Changes in Mangrove/Salt-Marsh Distribution in the Georges River Estuary, Southern Sydney, 1930-1970

Aerial photographs taken between 1930 and 1970 of the Georges River in southern Sydney show a rapid advance of mangroves landward (upslope) at the expense of salt-marsh, though the timing and extent of the advance was not the same in all locations. By 1970, in the central estuarine reaches, salt marsh had all but disappeared from the tidal wetlands of the bayhead fluvial-estuarine deltas, surviving only on the more open 'halfmoon' embayments adjacent to the main river, as well as on the Mill Creek delta. In contrast, there was little or no advance of the mangroves seaward onto tidal mud flats during this period, except in the one instance of upper Salt Pan Creek, which also contains the highest proportion of erodible shale in its catchment. Salt marsh survived best in wetland sites with least disturbance from urban development and engineering works, strongly pointing to increased urban runoff and nutrient-enriched sediment as key factors accounting for local differences in mangrove advance. The confined topography of bayhead sites produced the most complete and earliest changeover to mangroves, probably because they concentrated both runoff and nutrient in comparison to the more open sites near the main river. Mill Creek, with the least urbanised catchment, had areas that represented both geomorphic types, yet significant 80 areas of salt marsh survived in all parts of the wetland. The general upslope advance of mangroves after 1940 in the Georges River is undoubtedly partly associated with the degree and timing of catchment disturbance as Sydney spread southwards. However, despite this local variability, the mid to late 20th Century mangrove advance both in the Georges River and elsewhere in southeast Australia is so widespread that the influence of other forcing agents, such as global sea level rise and regional changes in temperature and rainfall, cannot be ruled out.