RELATIONSHIPS BETWEEN GEOMORPHOLOGY and VEGETATION in the MINNAMURRA ESTUARY, NSW * † Ronald J

Originally published in Wetlands (Australia), Journal of the Coast and Wetlands Society Inc., 8(2), 1989: 61-68.

RELATIONSHIPS BETWEEN GEOMORPHOLOGY AND VEGETATION IN THE MINNAMURRA ESTUARY, NSW * † Ronald J. Carne

ABSTRACT This study examines landform-vegetation relationships in the Minnamurra estuary south of Wollongong, N.S.W. Here, the formation of coastal barriers, the subsequent reworking of barrier sands, and the infilling between barriers with marine and fluvial sediments have created surfaces for plant colonization, while the barriers themselves have provided protection from wave attack. Within the estuarine environment diverse habitat conditions exist. These reflect the operation of contemporary geomorphic processes and differences in the evolution of land surfaces. Two areas have been examined in detail. The first has formed as a result of estuarine infilling over old beach deposits, and the second has resulted from the river's lateral migration. The contrast in the spatial expression of the major vegetation types (mangrove, saltmarsh, swamp-oak forest) is directly related to differences in surface form, which, in turn, are a consequence of the very different geomorphic evolution of the two areas. The latter also exhibit marked differences in regard to the composition of surface sediments, and these produce differences in drainage conditions, soil salinity levels and degree of soil waterlogging. These differences again reflect the contrasting geomorphic evolution of the two areas, and account for some of the variations which occur in the communities which comprise the respective vegetation types.

Introduction Estuarine vegetation patterns are closely related to geomorphology through the landform attributes of microtopography and substrate composition. The former exerts controls on the frequency, duration and extent of tidal flooding, and the latter is a major control on soil moisture conditions. Together, these variables have a significant influence on soil salinity and waterlogging, both of which have a direct effect on plant physiology (see Chapman 1974, Etherington 1975) and therefore on the distribution of species. Clearly, a thorough understanding of estuarine vegetation requires an examination of the geomorphic processes, which, by creating variation in surface form and substrate conditions, have acted to differentiate plant habitats.

This study examines vegetation pattern in the Minnamurra estuary, and attempts to relate the spatial expression of major vegetation types, and the species composition within each type, to geomorphic processes and landform evolution.

The study area The Minnamurra estuary (34°37ʹ S, 150°51ʹ E) lies approximately 25 km south of Wollongong (Fig. 1). It is situated at the southern end of the Illawarra coastal lowlands; this region extends for about 50 km, bounded in the east by the Pacific Ocean and in the west by a cliff-lined escarpment. The Minnamurra River drains off the escarpment and flows across the coastal plain, which at this point is about 12 to 14 km wide. The drainage basin has an area of 142 sq. km.

The region has a temperate marine climate (Cfb in Koppen's classification). At Wollongong, mean monthly temperature ranges from 22°C in January to 13°C in June, with a mean maximum of 26°C and a minimum of 8°C. Average rainfall varies between 1000 mm and 1277 mm on the coastal plain but due to orographic effects rises to 1500 mm at the top of the escarpment (Young & Johnson 1977). Tidal range at Port Kembla, from mean high to mean low water springs is 1.2 m, from highest to lowest astronomical tide 2.0 m.

This study focuses on the estuarine environment 1 to 2 km inland from the seaward entrance of the Minnamurra River where the channel begins to meander around coastal barriers composed predominantly of siliceous sands (inset A Figure 1 and Plate 1). In this area the river is microtidal with some degree of salt-freshwater mixing.

* This paper is a short summary of the monograph Landform-Vegetation Relationships in the Minnamurra Estuary, NSW published in 1991 (Monograph Series No.6, Dept. of Geography and Oceanography, University College, UNSW, ADFA). It represents an early study in what was, in the 1980s, the incipient field of ‘ecogeomorphology’. The latter is now a well-established part of geomorphology [see Butler, DR & Hupp, CR 2013, ‘The role of biota in geomorphology: Ecogeomorphology’, in J Shroder, J. (editor in chief), DR Butler & CR Hupp (eds), Treatise on geomorphology, Academic Press, San Diego, CA, vol. 12, Ecogeomorphology, pp. 1–5.] † All figures in this digital version have been redrawn by the author. Aerial images were scanned from the original photographic print (Plate 1) and colour slides (Plates 3 to 5). Plates 2, 3 and 4 were not included in the original publication. The base aerial image (Plate 2) was obtained from the Spatial Information Exchange (NSW Govt). An abstract has also been added.

