and marine sediment supply. Five distinctive estuary types are identified on the basis of contemporary morphodynamics. These are categorised into three types of normally open estuary Žthat maintain a semi-permanent connection with the open sea. and two types of normally closed estuary, which are separated from the sea for long periods by a continuous supratidal barrier. The contemporary morphodynamics and sedimentary environments of each estuary type are discussed. Open estuaries include barrier–inlet systems maintained by fluvial discharge Žtermed river-dominated estuaries. and tidal discharge Žtermed tide-dominated estuaries.. A third category of open estuary lacks a supratidal barrier due to inadequate marine sediment availability. Closed estuaries receive marine influence via barrier overwash and occasional breaching but are typically enclosed behind a continuous supratidal barrier. Two categories of closed estuary are identified: perched and non-perched. Perched estuaries develop behind high berms and maintain a water level above high tide level in the open sea. They are often dominated by fresh to brackish water and breach and drain periodically. Non-perched estuaries are developed behind low-elevation barriers fronted by wide dissipative beach profiles. High overwash frequency introduces marine water into such systems. Breaching of such estuaries does not produce dramatic draining as the water level may only drop according to the stage of the open sea tidal cycle. The variety of estuary types suggests several potential pathways may exist in estuarine development and that the progressive infilling model is an over simplification. q 2001 Elsevier Science B.V. All rights reserved. Keywords: Estuary; South Africa; Lagoon; Microtidal

1. Introduction Coastal water bodies exhibit considerable morphological and dynamic variability and have been variously termed estuaries, deltas, river-mouths, la-

goons. While a number of definitions have been provided and a number of comparative studies have been undertaken, the relationships between the various water body types are poorly understood ŽCooper, 1995; Perillo, 1996.. This stems in part from the

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incomplete basis of description of coastal water body types and of the controls on their development. The diversity of estuary types is well known from the literature and has been assessed in relation to tidal range ŽHayes, 1979., sedimentary infilling status ŽNicholls, 1989. and the relative influence of wave, tidal and fluvial processes ŽDavies, 1980; Cooper, 1994.. In addition, a number of regional synopses have been undertaken that seek to classify and describe the range of estuaries present within a geographic region ŽLankford, 1977; Roy, 1984; Hume and Herdendorf, 1988; Cooper et al., 1999; Kench, 1999.. While numerous descriptions of individual estuaries and regional synopses exist, the context of estuarine development needs to be addressed more widely if geomorphologists are to fully understand the dynamics and sedimentology of estuaries. In this regard, this regional study of the South African coast ŽFig. 1. eliminates three factors that contribute to estuarine variability globally, i.e. tidal range, wave energy and sea-level history, and enables insights to be made into the other controlling factors.

In this context, this paper aims Ži. to describe the types of estuary present on the South African coast; Žii. to examine the morphodynamics of each estuary type; Žiii. to discuss the controls on development of each estuary type; Živ. to provide a conceptual classification of microtidal estuaries; and Žv. to comment on potential evolutionary pathways and linkages between estuary types. In the study area, the range of controlling variables on estuary morphology is highly diverse, as the southern African subcontinent spans a number of climatic and morphological zones and is subjected to a range of marine conditions. However, there are several factors that may be regarded as virtually constant around the coast. These include the relatively low tidal range, high wave energy Žalthough a gradient does exist., the predominantly bedrock coast Žgenerally lacking a coastal plain. and a consistent

Fig. 1. Location map of the South African coast.

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sea-level history. Variability in factors that contribute to estuary morphology arises from climatic variation Žarid to humid and subtropical to temperate., discharge variation in rivers, hinterland gradient and coastal gradient, sediment supply rates from rivers, fluvial sediment texture and sediment availability and texture in the coastal zone. The range of variation among these and other factors potentially produces a wide range of estuary types. An estuary has been defined Žin a South African context. as ‘a coastal body of water in intermittent contact with the open sea and within which sea water is measurably diluted with fresh water from land drainage’ ŽDay, 1980.. This definition, which is a variation on that of Cameron and Pritchard Ž1963., seeks to take account of the non-permanent nature of many South African estuary mouths. In doing so it also includes a number of systems that may be regularly hypersaline or even dry for prolonged periods. Since such systems are inhospitable for most forms of life normally associated with estuaries, Žalthough notably not necessarily the avifauna. they are best regarded as distinct from estuaries. Estuaries have long been recognised as an important element in the coastal geomorphology of South Africa. They are numerous and are frequently the focus of sand accumulations Žsand barriers. that have achieved great importance as foci of recreational activity Žoften in conjunction with the adjacent water bodies both landward and seaward of the barrier.. Estuaries too have achieved recognition through their ecological importance and are widely regarded as critical in terms of their contribution to marine fish and invertebrate stocks. As habitats in themselves they have received less attention, although in recent years much more attention has focused in this area of research. Geomorphologically, estuaries occupy a transitional position between land and sea and act as an interface between terrestrial and marine environments. As such, they are affected by variations in the intensity of both sets of processes. This renders many estuaries highly dynamic environments in which geomorphological change may occur at timescales that range from almost instantaneous Žas, for example, during river floods. to progressive change due to sediment infilling and sea-level rise. In all estuaries an interface exists between fluvial

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and marine processes, although the nature and intensity of processes operating at this interface may vary considerably. In some systems, sea water may extend many kilometres upstream while in others it may be restricted to inputs from barrier overwash and be confined to areas adjacent to the barrier. A strong seasonal imprint exists in some estuaries such that they may be tidal for long distances during the dry season and freshwater extends right to the barrier Žor beyond. during river floods ŽDay, 1981a.. In others, a seasonal imprint is produced by variation between closed conditions in the dry season and open conditions during the rainy season. In a sedimentological context, sediment that moves seaward from rivers must pass through an estuary and vice versa. Estuary morphodynamics therefore reflect the relative balance between these two competing sets of processes. The seaward transfer of sediment from rivers is not constant in time or space. Those that have steep readily eroded hinterlands are more likely to yield large quantities of sediment than those that are better vegetated and less steep. The former are more likely to fill their estuaries with fluvial sediment than the latter. Sediment moving landward from the sea into estuaries is a common phenomenon and is driven by the tidal asymmetry that exists in constricted inlets. Flood tides in these inlets have a shorter duration and hence faster current regime than ebb tides and, thus, sediment accumulates preferentially in the estuary in the form of a flood-tidal delta. In many instances, these flood-tidal deltas gradually extend upstream. In others they are poorly developed or absent entirely. Microtidal estuaries are widely regarded as sinks Žnet accumulation zones. of sediment ŽRoy, 1984.. They receive sediment from a variety of sources including marine and fluvial inputs as well as detritus from fringing vegetation, material Žmainly organic. generated within the estuary itself, aeolian sediment input and human-induced inputs ŽCooper, 1995.. Estuaries are thus often viewed as areas that should exhibit progressive shallowing and reduction in area ŽRoy, 1984.. This is, in fact, not always the case. River floods periodically scour estuaries and remove accumulated sediment thus ‘resetting the evolutionary clock’ ŽCooper, 1994.. In addition, some estuaries function mainly as ‘conveyor belts’ by which excess sediment is transferred through them

