A geophysical investigation of the Derwent Estuary

The Derwent River is one of the largest rivers in Tasmania by any measurement standard. It drains Lake St Clair in the southern central highlands, and meanders over 200km down to Storm Bay south of Hobart where it eventually joins the Tasman Sea. The modem river deposits most of its suspended sediment load between New Norfolk and Bridgewater, and an area of fluvio-deltaic mud flat is developed north of the Bridgewater causeway. South of the causeway is the Estuary proper, and this extends to the entrance of Storm Bay, marked by the significant widening of the water body just south of the Iron Pot. The length of the Estuary between Bridgewater and Storm Bay is nearly 60km, and it is up to 6km wide. The bulk of this study was conducted in the southern portion of the Estuary, between Hobart in the north and Betsey Island in the south, excluding Ralphs Bay to the east and North West Bay to the West. Some seismic data were collected further upstream, as far north as the Bowen Bridge. See the location maps for details. (Since the city of Hobart is built on the banks of the Estuary, access is excellent. There are major roads and numerous boat launching and mooring points along both shores of the river. The boats used for this study were launched near Electrona in North West Bay, at the Regatta Grounds near the Cenotaph and at the Derwent Sailing Squadron (DSS) in Sandy Bay. Access to the DSS ramp and mooring facilities was arranged by Dr. Michael Roach. The other launching points are freely accessible to the public, via good quality sealed roads.