Originally published in Wetlands (Australia), Journal of the Coast and Wetlands Society Inc., 8(2), 1989: 61-68.

Figure 1: Location of study area

Plate 1: Study area showing location of transects (approx. scale 1: 32 000; photo courtesy of NSW Department of Lands, Sydney).

Geomorphic background The origin and evolution of estuaries on the NSW south coast is intimately related to Quaternary sea-level movements and associated phases of erosion and deposition (Roy 1984). In the period spanning the late Pleistocene and present-day four major phases can be recognised (Roy & Crawford 1977). These include:

Originally published in Wetlands (Australia), Journal of the Coast and Wetlands Society Inc., 8(2), 1989: 61-68.

1) river erosion and re-excavation of coastal valleys during the last glacial low sea-level; 2) initiation of shoreward migrating coastal barriers during the Post-glacial Marine Transgression (17000 to 6000 yrs. B.P.); 3) the creation of estuaries in drowned river valleys behind coastal barriers, and the deposition of terrestrial sediments resulting in a progressive infilling of the estuary; 4) the downstream progradation of fluvial sediments over estuarine deposits culminating in the infilling of the coastal embayment.

At Minnamurra, C14 dates indicate a progressive infilling of the embayment from both landward and seaward during the Holocene. Oyster shells collected approximately 9 km upstream from the seaward entrance near the present limit of tidal influence, and buried under 2 to 3m of floodplain sediments encroaching downvalley, were dated at 5,950 ± 120 yrs. B.P. (Carne 1981). Sediment movement from seaward during the mid-Holocene ‘stillstand’ stage (6000 to 3000 yrs. B.P.; see Thom, Polach & Bowman 1978) is suggested by a C14 date of 3450 ± 95 yrs. B.P. on shell material obtained from an in situ carbonate mass (see Fig. 6) in the most landward barrier (denoted ‘a’ in Fig. 2). The two seaward barriers (b and c; Fig. 2) must be younger than 3450 yrs., with the active barrier on the ocean beach being the most recent.

Since large scale eolian sand transport and transgressive dune development are not apparent within the estuary, it seems that the period following barrier construction has been one of dune stabilisation. The late Holocene has also seen a continuation of the infilling process through estuarine and fluvial deposition. Today, substrates built by these depositional processes lie some 2 to 3m below the general elevation of the barriers. Those areas which are vegetated and subject to direct tidal inundation and/or influence through elevated water tables are denoted ‘wetlands’ (W) in Plate 2.

According to Roy (1984) three basic estuary types can be recognised. These include drowned river valley estuaries, barrier estuaries, and saline lagoons. The Minnamurra estuary has entrance conditions, in particular a narrow elongated channel, which are characteristic of the ‘barrier’ type (see Roy 1984, pp. 106-107). The sinuous channel with smooth levee banks, characteristic of the final stages (stage D) of infilling of this estuary type (Roy 1984, pp.111- 112) are not fully developed at Minnamurra. Instead, the two sub-embayments which lie between barriers a and b (Fig. 2 and Plate 2) give the channel an irregularity suggestive of evolutionary stage C.

Major features of the vegetation pattern The major vegetation types within the study area include mangrove, saltmarsh, swamp-oak forest, and eucalypt forest (Fig. 3). The first three types occupy wetland areas, and the eucalypt forest barrier ‘a’ (Fig. 2). In the northern part of the study area (Tidal Plain A; Fig. 3 and Plate 3) mangrove fringes the river channel and landward is sharply demarcated from saltmarsh. Towards the rear of the tidal plain, swamp-oak forest intergrades with saltmarsh and merges landward with eucalypt forest (transect A-B; Fig. 3 and Plate 4).

Figure 2: Surface morphology – a sequence of 3 sand barriers (denoted a, b and c) represent the progressive infilling of the Minnamurra embayment by sediment movement from seaward.

Originally published in Wetlands (Australia), Journal of the Coast and Wetlands Society Inc., 8(2), 1989: 61-68.

On the alluvium within the two migratory channel bends (MB1 & 2; Fig. 3) there is a quite different vegetation pattern. Much of the surface, except for a series of low ridges which support swamp-oak forest, is covered by mangrove interspersed with small patches of saltmarsh (Fig. 3 and Plate 5).

The most southerly extent of the study area (Tidal Plain B) may have originally been very similar to Tidal Plain A except that MB2 has migrated into, and partially altered, the zonation. The bend appears to have eroded through the mangrove community and into the saltmarsh which is now found fringing the river channel (Fig. 3).

