The role of ant activity in limiting the effectiveness of erosive overland flow in eucalypt forests, Central Tablelands, south-east Australia

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Using tracer based sediment budgets to assess redistribution of soil and organic material after severe bush fires

In the summer of 2001, a combination of severe wildfires and moderate intensity rainfall events swept through the catchment of Lake Burragorang, NSW, Australia. Beryllium-7 and 210Pbex budgets were used to assess the redistribution of soil and organic debris that occurred on hillslopes in this catchment after the wildfire. In the ?90-ha study site, the budgets showed substantial losses of 7Be from the upper plateau (10 � 2%) and side slopes (26 � 5%), and deposition on the lower footslopes and alluvial fans (2 � 4%). Overall, it was possible to account for ?65% of the initial amount of Be expected to be present with the balance, 35 � 6%, presumably being exported offsite. Construction of a 210Pbex budget showed a similar pattern of internal redistribution; some 28 � 6 % of the total amount was exported from the site. Analysis of litter, soil and sediment samples shows that both 7Be and 210Pbex were: (a) preferentially retained near the soil surface (due to their constant deposition in rainfall), and (b) exhibited an affinity with organic material. It appears that 210Pb and 7Be budgets can reveal much about the redistribution of soil, organic (and nutrient) material after fires in these systems.8 page(s

Impacts of wildfire on effective sediment particle size: implications for post-fire sediment budgets

Recent work in an Australian catchment has shown that severe wildfires followed by rainstorm events lead to redistribution of topsoil and export of sediment to the local river network. Considering the affinity of nutrients and other contaminants for fine sediment, and the potential for considerable downstream impacts, a sound understanding of the fine-sediment delivery from burnt systems is required. However, past work has shown that fire can modify the particle size distribution of the soil through formation of robust sand-sized aggregates comprising fine clays and silts. Image analysis confirmed the presence of fire-modified soil aggregates in our study area. Analysis of aggregate form and fluvial behaviour showed that fire-modified aggregates are fused, dense and inorganic in nature with settling velocities of an order of magnitude faster than unburat soil aggregates or classic riverine flocs. This implies an increased potential for storage of nutrient-rich fine sediment within slope units, flood plains and river channels. The unusual behaviour of these composite particles should be considered in the construction of post-fire fine-sediment budgets.8 page(s

Structural characteristics and behavior of fire-modified soil aggregates

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Effects of contrasting wildfire severity on soil water repellency and implications for hydro-geomorphological response

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Using geochemical stratigraphy to indicate post-fire sediment and nutrient fluxes into water supply reservoir, Sydney, Australia

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Valley side-wall retreat via extreme erosion events, south-west Sydney Basin

The spectacular incised valleys of the Blue Mountains Plateau in the west and south-west of the Permo-Triassic Sydney Basin are the focus of this paper. The evolution of these valleys has been shown to be controlled by knickpoint retreat following uplift along the Lapstone Structural Complex. However, very little investigation has been conducted into the processes which dominate after the knickpoint has past, in particular those processes which facilitate valley widening. Investigations in the incised Nattai River valley, a tributary of the Wollondilly River in the south-west of the Basin, found hillslopes which are characterized by mass movement, particularly rock fall, debris flows, rotational slumping and large landslides. To determine the relative importance of mass movement in denudation, we compared modern rates of sediment yield from hillslope plots and suspended load in the Nattai catchment with long term rates of sediment yield and denudation calculated from incision below Tertiary basalts, cosmogenics and apatite fission track ages determined for the Blue Mountains Plateau. The comparison revealed that the average modern background denudation rate is an order of magnitude less than the long term average, which implies a very low sediment yield for the majority of the time and compares well with field observations. The background rates are episodically punctuated by short term sediment yield events which are at least an order of magnitude higher than the long term average and higher than erosion rates measured following the 2001/02 bushfires. We suggest that following knickpoint retreat, the primary process of denudation in the Nattai valley is extreme erosion events on hillslopes which facilitates valley side-wall retreat. Sediment generated through mass movement directly enters the stream network or is stored on the lower slopes and valley floor to be progressively reworked by floods capable of remobilizing sand to boulder sized material. The mass movement events appear to be linked to lithology which suggests that mass movement and side-wall retreat is an important geomorphic process in the evolution of the incised valleys of the Blue Mountains Plateau.1 page(s