RIVER STABILIZATION DUE TO CHANGING CLIMATE AND VEGETATION DURING THELATE QUATERNARY IN WESTERN TASMANIA, AUSTRALIA

The Stanley River in western Tasmania, Australia, contains sub-fossil rainforest logs within the channel and floodplain. Of the more than 85 radiocarbon dates obtained, all but 3 date from 17 ka to the present and permit an interpretation of fluvial and related environmental chan ges over this period. Particular attention is focused on the interacti ve relationship between the river and its riparian rainforest. Followi ng the Last Glacial Maximum, the Stanley River was a laterally active gravel-load system reworking most of its valley floor in the upstream reaches. With ameliorating conditions at the end of the Pleistocene, c limate became less seasonal and flow regimes less energetic. Huon pine s already present in the catchment, re-asserted themselves in the form of dense tree cover along the river banks and floodplains with basal floodplain deposition shifting from gravels to coarse sands and granul es. By about 3.5 ka, a further change in climate reduced stream discha rges substantially. As a result the channel reduced in size, transport ed finer sediment, became laterally stable, and the floodplain accrete d with overbank deposits of sand and silt. Huon pines falling into the channel formed obstructions of woody debris, some surviving for 2 ka. These have reduced stream power and boundary shear stress, further co ntributing to channel stability. Generational sequences of Huon pines on the river banks, some extending back 1-2 ka, are additional evidenc e of this stability. Since the Pleistocene, changing climate and the r e-establishment of dense riparian rainforest appear to have stabilised the river channels and floodplains of western Tasmania.