Plate 2: Wetlands, labeled ‘w’, are low-lying vegetated areas subject to tidal inundation or influence. The boundaries shown are approximations (image courtesy of NSW Dept. of Finance and Services Spatial Information Exchange).

Plate 3 (below): Mangrove fringing the river channel on Tidal Plain A (a = A. marina trees; b = A. marina shrubs) (Photo: R.J. Carne, 1980; photo viewpoint P2 in Fig. 3 opp.)

Figure 3 (opposite): Distribution of major vegetation types.

Originally published in Wetlands (Australia), Journal of the Coast and Wetlands Society Inc., 8(2), 1989: 61-68.

Plate 4: Looking west across Tidal Plain A in the vicinity of transect AB ( a = A. marina trees; b= A. marina shrubs; sm = salt-marsh; sof = swamp-oak forest; e = eucalypt forest) (Photo: R.J. Carne, 1980; photo viewpoint P3 in Fig. 3)

Plate 5: In Meander Bend 1 swamp-oak forest (grey-green vegetation, denoted ‘c’ = Casuarina glauca) forms a distinctive pattern coincident with a series of low ridges (Photo: R.J. Carne, 1980; photo viewpoint P4 in Fig. 3 above).

Tidal Plain A and Meander Bend 1: geomorphology and vegetation Within the estuarine environment diverse habitat conditions exist and reflect the operation of contemporary geomorphic processes and differences in the evolution of the land surfaces. Two representative areas were

Originally published in Wetlands (Australia), Journal of the Coast and Wetlands Society Inc., 8(2), 1989: 61-68.

studied in detail. The first is Tidal Plain A traversed by transect A-B (Fig. 3), and the second is Meander Bend I traversed by transect C-D (Fig. 3). In the following, the geomorphology and vegetation of each area is described.

(a) Tidal Plain A Tidal Plain A is a low-lying wetland area situated seaward of the most landward barrier (Fig. 3) and has developed as a result of the deposition of 1 to 1.5m of estuarine sediments over old beach deposits (Carne 1981). This is likely to have occurred as a consequence of the partial enclosure of the estuary by a more seaward barrier, possibly barrier ‘b’ (Fig. 2). The muddy estuarine sediment provides the substrate for mangrove, saltmarsh and swamp-oak forest. Nearshore shelly sands underlie the beach and barrier deposits, and overlie a basal clay unit. The clays lie approximately 10m below the tidal plain (Smith 1978) and sloping upwards are encountered at 5m beneath the barrier (Fig. 4).

The surface sediments contain significantly less silt and clay than that found further upstream; on Tidal Plain A silt-clay content averages 19% (n = 17), whereas on Meander Bend I (Fig. 3) mean silt-clay content is 69% (n = 20) (Carne 1981). These differences persist at shallow sub-surface depths (- 40 to - 50 cm), with silt-clay content averaging 13% and 36% for Tidal Plain A and Meander Bend I respectively. Differences are significant at the 0.05 level (Mann Whitney U test).

The tidal plain surface comprises two main morphological units. Following the nomenclature used by Davies (1977) these are termed the intertidal slope and high tide flat (Fig. 4). The surface of the high tide flat is around 50 cm above the lowest point of the intertidal slope, at an elevation which roughly approximates the HHW level (Fig. 4). Towards the river the intertidal slope incorporates a small levee bank which rises to an elevation approximating that of the high tide flat (Fig. 4). The levee extends some distance along the river's margin (approx. 640 m; transect A-B crosses the levee 200m from its northern extremity) and diverts tidal entry to the northern and southern ends of the tidal plain.

A B Tidal Plain Barrier

K3 KEY +4 5

Intertidal Slope High Tide Flat ES estuarine

e n

o sediments

+3 z

a BBS beach and

i t

- barrier sands

)

b u

m +2

S (

levee NSS nearshore

n o

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v +1 HHW

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E 2 carbonate 0 MSL ES deposits ±10cm -1 4 BBS borehole

-2

NSS -3 Datum: WL at time of survey C -4 VE x50

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0 200 400 600 800 1000

Distance (m) Figure 4: Transect A-B: Surface Profile and Stratigraphy

Mangrove, composed predominantly of a riverside fringe of Avicennia trees (6 to 8m tall) and a landward zone of Avicennia shrubs (0.5 m to 1.5 m tall), occupies the intertidal slope (Fig. 5). On the high tide flat, Sporobolus dominated saltmarsh occurs and is sharply demarcated from mangrove. Landward, and at a slightly higher elevation (5 to 10 cm), saltmarsh integrades with swamp-oak forest. The latter comprises mainly Casuarina

Originally published in Wetlands (Australia), Journal of the Coast and Wetlands Society Inc., 8(2), 1989: 61-68.

glauca with a ground cover dominated by Suaeda australis. Further landward, the eucalypt forest marks the rise in elevation assocation with the barrier (Fig. 5).