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and deposited in the sea Žsuch systems have no capacity to accumulate additional sediment.. In all estuaries, the presence and persistence of a mouth Žinlet or outlet. that permits a surface water connection between the estuary and the sea is dependent on the relative strength of inlet currents Žwhich act to maintain the mouth. and wave and tidal currents in the nearshore zone Žwhich act to close to mouth by depositing marine sand.. The balance between these processes determines the nature and persistence of an estuary mouth. The situation is variable with time. The persistence of a mouth has importance for the migration of flora and fauna between the estuary and the sea and is a key factor in producing variability between estuaries ŽWhitfield, 1983, 1994, 1998.. Wide geomorphological variability exists between the estuaries of South Africa ŽReddering, 1988; Whitfield, 1992; Cooper et al., 1999. in spite of the commonality of several factors that produce estuarine variability worldwide.

2. Coastal setting The South African coast ŽFig. 1. is highly variable both geomorphologically and climatologically. These two main categories of variability contribute much to the variation that exists between estuaries. There are, however, a number of factors that do not vary so considerably and may be regarded as consistent for the purposes of this paper. The tidal range around the South African coast varies comparatively little with most areas experiencing a spring tidal range between 1.8 and 2.0 m rendering the coast microtidal ŽDavies, 1980.. Neap tides are typically between 0.6 and 0.8 m. Wave energy is also consistently high around the South African coast ŽHRU, 1968., although a slight peak in wave heights Žmodal wave height: 2.1 m, period: 11 s. is evident in the southern Cape. Wave height and period diminishes slightly northward along both the east and west coasts Žmodal wave height: 2.07, period: 9 s at Richards Bay.. The entire coast is a high energy, swell-dominated environment. Generally, beach profiles become steeper as wave energy diminishes and grain size increases. Although wave characteristics do not vary appreciably around the South African coast, there is a regional scale fining of

littoral sediment southward. ŽThis results from sediment source material variability.. Thus, high wave energy and fine sand in the southern Cape generally produces wide, gently sloping dissipative beaches with multiple offshore bars while the coarser sediment of KwaZulu–Natal and Namaqualand produces characteristically steeper and narrower beaches with high berms. A further common factor is that almost all estuaries in South Africa are located in incised bedrock valleys and thus are laterally confined. Some estuary channels fill their entire bedrock valley while others have a substantial floodplain, but most are nonetheless confined by their bedrock valley. Only a few examples of coastal plain estuaries Ži.e., estuaries formed in semi-consolidated alluvium on coastal plains. are present on the South African coast. These are mainly confined to northern KwaZulu–Natal and to the Wilderness Lakes area and in both instances produce estuaries that are linked to substantial water bodies Žlocally termed coastal lakes; Whitfield, 1992.. Factors that contribute to variability among estuaries around the South African coast include catchment size and gradient, fluvial sediment supply, marine sediment availability, climate and fluvial discharge. The South African coastal hinterland is typically gently sloping in the west and very steep in the east, with the south coast exhibiting variations in gradient according to whether mountain ranges intersect the coast or not. The hinterland gradients on the east coast are among the steepest found in the world. Climatologically, the subcontinent may be divided into several zones. The east coast is a subtropical humid zone that experiences a peak in summer rainfall. The west coast is a highly arid zone with extremely erratic rainfall. The south coast experiences varying rainfall regimes with some areas exhibiting a summer peak and others a winter peak and some are bimodal ŽHeydorn and Tinley, 1980.. A consequence of this climatological variability is a variation in rainfall and river runoff.

3. Estuary morphodynamics In order to address estuarine morphodynamics around the South African coast, the major variability

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in estuarine form was assessed during a 7-year field data-collection programme during which 280 estuaries were visited. A provisional classification based upon the main forms of morphological variability among estuaries of the South African coast was developed and is reported here. At the highest level, the major variation between estuaries is based upon the frequency of connection with the sea via a surface channel. This divides estuaries into those that are normally open Ži.e., they have a surface water connection with the sea. and those that are more commonly closed by a barrier and which accomplish fluvial discharge via barrier seepage and evaporation losses. This division, which is theoretically entirely gradational, appears in practice to identify two groups of estuary. In KwaZulu–Natal, where the most extensive set of observations on frequency of mouth opening exists, estuaries divide clearly into those that are open more than 70% of the time and those that are open less than 30% of the time ŽBegg, 1984.. There is clearly longer term variability in this as drought cycles ŽTyson, 1987. are more likely to be associated with reduced freshwater discharge and may cause estuaries to close for longer periods during droughts. The categories ‘normally open’ and ‘normally closed’ will be discussed below and further subdivisions are described within these two major categories, based upon geomorphology and morphodynamic behaviour of various types of system.

