Abstract:Wild®res raise concerns over the risk of accelerated erosion as a result of increased overland¯ow and decreased protection of the soil by litter and ground vegetation cover. We investigated these issues following the 1994 ®res that burnt large areas of native Eucalyptus forest surrounding Sydney, Australia. A review of previous studies identi®es the ®re and rainfall conditions that are likely to lead to increased runo and accelerated erosion. We then compare runo and erosion between burnt and unburnt sites for 10 months after the 1994 ®res.At the scale of hillslope plots, the 1994 ®re increased runo by enhancing soil hydrophobicity, and greatly increased sediment transport, mainly through the reduced ground cover, which lowered substantially the threshold for initial sediment movement. However, both runo and sediment transport were very localized, resulting in little runo or sediment yield after the ®re at the hillslope catchment scale. We identify that after moderately intense ®res, rainfall events of greater than one year recurrence interval are required to generate substantial runo and sediment yield. Such events did not occur during the monitoring period. Past work shows that mild burns have little eect on erosion, and it is only after the most extreme ®res that erosion is produced from small, frequent storms. #

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This paper examines the patterns of sediment transport in rivers in terms of the sources of sediment and its transport and deposition through the river network. The analysis is in the context of dramatic human influences on river sediment transport and how they might influence freshwater ecosystems. The review of Australian work shows that erosion of hillslopes and stream banks has greatly increased in historical times, supplying vast quantities of sediment to rivers, much of which is still stored within the river system. The stored sediment will continue to effect in-stream and estuarine ecosystems for many decades. In most Australian catchments the dominant source of sediment is streambank erosion. An analysis of historical channel widening suggests that a conceptual framework of relative stream power can explain the diversity of behaviour observed in the numerous case studies. Sediment delivery through catchments is considered first in a generic whole network sense, which emphasizes the crucial role played by riverine deposition in determining catchment sediment budgets. A method is then presented for analysing the diverse spatial patterns of sediment storage in any river network. Finally, the paper considers the temporal changes to channel morphology in response to a human-induced pulse of sediment.

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