1 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. 2 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: 3 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. 4 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).