4. Open estuaries Those systems that are normally open display marked variability in size and in mouth dynamics. They may be subdivided into systems that are essentially unbarred Ži.e., they lack a sand accumulation at the mouth that is exposed above high tide. and those that have a supratidal barrier with a surface drainage channel. Further subdivisions may be made on the basis of the size of the individual estuary. 4.1. Barred open estuaries Barred estuaries Žthose with a supratidal barrier. vary markedly in size and in the volume of fluvial discharge. Since the volume of the bedrock valley in

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which the estuary is formed is largely dependent on the amount of fluvial discharge, large catchment rivers tend to form bigger bedrock valleys. Barred estuaries that are normally open range in discharge from small, localised stream catchment systems to large systems such as the Orange ŽGariep., Great Fish and Tugela ŽThukela. that drain large sections of the subcontinent. The smaller systems are incapable of maintaining large tidal prisms and are thus maintained in an open condition by river discharge. This is often assisted by the outlet whereby a bedrock ledge permits enhanced scouring. Such outlets frequently form in the lee of a headland where wave energy is reduced. These systems seldom exhibit a flood tidal delta and many appear to operate almost exclusively as outlet channels. Estuarine characteristics are however imparted by the frequency of overwashing and by occasional surges that increase tidal penetration into the estuary. These systems tend to have short barriers that reduce the volume of discharge that can be accommodated by seepage through the barrier such as was noted in the barriers of SE Ireland by Carter and Orford Ž1984.. The typically open barred estuaries span a range of discharge characteristics and are divisible into two types. 4.1.1. Tide-dominated estuaries Discussion. Tide-dominated estuaries are defined here as those that, in spite of their low tidal range have sufficient tidal prism to permit their inlets to be maintained by tidal currents against longshore and cross-shore wave-driven littoral sediment transport. Such estuaries have been described from a number of localities globally ŽRoy, 1984; Reddering and Esterhuysen, 1987a; Cooper, 1993a; Nichol et al., 1994. and appear to be regarded as the ‘typical’ microtidal estuary. Facies arrangements in such estuaries ŽFig. 2. essentially follow those described by Hayes Ž1979. in microtidal barrier island environments, in that high wave energy usually tends to minimise the development of ebb-tidal deltas, whereas flood-tidal deltas are well developed landward of the barrier. If sufficient sediment is available, delta growth may be sustained for long periods. Reddering and Esterhuysen Ž1981., for example, documented progressive growth of flood tidal deltas into the Sundays Estuary that is indicative of flood-

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Fig. 2. Morphology and sedimentary facies of tide-dominated microtidal estuaries. Ža. Nahoon Žafter Reddering and Esterhuysen, 1987a., Žb. St Lucia Žafter Wright and Mason, 1993.. Note the Tripartite division into lower sandy reaches, deep, muddy middle reaches and a fluvial delta upstream.

dominance and the ability of such inlets to act as sinks for marine sediment. Wright and Mason Ž1993. also noted the progressive accumulation of sand on prograding food-tidal deltas in St Lucia Estuary, South Africa ŽFig. 3.. Similar landward bedload sediment transport into estuaries was recognised by Meade Ž1969. and Hayes Ž1975. in US East coast estuaries and is ascribed to flood-tide dominance. Tide-dominated estuaries thus act as sinks of marine sediment through flood-tidal dominance at the estuary inlet. If the estuary forms an extensive shoreparallel extension at the coast, inlets may migrate. This process and the associated stratigraphic sequence was described for the Keurbooms estuary ŽReddering, 1983..

Upstream of the tidal deltas, tide-dominated estuaries tend to be deep and dominated by suspension settling of fine-grained sediment. The Mtamvuna estuary, for example, maintains depths of 13 m ŽCooper, 1993a.. At the tidal limit of the estuary, fine-grained sedimentation is replaced by deposition of fluvial sediment in the form of a delta that may vary in texture according to the catchment geology of the river catchment. Many estuaries in the eastern Cape Province, South Africa maintain this pattern ŽReddering and Esterhuysen, 1981, 1984a, 1987a.. The evolution of such systems is typically viewed as that of downstream delta growth and gradual infilling by sediment from fluvial, marine and intra-estuary sources ŽRoy, 1984; Cooper, 1993a.. Nicholls Ž1989.

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Fig. 3. Advance of flood-tidal delta front into St Lucia estuary Žafter Wright and Mason, 1993.. This growth is supplied by sand derived from seaward of the inlet that is, in turn, supplied by the adjacent, river-dominated Mfolozi estuary.

examined the response of such systems to changes in sea level and sedimentation rates and indicated that they could be sustained if sea-level rise and sedimentation rates were balanced. Those systems maintained by tidal flow are recognisable by virtue of a distinctive morphology. They have well developed, often transgressive flood-tidal deltas ŽReddering and Esterhuysen, 1981; Cooper, 1993a; Wright and Mason, 1993.. This is accomplished as a result of flood-dominance at the tidal inlet and by the enhanced suspension of sediment by wave action, that is then entrained on the flood current. Wave action does not assist in the return flow and thus net accumulation takes place ŽFig. 3.. Landward of the tidal deltas is a relatively deep section characterised by fine-grained sedimentation, and upstream is a coarse grained fluvial delta that progrades into the estuary, gradually reducing its volume. Riverine floods in such systems serve an important geomorphological function in removing accumulating flood-tidal deltas.

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Inlets at tidally maintained systems may close through the effects of extreme marine events. The Kosi Estuary in KwaZulu–Natal ŽWright et al., 1997. with a tidal prism in excess of 350 million m3 , has closed Žon August 17, 1965. and this was probably as a result of a storm event. After tropical cyclone Claude Žin January 1966., water levels rose by 1.6 m ŽBreen and Hill, 1969.. The system, however, did not open of its own accord and was artificially opened to save the mangrove community. Smaller tidally maintained outlets may close as a result of less intense storms as the tidal flow that is required to be overcome is smaller. These systems may then be reopened by renewed freshwater discharge that reestablishes the tidal inlet and permits tidal currents to be reinstated. Discussion. Tide-dominated microtidal estuaries exhibit a distinctive tri-partite facies arrangement based on marine inputs at the inlet, fluvial inputs at the tidal head, and a quiet water suspension-settlingdominated zone in the middle reaches ŽFig. 4.. It has long been recognised ŽMeade, 1969. that landward transport of bedload in micro and mesotidal estuaries via flood-dominant tidal flow leads to the development of distinctive flood-tidal deltas ŽHayes, 1979.. Such inlets thus act as marine sediment sinks and their dynamics are driven by tidal currents, that are themselves dependent on an adequate tidal prism. Under such circumstances, tide-dominated estuaries may persist even in the absence of fluvial discharge. Inlets of tidally maintained systems may close following marine storms. Observations in Nahoon ŽReddering and Esterhuysen, 1987a. and St Lucia ŽWright and Mason, 1990, 1993. indicate that riverine floods too play a role in developing a balance between progressive flood-tidal deposition and erosion in such estuaries, whereas in others Že.g., Mtamvuna; Cooper, 1993a. that have a limited marine sediment supply, riverine floods cause seaward reworking of a finite sediment volume. 4.1.2. RiÕer-dominated estuaries River-dominated estuaries have insufficient tidal prisms to maintain an inlet against nearshore wave and tidal action. This may arise through either preexisting morphological constraints Žsteep gradient, high sediment supply during transgression and narrow bedrock valleys. or through sedimentation and

