Reconstructing contemporary erosion of upland swamps in a fire prone forested landscape, southeastern Australia.

Upland swamps on the Woronora Plateau, located to the south of Sydney, are important sediment storage features. Many of the swamps are situated within water supply catchments for Sydney and Wollongong thereby acting as a filter and buffer to produce clean, constant flows. The recent erosion of several swamps has resulted in the release of sediments and nutrients into reservoirs downstream raising water quality and quantity concerns within managing agencies. Analysis of aerial photographs dating back to the late 1940’s and covering three eroding swamps, revealed a common sequence of erosion, commencing with the formation of discontinuous scour pools cut into the swamp sediments, followed by the gradual linking of the pools (starting at the downstream end) through knickpoint retreat, leading to the eventual gully erosion through the swamp. The gully erosion results in the removal of a small slice of the swamp, rather than the whole scale removal of sediments, whereas associated impacts on swamp hydrology and vegetation are more widespread. The potential triggers of the scour pools and gully erosion are considered including wildfires, severe rainfall events and human disturbance which occurred over the same period as the air photo record (i.e. 1940’s to the present). Analysis of sediments exposed within the gully walls of the swamps revealed previous episodes of scour and gully erosion indicating that cut and fill processes are an important part of long-term swamp development and evolution over thousands of years. This is reinforced by the recent episode of erosion which shows that despite extensive disturbance and severe climatic conditions (e.g. drought, wildfires) the swamps continue to remain as resilient long-lived features in the landscape.