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Geomorphology of estuaries

Estuaries are low-lying areas of non-marine origin that have been invaded by the sea. Australia is a flat, dry continent with mainly small coastal catchments, and it has few of the large drowned river valleys (and no drowned glacial valleys) that dominate the coastal landscapes of, for example, north-western North

America. Only the Hawkesbury, Georges Rivers, Port Hacking, and Port Jackson (in New South Wales) and the Derwent River (in Tasmania) represent this scale of drowned river valley topography. The postglacial marine transgression (about 20 000 to 6000-7000 BP) affected the whole coastline, drowning low embayments, large gulfs, coastal floodplains, and numerous small river valleys. During the sea level oscillations of the Pleistocene, drowning of valleys and coastal lowlands occurred repeatedly, together with a variety of erosive and sedimentary processes. Infilling of these drowned features has varied greatly around the continent, because of differing river and tidal flow, sediment supply from the land and the continental shelf, and pre-existing topography. Figure 2.32, adapted from Chapman et al. (1982), gives two idealised examples from New South Wales.

Table 2.6 Distribution of estuary types in Australia

Region

Annual rainfall (mm)

Total runoff (IQ10 m3)

Tidal range variation (m)

Dominant morph, estuary type

Seasonal mixing regime

Timor Sea

600-1200

0.138

2.0-10.5

Macrotidal drowned river valleys. Mature.

Dry: inverse estuary Wet short lived freshwater flushes

Gulf of Carpentaria

600-1600 summer max.

0.099

2.2-7.7

Macrotidal drowned river valleys. Mature.

Dry: inverse estuary Wet short lived freshwater flush may occur

North-east

coast

800-4000

0.183

1.6-6.3

Barrier drowned river

Dry: well mixed.

Wet highly stratified in high flows

South-east

coast

600-1600

0.136

1.1-1.8

Barrier drowned river

Well mixed; short period erf stratification after flood

Tasmania

600-3200

0.690

0.9-2.5

Drowned river

Salt wedge

Sth Australian Gulfs

300-800, winter max.

0.007

0.5-2.0

Barrier

No data

South-west

coast

400-120 winter max.

0.052

0.4

Barrier

Dry: highly stratified. Wet freshwater flush

Indian Ocean

200-400

0.012

0.5-5.8

Drowned river

Permanent Inverse circulation

Source: after Digby et al. 1999, p. 4

River Row, tides and mixing

Fluvial discharges to Australian estuaries are low and highly variable (see table 2.6). For example, the Burdekin River in Queensland is one of the largest and most reliable rivers in the Australian context, but in world terms it is not a major river. There is regular seasonal variation across the continent – summer flows in the north, winter flows in the south – but interannual change associated with El Nino cycles adds great variability.

Variation of conditions depends on estuary form, and on variations in both tidal and river flow. Where there is a strong river flow and a low tidal range, 'salt wedge' estuaries may form, with the lighter fresh water flowing out over the denser salt water into the open ocean. 'Partly mixed' and 'fully mixed' types also occur, depending on the relative power of river and tide. Thus, many New South Wales estuaries vary according to tidal flows, since river flows have little seasonal change, whereas estuaries in south-west Western Australia vary greatly following large seasonal variation in rainfall and river discharge. Some estuaries with minimal river input may become more saline at the landward end due to poor circulation and high evaporation; the 'reversed estuary' of Gulf St Vincent in South Australia is one such case.

 
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