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Fig. 4. Generalised morphology of tide-dominated estuaries in plan Ža. and cross-section Žb.. Note the flood-tidal deltas which may show landward progradation and the small ebb-tidal delta which is confined by high wave energy. The fluvial delta marks the downstream limit of coarse-grained riverine sediment.

infilling of formerly tidally maintained systems. These systems are especially well developed in KwaZulu–Natal and have been termed ‘riverdominated’ estuaries ŽCooper, 1993b, 1994; Cooper et al., 1999.. Morphologically these systems differ from tidally dominated systems and in them floodtidal deltas are much reduced in size or absent. Fluvial sediment typically extends to the barrier ŽFig. 5. and tidal inflow is frequently minimised by elevated bed levels. Under lower wave energy conditions these systems would probably form deltas, but in the present setting, sediment dispersal occurs sea-

ward of the river mouth. The only examples of such deltas on the South African coast are those of the Orange ŽGariep. River Žsubmerged. ŽVan Heerden, 1986. and the Tugela ŽThukela. and Berg which have offshore mud depocentres and a series of beach ridges located north Ždowndrift. of the estuary mouth. River flow in such systems is critical to the maintenance of an outlet channel. Under drought conditions inlets may close for prolonged periods. The impacts of impoundments are especially notable in these types of systems. For example, the Mgeni estuary which was formerly open for more than 90% of the year is now closed for prolonged periods since the completion of Inanda Dam, some 35 km upstream of the mouth. Stratigraphical evidence suggests that many river-dominated systems have been in such a state throughout much of the Holocene period ŽCooper, 1993b.. While many of these systems have relatively large surface areas, many are comparatively short, due to the steepness of the hinterland and this limits tidal penetration and hence tidal prism. The role of river floods in these systems is important in eroding accumulated sediment and temporarily deepening the channel ŽCooper et al., 1990.. Erosion during extreme river floods appears to be distributed throughout the channel and even cohesive sediments may be eroded. The Mgeni for example in 1987 lost an entire, vegetated and mangrove-fringed island from its lower reaches as a result of a riverine flood. Sedimentary facies in river-dominated estuaries are essentially fluvial, although an estuarine fauna may produce distinctive trace fossil and occasional body fossil assemblages ŽCooper, 1988; Cooper and McMillan, 1987.. Analysis of valley fill sequences ŽOrme, 1975. shows the incised valleys to be entirely filled by successive scour and fill events associated with periodic floods. Discussion. River-dominated estuaries are defined as those in which an inlet is maintained by fluvial discharge and in which the tidal prism is too small to generate currents adequate to overcome wave-induced sediment transport in the nearshore that acts to close the barrier inlet. Such estuaries in South Africa ŽFig. 6. are characterised by shallow or intertidally exposed back-barrier areas in which fluvial sediment extends close to, or directly to the landward margin of the estuary barrier. This is attributed to high fluvial sediment supply from steep hinterland and

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Fig. 5. Morphology of two river-dominated estuaries. Ža. Mvoti Žafter Cooper, 1993b. and Žb. Orange Žafter van Heerden, 1986.. The floodplain limits are shown. In the Mvoti, a narrow, braided channel flows across the vegetated floodplain and former channel positions may be seen from lineations. In the Orange, the floodplain is filled largely by a braided channel with vegetation on some braid bars.

deeply eroded catchments. Ebb-tidal deltas are poorly developed or absent due to strong wave energy and flood-tidal deltas are small or absent due to the weak tidal currents and lack of accommodation space in the sediment-filled channel. Cooper Ž1988. recorded a small flood-tidal delta in the Mgeni estuary, while no flood-tidal delta is present in the Mvoti ŽCooper, 1993b. or Orange ŽVan Heerden, 1986. estuaries. In such filled channels, additional fluvial sediment passes through the estuary and is deposited in the sea where it is dispersed without forming a delta. In some instances Že.g., Tugela, Berg. this sediment may accumulate on beachridge plains updrift of the estuary ŽCooper, 1991. if sufficient accommodation space exists to permit coastal aggradation. In the United States, Horne and Patton Ž1989. documented the bedload dynamics of the Connecticut River estuary and noted that it too exhibited net

seaward sediment transport and was essentially sediment filled. Such river-dominated estuaries generally act as conduits of sediment to the sea and are thus sources of sediment in the coastal zone rather than sinks for marine sand. They are thus fundamentally different in morphodynamic behaviour to tidedominated systems Žwhich accumulate marine sediment.. 4.2. Non-barred estuaries Several estuaries exist within drowned river valleys that have either no sand accumulation at their outlet or have a sand body that is only intertidally exposed and is, in effect, the upper surface of a flood-tidal delta. Such situations characterise the mouths of the Sout Estuary ŽMorant and Bickerton, 1982. and the Mtentu ŽConnell, 1974.. Incident wave

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Fig. 6. Generalised morphology of river-dominated estuary in plan Ža. and cross-section Žb.. Note the extension of riverine bars to the barrier and the small extent of flood- and ebb-tidal deltas.