Figure 5: Transect A-B: Vegetation

(b) Meander Bend I The low-lying depositional surface in Meander Bend I is a result of the river's lateral migration, during which a large portion of barrier ‘a’ appears to have been eroded (Fig. 3). The stratigraphic sequence indicates the replacement of barrier sands by a 4m thick deposit of floodplain sediment (Carne 1981) comprised mainly of reworked barrier and nearshore sands (Fig.6). The upper 0.5m to 1m of these alluvial deposits (Fig. 6) consists of muddy sands supplied by overbank river and tidal sedimentation, and this provides the substrate for the meander bend vegetation (mangrove, saltmarsh and swamp-oak forest). As noted previously, the silt-clay content of this surficial deposit is significantly higher than that found further downstream on Tidal Plain A.

The microtopographic variations on the meander bend are concordant with an underlying ridge and swale topography (Fig. 6). Although the mechanism by which the ridges form is unclear, they must in some way be related to the process of lateral migration (each ridge probably marks the former location of the convex bank, for immediately behind and adjoining the modern point bar the most recent ridge is clearly evident; Fig. 6). The vegetation-elevation relationship in the meander bend is similar to that on the tidal plain, that is, mangrove occupies the lowest surfaces, saltmarsh occurs slightly higher (10 to 20cm) and swamp-oak forest occupies the highest elevations (40 to 80cm above the mangrove; Fig. 6).

Originally published in Wetlands (Australia), Journal of the Coast and Wetlands Society Inc., 8(2), 1989: 61-68.

B C ) m

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Salt-marsh species Juncus kraussii Phragmites australis Sporobolus virginicus Suaeda australis Samolus repens Sarcocornia quinqueflora Triglochin striata

KEY

OB Overbank deposits Carbonate deposits A Aegiceras corniculatum B Avicennia marina

BS Barrier sands (barrier a) Borehole C Casuarina glauca D Melaleuca styphelioides LAD Lateral accretion deposits Salt-marsh (reworked barrier & nearshore sands) V Avicennia seedlings NS Nearshore sediments

Datum: WL at time of survey; VE x12.5

Fig 6: Transect CD. Vegetation and Stratigraphy

Differences between meander bend and tidal plain vegetation and their relationship to geomorphology Differences in the spatial expression of vegetation types (mangrove, saltmarsh, and swamp-oak forest) occur between the tidal plain and meander bend (Fig. 3). On the tidal plain, the well defined zonation is associated with the gradual rise in elevation towards the barrier (Fig. 4). On the meander bend, the formation of floodplain ridges has created slight but abrupt topographic changes across an otherwise level surface (Fig. 6). This creates abrupt variations in environmental conditions and hence prevents the development of an orderly zonation.

The differences in surface form, and hence in the patterning of vegetation types, reflect differences in the evolution of the meander bend and tidal plain. The abrupt microtopographic variations across the meander bend appear to be related to the process of lateral migration. On the tidal plain, estuarine infilling has built a gently sloping surface which increases in elevation towards the barrier (Fig. 4). The low elevations associated with the intertidal slope represent a less advanced stage of this infilling process. The only abrupt microtopographic variation is that introduced by a small riverside levee (Fig. 4).

The two areas also exhibit differences in species which comprise the respective vegetation types. On the meander bend, the mangrove community is characterised by a dense understorey of Aegiceras corniculatum and the Avicennia shrubs are absent (Fig. 6); on the tidal plain Aegiceras is mainly restricted to riverside locations, while landward Avicennia shrubs are abundant (Fig. 5). The saltmarsh in the meander bend is dominated by Juncus kraussii (Fig. 6), whereas on the tidal plain Sporobolus virginicus is the dominant saltmarsh species over much of the transect (Fig. 5). Differences occur between swamp-oak communities in terms of the understorey trees associated with Casuarina glauca; Melaleuca styphelioides in the meander bend community, Myoporum accuminatum and Glochidion ferdinandi in the tidal plain community.

