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This section summarizes the meteorology and hydrology of the field sites. Firstly, the common aspects of both estuaries will be described due to their close geographical location. Secondly, more specific characteristics of each river and its catchment will be presented.

3.3.1 Climate of the sites

Both sampling estuaries on the south coast of NSW are temperate, with typically mild summers and cool winters and are slightly humid. Mean monthly sea temperature changes latitudinally along the coast of NSW. The sea surface temperatures in waters off Sydney range from 17.7 – 23.5ºC, with an annual average of 20.6ºC. Off Batemans Bay, they range from 16.3 – 22.7ºC, with a annual average of 19.5ºC (BOM, 2006). Using a linear interpolation between these two sites, it is expected that the temperature range of the ocean off Shoalhaven should be 16.9 – 23ºC with an approximate annual average of 19.9°C. Prevailing winds along the coast tend to change direction seasonally; in winter they are south-westerly to westerly whilst in summer winds are mainly north to south-easterly (observations during fieldwork).

Figure 3-4. Average annual precipitation in NSW for the period 1900-2006 Source: NSW Bureau of Meteorology

The NSW average annual rainfall is 950-1100 mm (Figure 3-4) with fairly even monthly distribution of rainfall (Turak and Waddell, 2000; BOM, 2006) despite the fact that most of the NSW estuaries are characterized by variable water flow due to variable rain patterns. However, during the sampling period of this thesis, rain patterns fluctuated slightly from month to month and from the first to the second year (see Figure 3-5 and Figure 3-6) in the two studied estuaries.

Overall rainfall levels at the different rain gauges through the catchment correlate well with water flow so that the latter parameter will be used in the data analysis as described in Chapter 5. Rainfall variability in recent years might be a consequence of the changes in climate. Rainfall is dominated by ENSO and other climatic events, which may operate on decade-long cycles (Power et al., 1999). This should be taken into consideration especially if rainfall has any effect on oyster production in NSW.

Figure 3-5: Monthly rainfall by year for two rain gauges in the lower Clyde River catchment: a) B. Bay Town (at the Catalina Club, Station Number 69134); b) Brooman (at Carisbrook, Station Number 69121) for the period 1992-2005

Figure 3-6: Monthly rainfall for three rain gauges in the Shoalhaven River catchment: Nowra Town (Station Number 68072); Moss Vale (Station Number 68239) and Bomaderry creek (Station Number 215016) for the period 2003-2005

3.3.2 Hydrology of the sites

Australian rivers have a distinctive hydrology compared to other rivers in the world due to highly variable annual flows (Eyre, 1998; Eyre and Balls, 1999). The Clyde R. and the Shoalhaven/Crookhaven R. are temperate estuaries due to their geographical location and hydrology. Temperate estuaries are characterized by the lack of a well-defined seasonal variation in the flow compared to, for example, subtropical estuaries which are characterized by summer floods and winter droughts (Rochford, 1959). Temperate estuaries have some slight

stratification for most of the year due to small-scale constant freshwater supplies. However, if a large discharge event takes place, strong salinity stratification may occur. This generates buoyant freshwater plumes or, under special circumstances, salt-wedges depending on the structure of the estuary and the inflow volume. In general, salt-wedges do not develop, as the rainfall events are short and the tidal exchange breaks down the vertical stratification and induces mixing of waters of different salt content (Eyre, 1998). Water stratification has an important influence on the ecology of the estuary. Stratification can potentially decouple benthic and pelagic interactions occurring in the estuaries, and therefore, could change the biological processes governing nutrient availability.

3.3.2.1 Clyde River hydrology

The Clyde River is one of the few coastal rivers in NSW that transport fluvial sediment to the ocean. The beaches close to the mouth of the Clyde are supplied with sand from this river. During the last decade, Clyde oyster growers claim that the entrance to the estuary, which has a mobile and shifting sand bed, has silted up, reducing the oceanic water influence in the river (Clyde oyster growers, pers. comm. 2003). The southern side of the entrance is relatively stable with a single training wall, while more instability occurs along the northern side (WBM Oceanics Australia, 1999a).

WBM Oceanics (1999) developed a hydrodynamic model for the NSW Estuary Management Policy. This model was used to examine sediment and hydrodynamic processes occurring along the Clyde River. It has been estimated that the training wall constructed in 1899 has trapped approx 800,000m3 _{of sand corresponding to 80% of the net supply to the inner bay}

over the past 100 years. This sand volume may otherwise have remained in the sand circulation system of the Bay (WBM Oceanics Australia, 1999a).

While the Clyde River catchment remains nearly pristine and the unregulated, local oyster farmers are concerned about the potential downstream impacts of modifications in the upper catchment, through logging and/or water extraction (Clyde oyster growers, pers. comm. 2006). As a result, currently, management plans and environmental management systems are being developed for the whole catchment to protect downstream users.

3.3.2.2 Shoalhaven River hydrology

Before European settlement, the river at Shoalhaven Heads, was mostly open to the ocean, so that there was an interchange of estuarine and oceanic waters. In 1822, James Berry, a European settler had a channel excavated by convict labour through a sand bar to improve boat access to the river. This channel has eroded due to tidal flow into ‘the Berry Canal’, which scoured it deeper and wider. This is now the main river channel discharging into the ocean. The opening of this Canal reduced the water flow through the Shoalhaven Heads allowing a sand bar

to develop and close this entrance almost permanently. Oceanic exchange now occurs through two entrances; the intermittently open and untrained entrance at Shoalhaven Heads and the permanently open Berry’s Canal with a single training wall at Crookhaven Heads.

This estuary is influenced primarily by river energy, so the river has a wave-dominated delta, with low sediment trapping efficiency, naturally low turbidity, and the occasional presence of a salt wedge. There is therefore a low risk of habitat loss due to sedimentation (Umwelt Pty Ltd, 2005). As a consequence of being a ‘barrier estuary’ (Roy et al., 2001), the Crookhaven/ Shoalhaven River is also one of the few NSW rivers discharging fluvial sand into the ocean. Sand dynamics in this river system are active and intensive, quickly changing the location and extent of most of the sand flats.

The hydrology of the Shoalhaven was altered dramatically by the construction of various upstream dams (section 3.2.2) The presence of these dams has decreased downstream flows, reduced minor flood events, altered water and environmental characteristics (Healthy Rivers Commission, 2003) and created a barrier to the migration of native fish species (Umwelt Pty Ltd, 2005). Water discharges from the dams decrease downstream water temperatures and increase turbidity in the areas 4km downstream of the dam. It has been suggested that tidal processes dominate areas further downstream from the dam (The Ecology Lab Pty Ltd, 1996). Hydrodynamic modelling of the river has estimated that a flow of 43,000 ML will be needed to flush the whole estuary after which salinity values would re-establish in a period of 2-3 weeks (The Ecology Lab Pty Ltd, 1996).

The presence of dams in the upper regions of an estuary could severely affect oyster- growing estuaries by decreasing upstream nutrient discharges, especially in those oyster estuaries that rely on upstream nutrient delivery.