energy, which may be high, is attenuated on these intertidal sand bodies ŽFig. 7. and the main channel maintains comparatively quiet water conditions. In some instances, however, such as the Steenbras ŽHeinecken et al., 1982; Harrison, 1998. no sand body occurs at the inlet due to a lack of marine sediment and wave action penetrates along the bedrock valley to be dissipated against the leading edge of the fluvial delta ŽFig. 7.. The middle reaches of these types of system tend to be relatively deep Ž10–20 m in the Steenbras and 4–5 m in Mtentu. and are bottomed by bedrock, gravel or sand depending on fluvial sediment supply. The upper reaches typically comprise a fluvial delta

in which bioturbation by burrowing estuarine mudprawns ŽUpogebia africana. and sandprawns Ž Callianassa kraussi . occurs. Salinity varies along such systems and stratification is common with marine waters at the bed of the estuary and freshwater on the surface. Two large variations on this type of system exist; Knysna estuary ŽReddering and Esterhuysen, 1987b. and Langebaan Lagoon ŽFlemming, 1977.. In both instances a scarcity of marine sediment has prevented barrier development. In the Knysna estuary ŽFig. 7., barrier development is impeded by a lack of accommodation space between the rock headlands that bound the tidal inlet. The tidal inlet is 15 m deep and 120 m wide. The inner estuary is, however, largely sediment-fringed and comprises a meandering channel Ž2–4 m deep. flanked by broad intertidal flats up to 2 km wide. Reddering and Esterhuysen Ž1987b. noted that fluvial and marine sediment characterised the upper and lower reaches, respectively, and concluded that low fluvial and marine sediment supply rendered the Knysna estuary relatively youthful in terms of its sedimentary evolution. Discussion. The non-barred estuaries present on the South African coast exhibit a common lack of a supratidal barrier that precludes inlet closure. In this regard they are unusual on this high energy coast where almost all estuaries have the potential to close under certain Žalbeit extreme. wave conditions. Sediment accumulations exist in the inlets of some of these estuaries as intertidal bodies of marine sand ŽFig. 8.. They represent the upper surfaces of floodtidal deltas that are restricted in upward growth by absence of additional marine sediment. Where present, such sand bodies act to attenuate wave action and induce tranquil water conditions in the estuary. In cases where no sand body is present, waves may propagate into the estuary Že.g., Steenbras. and produce higher energy conditions while in estuaries with subtidal sand bodies, wave energy dissipation may take place. All such estuaries occur on rocky, sediment starved coastal sectors. Channel environments in these estuaries do exhibit some variability in that some are bedrock-confined ŽSteenbras, sections of Mtentu. while others contain intertidal or subtidal sediment bodies ŽKnysna. or supratidal fringing sediments Že.g., Mtentu.. Some contain distinct fluvial deltas at the tidal head. The Knysna shows a fluvial

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Fig. 7. Non-barred estuaries Ža. Mtentu Žafter Connell, 1974., Žb. Steenbras Žafter Heinecken et al., 1982., Žc. Knysna Žafter Reddering and Esterhuysen, 1987a.. Whereas wave energy is attenuated on the intertidal barrier of the Mtentu, it propogates up the Steenbras to the fluvial delta. The sand within the Knysna channel is relict.

delta at the head and an area of marine sand in the lower reaches while the middle reaches are underlain by relict aeolian sand. This variability in back-barrier morphology is a response to both the contemporary

fluvial sediment supply and the antecedent bedrock morphology. Wider bedrock valleys provide accommodation space for accumulation of fluvial sediments while narrow, steep sided valleys do not.

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with minimal marine influence. None of these has been described geomorphologically.

5. Closed estuaries The normally closed estuaries of South Africa span great variation in size from tiny systems of - 0.2-ha surface area to systems such as the Bot and Klein that exceed 1200 ha in area. Two main forms of morphodynamic behaviour are exhibited by these systems, based on whether the back-barrier water level is higher than open sea tidal levels or within the range of open sea tidal elevations. This, in turn, is related to the elevation of the berm crest of barriers that enclose these systems. These normally closed systems may be subdivided into perched and non-perched categories. These are discussed below. 5.1. Perched closed estuaries Fig. 8. Generalised model of non-barred estuary showing plan ŽA. and cross-sectional ŽB. perspective. Note the intertidal barrier that is submerged at high tide and emergent at low tide. In such systems, insufficient sediment is available to permit mouth closure. The fluvial delta upstream marks the limit of coarse-grained riverine sediment.

These estuaries share some features in common with what Roy Ž1984. termed ‘drowned river valley’ estuaries in NSW Australia and in which marine processes Žtidal and wave. penetrate long distances up the estuarine channel ŽKench, 1999.. In an evolutionary context, these estuaries probably represent various stages of infilling. Given the lack of marine sediment availability, each could evolve via fluvial delta progradation such that marine influence is reduced and the bedrock valley is filled by a delta topped by a terrestrial drainage channel. Under these conditions, wave-reworking of the delta front could produce a barrier composed of fluvial sediment. Potential examples of such systems occur locally on the South African coast Že.g., Crooks, Kranshoek, Grooteiland between Knysna and Plettenberg Bay. where barriers are composed of wave-reworked coarse-grained fluvial sediment. Back barrier environments comprise a variety of water body types from isolated ponds to fluvial channels, all of which contain freshwater drainage channels

Those systems that have a high berm, produced as a result of coarse grained barrier sediment and relatively low wave energy Žin a South African context. impound water levels behind them at elevations above the levels of most high tides. Several such systems have been described ŽFig. 9. including Mhlanga ŽCooper, 1989., Mdloti ŽGrobbler et al., 1987. and uMgababa ŽGrobbler et al., 1988., all of which are in KwaZulu–Natal. Mhlanga Lagoon is impounded behind a normally continuous supratidal sandy barrier that is 700 m long and whose crest elevation is between 1 and 2 m above high tide. The back-barrier area comprises a channel that is ca. 1.5 m deep, 100 m wide and extends for ca. 1500 m landward of the barrier. It is fringed by a Phragmites reedswamp. Back-barrier sediments comprise fine-grained sand and mud derived from the river catchment, with an apron of marine sands derived by barrier overwash, deposited adjacent to the barrier. When the barrier is breached, the elevated bed level in these lagoons means that they almost drain, exposing formerly submerged areas of the bed subaerially ŽFig. 9.. Breaching associated with river floods may produce large bedforms on the lagoon bed Že.g., Mhlanga, Fig. 9., generated by strong seaward-flowing currents. These are potentially pre-

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Fig. 9. Examples of perched estuaries when closed and post-breaching. Ža. Mdloti, Žb. Mdloti after flood-induced breaching Žafter Grobbler et al., 1987., Žc, d. Mhlanga before and after flood-induced breaching Žafter Cooper, 1989..

served when the estuary seals and fine-grained deposition is re-instated. Crevasse-splay deposits were formed adjacent to the flood-related channel in the Mdloti ŽFig. 9.. Breached barrier outlets normally close rapidly Žwithin ca. 10 days in the case of the Mhlanga.. Little seawater enters such estuaries during breached phases due to elevated bed levels and instead a freshwater stream discharges through the emergent estuary bed. Surveys of the bed level of the estuary

