A history of fire and sediment transport in the Cotter River catchment, southeastern Australia
Date
2013
Authors
Worthy, Martin
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Effective contemporary land management requires an understanding of how sediment transport in landscapes responds to changes of climate. This study aims to contribute to this understanding by investigating the fire sediment transport relationship and its palaeo-history in the Cotter River catchment, southeastern Australia. It is shown that large landscape fire had during the Holocene an integral link with the rate of sediment transport in forested catchments, and that short-lived sediment transport events were linked to major climatic shifts. There are no other detailed histories of Holocene sediment transport events in Australia. The study provides a model for future investigations of the history and causes of change in local environmental conditions in southeastern Australia. Forested lands that effectively bind soils and reduce runoff volume and velocity are fire prone and hence subject to episodic post-fire erosion influences. In the summer months of 2003 about 98% of the Cotter River catchment, in the Australian Alps, was burnt by high intensity fire. This large landscape fire consumed vegetative litter and ground covering vegetation, increasing the vulnerability of the soils to erosion. In the months following the fire, high intensity rainfall eroded soils and increased runoff on hillslopes and downstream before vegetation could be re-established. Extensive surveys of the Cotter River were undertaken to identify alluvial deposits and to construct an archive of catchment fire and sediment transport history. Detailed stratigraphic records were dated using a large Optically Stimulated Luminescence (OSL) and radiocarbon dataset. These were employed to elucidate a detailed fire and sediment transport record. The major finding from the analysis of these data is a demonstrable relationship between fire and the occurrence of sediment transport events in the Cotter River catchment over the Holocene. Moreover, evidence was found in the palaeo record of a number of sediment transport events similar to, as well as larger than, the major sediment transport event observed following the 2003 fire. This fire-sediment transport association is based on a new approach to interpreting OSL datasets that uses sub-populations of the overall single-grain dataset to identify sediment transport events in the catchment. The second key finding is that climate is a controlling force on the occurrence of fire. This was confirmed by demonstrating that the trends of broad scale climatic records mimic those of biomass burning and sediment transport over the Holocene period. The implications for management in a landscape where the likelihood of recurring fire is high, is that sediment transport in the months following fire requires consideration in the context of the many values associated with the Cotter River catchment. With climate projections indicating more frequent fire conditions, a likely increase in the occurrence of fire may change the form and function of the alluvial zone as well as the sediment export regime.
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