Originally published in Wetlands (Australia), Journal of the Coast and Wetlands Society Inc., 8(2), 1989: 61-68.

These differences are associated with marked differences in the environments of the two sites. The generally higher soil salinities in the meander bend (most values greater than 30%) as compared to the tidal plain (most values less than 30%; Table 1) must be largely due to poor drainage conditions consequent on the high silt-clay in the meander bend substrate. Waterlogging is also likely to be more extreme for similar reasons. This muddier, more poorly drained and more saline environment is directly related to the very different evolution of the meander bend reflecting its higher alluvial, as opposed to marine component. This is in turn reflected in the vegetation, for Aegiceras corniculatum, which is abundant in the meander bend, appears to favour waterlogged muddy substrates (Clarke and Hannon 1970). However, since Sporobolus and Juncus have similar tolerances in regard to salinity and waterlogging (Clarke and Hannon 1971) and yet are not equally represented on the tidal plain and meander bend, the differences between saltmarshes must be due to other factors. Minor differences in site elevation and the concomitant variation in the number of inundations each community receives annually may be important (Chapman 1974).

Table 1: Soil salinity: Tidal Plain A and Meander Bend I.

Tidal Plain A Meander Bend 1

Distance along transect A-B Salinity (%) Distance along transect C-D Salinity (%) (m) (m)

100 8 55 30 110 24 128 45 150 29 170 50 200 31 183 100 300 26 190 46 400 21 200 86 525 17 225 38 575 14 250 98 600 4 275 76 288 37 320 21

The reason for the absence of Myoporum accuminatum and Glochidion ferdinandi from the meander bend is difficult to ascertain. While a preference for dry non-saline substrates is suggested by their frequent occurrence in the eucalypt forest on barrier ‘a’ (Fig. 3), their presence in the tidal plain community indicates some degree of tolerance to moist saline conditions. Whether or not the tolerance limits for these species is exceeded in the meander bend is clearly a matter which requires autecological investigation.

Conclusion The contrast in the spatial expression of the major vegetation types (mangrove, saltmarsh, swamp-oak forest) on Tidal Plain A and Meander Bend I is directly related to differences in surface form, which, in turn, are a consequence of the very different geomorphic evolution of the two areas. The extent to which the differences in species composition within each type are related to geomorphic conditions is less clear. However, it would appear that for the mangrove communities, the abundance of Aegiceras corniculatum in Meander I as opposed to Tidal Plain A is related to differences in substrate conditions, which again is a reflection of differences in geomorphic evolution.

Originally published in Wetlands (Australia), Journal of the Coast and Wetlands Society Inc., 8(2), 1989: 61-68.

REFERENCES Carne, RJ 1981, Landform-vegetation relationships in the Minnamurra Estuary, NSW, unpublished Honours Thesis, Dept. of Geography, University of Wollongong. Chapman, VJ 1974, Salt marshes and salt deserts of the world, 2nd edn, Cramer, Lehre. Clarke, LD & Hannon, N J 1970, ‘The mangrove swamp and salt marsh communities of the Sydney District, III. Plant growth in relation to salinity and waterlogging’, Journal of Ecology vol. 58, pp. 351-369. Clarke, LD & Hannon, NJ 1971, ‘The mangrove swamp and salt marsh communities of the Sydney District, IV. The significance of species interaction’, Journal of Ecology, vol. 59, pp. 535-553. Davies, JL 1977, Geographical variation in coastal development, Longman Group Ltd, London. Etherington, JR 1975, Environment and plant ecology, John Wiley and Sons, Chichester. Roy, PS 1984, ‘New South Wales estuaries: their origin and evolution’, in BG Thom (ed), Coastal geomorphology in Australia, Academic Press, Sydney, pp. 99-121. Roy, PS & Crawford, EA 1977, ‘Significance of sediment distribution in major coastal rivers, northern NSW’, Proceedings of 3rd Australian Conference in Coastal and Ocean Engineering, pp. 177-184. Smith, V 1978, Sand resources of the Wollongong area, Geological Survey of NSW, Dept. of Mineral Resources and Development. Thom, BG, Polach, HA & Bowman, GM 1978, Holocene age structure of coastal barriers in NSW, Australia, Dept. of Geography, Faculty of Military Studies, University of NSW. Young, RW & Johnson, ARM 1977, ‘The physical setting: environmental hazards and urban planning’, in R Robinson (ed), Urban Illawarra, Sorrett Publishing, pp. 38-57.