ŽMason et al., 1989. after a severe river flood, however, indicate that the base of the channel in the estuary falls to y0.4 m MSL for 600 m upstream of the barrier. This permitted limited tidal inflow during the post-flood open phase via a 20- to 30-m wide channel. The small tidal prism associated with this was not sufficient to sustain the outlet and it rapidly closed after breaching. During closed phases, the base level of the river is graded to a level above sea level. When the system

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breaches, whether at high or low tide, sufficient elevation difference exists between the estuary water and the sea for scouring to produce a channel in the sandy barrier. If the freshwater discharge is prolonged, scouring of the bed of the estuary is possible as the base level of the system has effectively dropped. In some instances an ephemeral delta is produced at the seaward terminus of the breach channel. Cooper Ž1989. noted that generally, only fine sand and organic matter were removed from the Mhlanga during breaching, although a sustained riverine flood produced significant modification of the estuary bed. Salinity of water in these estuaries varies between totally fresh and near marine with persistent stratification ŽHarrison and Whitfield, 1995.. Long term average low salinities are indicated by Phragmites reeds and freshwater-associated trees ŽBarringtonia and Hibiscus. that exhibit limited salt tolerance. Water level in the estuary is perched above sea level and remains at a near constant level due to a balance between freshwater inflow and losses via seepage and evaporation. In spite of the low salinity, estuarine invertebrates Žincluding C. kraussi . colonise the bed of the estuary and cause extensive bioturbation. Discussion. The enclosed perched water bodies ŽFig. 10. exhibit a long-term balance between inputs from freshwater inflow, barrier overwashing and rainfall, and outputs via evaporation, seepage and evapotranspiration by fringing vegetation. Vegetation within such systems is typically graded to a perched water level, as is sedimentation such that the bed of the estuary may be elevated above tidal levels. In such instances, tidal inflow during open phases may be precluded even at high tide. Quiet water conditions generally prevail in these estuaries and this promotes suspension settling and mud deposition. Organic matter accumulates on the bed of such estuaries during closed phases and is eroded during open phases. Breaching occurs when inputs via overwash and freshwater discharge exceed outputs and a surface channel is cut to permit discharge. When this occurs, the elevated water level provides a marked hydraulic head and rapid downcutting occurs into the berm Žits depth is dependent on the state of the tide when breaching occurs.. Under such conditions, such systems may drain within a few hours and a formerly large surface area

is reduced to a narrow shallow channel through which fluvial discharge is effected. Barrier breaching of such systems typically deposits an ephemeral delta from which sediment is eroded and transported by cross-shore transport to close the breach, whereafter the estuary fills and attains equilibrium once again ŽCooper, 1990.. Salinity in such systems is typically lowered and may even be totally fresh, dependent on the volume of overwash received. Minor variability between perched estuaries occurs as a result of variability in the elevation of the estuary bed. Although most are at or slightly above sea level Žas their base level is effectively slightly above sea level. some still retain part of their channel below sea level and this may permit tidal incursion. The distribution of these estuaries is mainly on the east coast where marine sediment is coarser grained and here too, high fluvial sediment yields frequently fill the estuarine channel. 5.2. Non-perched closed estuaries Those systems that do not have a high berm but which commonly lack a surface channel are impounded at or close to high tide level. The beaches that front these systems have a dissipative Žlow gradient. profile and are characterised by wide surf zones. The lack of a berm and high wave energy means that barrier overwash is more frequent in these systems and consequently they are typically saline. In addition, in areas of low coastal gradient, saline influences may extend for several km upstream even in the absence of tidal inflow. These systems often exhibit salt marsh vegetation as a result. No definitive study has been undertaken of this type of system, however, a number of observations over the past 10 years by the author, together with published descriptions of the Kabeljous ŽReddering and Esterhuysen, 1984b; Bickerton and Pierce, 1988., and Seekoei ŽBickerton and Pierce, 1988., permit some insights to be gained on their behaviour ŽFig. 11.. Day Ž1981b. reports additional observations on the west Kleinemond estuary and the Mbanyana estuary north of the Bashee estuary both of which are normally closed and have a saline back-barrier estuary as a result of frequent overwashing while the Qinira estuary in the Eastern Cape ŽWiseman et al., 1993. exhibits similar characteristics.

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Fig. 10. Generalised model of perched estuary behaviour showing cross-sectional Ža–d. and plan Že. views. Under balanced conditions Ža., the stream inflow is matched by evapotranspiration and seepage. Overwashing Žb. may elevate water levels and salinity while increased streamflow Žc. may promote breaching. When breached Žd., the water levels are lowered and tidal flow may take place if bed levels are sufficiently low. In the plan view Že., the difference in water area during open and closed conditions is evident.

The lower reaches of these estuaries are dominated by marine sand Žderived by frequent barrier

overwash. and the upper reaches comprise sand of fluvial origin. Marine sand is deposited as washover

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Fig. 11. Non-perched estuaries. Ža. Seekoei Žafter Bickerton and Pierce, 1988., Žb. Kabeljous Žafter Reddering and Esterhuysen, 1984b.. The position of the breach during open phases is shown. Note the ephemeral flood tidal delta in the Seekoei that is active during open phases. Much marine sand is introduced to both systems via barrier overwashing.

fans and occasionally as flood-tidal deltas during open phases. The supratidal barriers are composed of fine-grained sand and have a low gradient, dissipative profile Ždipping both landward and seaward at

1–28 in the Kabeljous.. Crest elevations are approximately equal to spring high tide level so frequent overwashing occurs. Small ephemeral dunes may form on top of the wet sand bar surface and migrate

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Fig. 12. Generalised model of non-perched estuaries in cross-section Ža–c. and plan Žd.. Under balanced conditions Ža., streamflow is balanced by losses through evaporation and seepage. Under high wave energy Žb., overwashing introduces marine water and sediment. Under enhanced inputs from overwashing or streamflow, the system may breach. The depth of incision is low since the estuary water level is so close to sea level. In plan view Žd., the consistency of surface water area between open and closed phases is clear.

alongshore as starved bedforms over the barrier surface.

During river floods, barriers may be breached and an ephemeral inlet develops. Due to the low surface

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water elevation in the estuary, inlet formation only occurs during low tides when sufficient gradient exists to permit channel scour. Bickerton and Pierce Ž1988. observed that inlet channels formed particularly during spring low tides. Observations of seaward flow during high tide, indicate that this occurs via a shallow Ž- 10 cm. braided sandy channel developed on the surface of the barrier. Low estuary bed elevations during open phases permit floodtidal currents to develop in the ephemeral inlet and flood-tidal deltas are deposited in some of these estuaries such as the Kabeljous ŽBickerton and Pierce, 1988.. Salinity in such systems is typically fully marine with minor freshwater dilution. Although these estuaries are normally closed, overwashing delivers large quantities of seawater into the estuary. Landward percolation of seawater may also occur during high tide. Mixing of the water column in the shallow system means that stratification seldom develops. High salinity is promoted by the relatively arid hinterland in some instances and Whitfield and Bruton Ž1989. recorded hypersaline conditions Ž55 ppt. in the Kabeljous during a drought phase when freshwater dilution stopped and evaporation losses exceeded seawater inputs. Vegetation in these systems is consequently dominated by salt-tolerant species. In the Kabeljous Zostera capensis and Juncus kraussi are abundant. Bioturbation of the back-barrier sediments takes place by the burrowing sandprawn Ž C. kraussi .. Discussion. Relatively few non-perched estuaries have been studied in South Africa, although they are abundant in the southern and eastern Cape. These estuaries share several common characteristics that include a continuous supratidal barrier of low elevation such that overwashing by waves is a common occurrence ŽFig. 12.. This produces characteristic high salinities in such systems and may lead to colonisation by estuarine organisms and development of salt marshes and other salt tolerant vegetation in spite of a lack of surface water connection with the sea. Relative stability of water levels is promoted by the barrier crest elevation close to sea level and is indicated by the development of salt marshes. Following increased fluvial discharge, these systems may form a surface connection with the sea.

This may happen relatively frequently as the barrier is close to sea level and, thus, saturated which precludes seepage. When a surface channel forms it is typically shallow and broad and forms a braided pattern on the barrier surface. Water depths are typically a few centimetres as downward scour is precluded by the low water level in the enclosed estuary. With strong fluvial discharge that coincides with low tide, scouring of the barrier may occur and an ephemeral inlet may be produced that persists for periods up to 2 months, although it is likely much shorter under most circumstances. The low bed levels in these estuaries permits flood-tidal flow to develop in the estuary when open and temporarily active flood-tidal deltas are reactivated during such phases. When such systems begin to experience reduced water levels through drought, landward seepage may occur as evidenced by rill marks and microchannels and deltas on the landward slope of the barrier. In some instances these systems have been known to become hypersaline. The large surface areas of such systems coupled with a strong wind regime means that the water column is frequently well mixed. Since the surface channels are typically very shallow, tidal inflow is also reduced, although evidence of flood-tidal delta morphology in some systems of this type does suggest that under certain conditions Žprobably associated with positive surges andror low estuary water levels., tidal inflow may occur. A characteristic of these systems is the constancy of their water area and volume which provides a more stable habitat than the perched systems.

6. General discussion A variety of estuarine types that exhibit distinctive morphodynamic behaviour has been identified on the microtidal South African coast ŽFig. 13.. These may be classified initially according to whether they are open or closed Ži.e., whether they have a near-permanent or ephemeral inlet.. Each of these categories is further subdivided. Each estuary type contains a distinctive suite of sedimentary environments and geomorphological units that reflect con-

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Fig. 13. Conceptual morphodynamic classification of microtidal estuaries. The major division is based on whether an estuary is normally connected to the sea by a surface channel Žopen. or not Žclosed.. The open estuaries are further subdivided on the basis of whether a barrier is present and if so, whether the outlet is maintained by river or tidal influences. Closed estuaries are subdivided according to whether the back-barrier water surface is perched above sea level or not. Further subdivision may be made on the basis of within-group variability in for example sediment type and physical size.

temporary processes and are capable of preservation in the geological record. In addition to controls exerted at the site-specific level, the distribution of estuary types follows a broad pattern around the coast that reflects regional variability in climate, topography and sediment availability ŽFig. 14.. River-dominated estuaries occur largely on the northeast coast where a steep hinterland limits the potential length of estuarine incursion into drowned river valleys. The steep hinterland and subtropical climate promotes high fluvial sediment yields and high fluvial discharge, both of which contribute to rapid sediment filling and river maintained inlets. Many of the open estuaries in KwaZulu–Natal are river-dominated. Outside this zone, river-dominance occurs under two additional scenarios. Where large-catchment rivers with high fluvial sediment loads discharge through low gradient channels, an inlet may be maintained by river discharge. Several examples exist in this setting including the Great Kei, Great Fish and Orange estuaries, each of which has a catchment of several thousand km2 . In addition, a number of small, permanently open river-dominated estuaries exist in areas of high rainfall where a high runoff exists in relation to their catchment size. In the Cape Peninsula area, rainfall reaches a peak, and rivers also drain small, steep catchments. Here a number of small river-dominated estuaries occur.

Tide-dominated estuaries exist on coastal sectors with low fluvial sediment supply and low gradient. Here, low gradients promote tidal incursion far into estuarine valleys and large tidal prisms may develop. This may also occur in coastal plain settings where large intertidal areas may develop through shoreline erosion. These estuaries act as sinks for marine sediment and may exhibit upstream delta growth if sufficient marine sediment is available. They may close during marine storms or through progressive tidal delta accumulation, after which floods or human intervention is required to re-instate an outlet. While barred microtidal estuaries have been identified elsewhere ŽRoy, 1984; Kench, 1999. they are not often subdivided into tide- and river-dominated types. Rather, a sediment-filled variant is regarded as the advanced evolutionary stage of a tide-dominated estuary. In South Africa the steep hinterland and high fluvial sediment discharge creates conditions for river-dominated estuaries to persist throughout the Holocene ŽCooper, 1994.. Similar conditions occur in high topography areas that experience high wave energy including New Zealand ŽHume and Herdendorf, 1988. and Chile. Paterson et al. Ž2000. provide an account of river-mouth processes at a river-dominated estuary in New Zealand where they are termed ‘hapua’. Non-barred estuaries are restricted to a few areas of coast dominated by rock outcrop in which small

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Fig. 14. Generalised distribution of Ža. open and Žb. closed estuary types around the South African coast. River-dominated estuaries are most abundant on the steep hinterland east coast. Non-barred estuaries occur in localised coastal sectors of low sediment availability. Perched estuaries are characteristic of the coarse-grained coastal sectors of KwaZulu–Natal and localised areas of False Bay.

amounts of littoral sediment are available. This is driven into coastal embayments where sediment

availability may be insufficient to produce an emergent barrier. These estuaries are characterised by

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permanence of connection with the sea and typically high levels of rock outcrop in the estuarine channel. These types of estuary have been identified in NSW, Australia ŽRoy, 1984. and are similar to the rias, calas and calanques of the Mediterranean coast of Europe where insufficient marine sediment exists to build barriers at the mouths of drowned river valleys. Perched closed estuaries are largely confined to the NE coast where a typically coarser grain size characterises the littoral zone. This leads to the development of high berms and reflective beach profiles that permit development of perched estuaries in situations where insufficient fluvial discharge exists to maintain a tidal inlet. Periodic breaching occurs during which the estuaries drain and scour accumulated sediment. Tidal inflow following breaching is usually limited as most of these estuaries are sediment-filled and graded to a base level above sea level. Non-perched estuaries characterise most of the S and SE coasts where high wave energy and fine sediment leads to development of dissipative beach profiles. Such profiles developed across estuary mouths that have small tidal prisms or small fluvial discharge, cause separation from the sea. Since this area also has relatively low rainfall, overwashing often dominates and the water in these estuaries is typically close to fully marine salinities, despite the lack of a surface channel. Spillage of water across the barrier Žcoupled with evaporation and seepage. is usually sufficient to accommodate fluvial discharge without barrier breaching. While saline coastal lagoons have been identified in Australia, their subdivision into perched and nonperched has not been noted ŽRoy, 1984., although the literature reports both saline and freshwater varieties. Kench Ž1999. also notes that there has been a paucity of research on these types of estuary. A hierarchical classification system for South African estuaries is shown in Fig. 13. The first subdivision is made on the basis of whether a stream outlet constitutes an estuary or not. Non estuaries were selected on the basis of permanent separation from the sea, very small size, ephemeral surface water andror hypersalinity, and if they were waterfalls. For the remaining systems an estuarine function was determined on the basis of periodic communication with the sea and the permanent occurrence

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of fresh to brackish surface water in the estuary. These systems were then divided on whether they were normally open or normally closed. This assessment and classification of estuaries is based upon morphodynamics and provides a basis for estuarine classification. Further subdivision is possible on the basis of variability among these types of estuary, for example based on estuary dimensions, channel morphology, shoreline features. Since most estuaries in South Africa are located in bedrock valleys, the largest deviation in estuarine morphology is apparent in estuaries developed in the few coastal plain settings. These have been termed coastal lakes ŽWhitfield, 1992., however, they fall into a number of different morphodynamic categories as outlined here. Kosi ŽWright et al., 1997. and St Lucia estuaries ŽWright and Mason, 1993. for example, although they are linked to large coastal plain water bodies, operate as tide-dominated estuaries with flood-tidal deposition in the inlet and their inlets are maintained by tidal flow. Swartvlei ŽWhitfield et al., 1993., in contrast, is also a large coastal lake but it operates as a typically closed estuary that receives barrier overwash and occasionally breaches during river floods. 7. Evolutionary perspectives While the focus of this paper has been on the spatial variability in estuary type, the observations suggest that these estuary types may be linked in a space–time continuum that determines the potential evolutionary pathways for microtidal estuaries. Since much contemporary thinking on estuarine evolution is based upon progressive infilling models that ultimately transform deep, tide-dominated estuaries into coastal wetlands, the linkages between these estuary types requires further examination. Open estuaries that lack barriers may evolve through progressive fluvial infilling such that a fluvial delta extends to the mouth of the inlet at which stage the seaward margin of the delta may be reworked to form a barrier. The course taken by the estuary at this stage is dependent on the volume of fluvial discharge. If high, the system may evolve into a river-dominated estuary and if low it will evolve into a closed estuary, whose characteristics will be determined by coastal dynamics. Since such evolu-

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tion requires wave-reworking of fluvial sediment that is typically coarse, it is probable that such closed estuaries will be perched. River-dominated estuaries are only likely to change morphology if their discharge characteristics change through climate change or anthropogenic modification. Under reduced discharge they may change to closed estuaries. Tide-dominated estuaries may evolve through fluvial infilling such that they close Žif fluvial discharge is insufficient to maintain an inlet. or remain open as river-dominated systems if river discharge may maintain an inlet. Alternatively, tide-dominated estuaries may close through episodic storms that promote barrier aggradation. In the absence of a river flood, such a system may remain closed. With continued infilling, a closed perched estuary may become laterally confined such that water losses through evaporation are minimised. Under such circumstances, a surface outlet may form to accommodate fluvial discharge and the system may become a river-dominated estuary. Increased fluvial discharge would produce a similar change. A non-perched, closed estuary, with progressive infilling may similarly evolve into a river-dominated estuary. Since this may involve reworking of coarse-fluvial sediment at the barrier, the estuary may become perched, or, if fluvial discharge is sufficiently concentrated, it may be transformed into a river-dominated estuary. While research to date has identified several microtidal estuary types and potential evolutionary pathways, it is not possible with the present state of knowledge to definitively place every estuary in this scheme. Neither does enough evidence exist to examine the range of sedimentary infills that result from the projected evolutionary pathways. Further research based on geomorphological measurement and on sediment coring is required to enable these steps. Cooper Ž1994. presented evidence of change from tide-dominated to river-dominated conditions over the Holocene timescale in the Mgeni estuary. Grobbler et al. Ž1987. present evidence from cores that suggests a change from tide-dominated open conditions to perched closed conditions in the Mdloti estuary. These findings suggest that no single model of microtidal estuaries is universally applicable, either in terms of contemporary morphodynamics or Holocene evolutionary pathways.

Acknowledgements This work was undertaken with support from the South African Department of Environment Affairs and Tourism, the CSIR, and the University of Ulster over a number of years and was completed while the author was a visiting fellow at the Marine Geoscience Unit of the South African Council for Geosciences. The research benefited from discussions with Trevor Harrison of the CSIR ŽKwaZulu–Natal.. I am grateful to Killian McDaid and Mark Millar for drafting the figures.

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