Wayne D. Erskine

Land use effects on sediment yields and soil loss rates in small basins of Triassic sandstone near Sydney, NSW, Australia

Abstract Sedimentation surveys of dams in small sandstone drainage basins near Sydney, Australia, demonstrate that land use is the dominant factor determining sediment yields and soil loss rates. Cultivated basins produce an average sediment yield of 7.1 t/ha/year whereas grazed pasture and forest/woodland basins export averages of only 3.3 and 3.1 t/ha/year, respectively. Nevertheless, these yields are high by Australian standards. Sediment exports from grazed pasture and forest/woodland basins are similar because the forest/woodland basins are also grazed. Dam sediments are enriched in clay and organic matter in comparison to topsoils. Gullies and bank erosion are not active geomorphic processes in the drainage basins investigated so that the measured sediment yields could be validly compared to soil loss rates determined by empirical soil loss equations, Modified Universal Soil Loss Equation (MUSLE), Soiloss and Revised Universal Soil Loss Equation (RUSLE), which do not account for gully and channel erosion. These equations accurately predicted the measured sediment yields, with MUSLE being the most accurate. Although Soiloss is the only empirical equation to use Australian data, MUSLE performed marginally better, despite being a simplified version of the Universal Soil Loss Equation (USLE) that is used for teaching. RUSLE predictions of soil loss rates were also closely correlated with measured sediment yields.

Distribution, recruitment, and geomorphic significance of large woody debris in an alluvial forest stream: Tonghi Creek, southeastern Australia

The complex yet poorly understood interactions between riparian vegetation, large woody debris and fluvial geomorphology in an anthropogenically undisturbed reach of an alluvial, sand-bed forest stream in SE Australia have been determined. Riparian vegetation exhibits lateral and vertical zonation of understorey and overstorey species. The dominant riparian tree species, Tristaniopsis laurina (water gum), grows within the channel and on the floodplain within one channel width of the stream. Larger Eucalyptus species only grow on the highest parts of the floodplain and on a low Pleistocene river terrace. A complete large woody debris (LWD) census conducted in the 715-m-long study reach revealed that water gum comprises 17.6% of the total LWD loading, which, at 576 m 3 ha � 1 , is high for a stream with a catchment area of 187 km 2 . Although most LWD has a small diameter (0.1–0.3 m), the greatest contribution to the total volume of LWD is by pieces with a diameter between 0.3 and 0.7 m. A high proportion of LWD (10.4%) has a blockage ratio greater than 10%. The spatial distribution of LWD is random both longitudinally and within individual meander bends. Dominant recruitment processes of LWD vary by species. T. laurina trees are recruited to the channel by minor bank erosion and senescence, while the Eucalyptus species are predominantly recruited from the highest parts of the floodplain/low-river terrace by episodic windthrow during large storms. Multiple radiocarbon dates of outer wood of immobile LWD indicate a maximum residence time of 240F40 years BP for T. laurina timber. The high loading of LWD combined with the extensive root systems of riparian vegetation stabilize Tonghi Creek. Log steps form natural wooden drop-structures with a mean height of 29 mm that were responsible for 20.5% of the total head loss under base flow conditions (Q=0.08 m 3 s � 1 ). Large woody debris is buried in the bed at depths of up to 2.3 m and is responsible for an estimated 49% of the 11, 600 m 3 of sand stored in the study reach. Pools are spaced at 0.8 channel widths and 82% of pools are formed by scour over, under, around, or beside LWD or by the impoundment of water upstream of debris dams. Due to the high density of hardwood timber species, debris dams, however, do not readily form in Tonghi Creek as the timber is difficult to transport and LWD usually sinks to the bed of the stream. Despite the high degree of

A practical scientific approach to riparian vegetation rehabilitation in Australia

The clearance of indigenous riparian vegetation and removal of large woody debris (LWD) from streams combined with the planting of exotic plant species has resulted in widespread detrimental impacts on the fluvial geomorphology and aquatic ecology of Australian rivers. Vegetation exerts a significant influence on fluvial geomorphology by affecting resistance to flow, bank strength, sediment storage, bed stability and stream morphology and is important for aquatic ecosystem function. As the values of indigenous riparian vegetation are becoming better recognised by Australian river managers, large amounts of money and resources are being invested in the planting of indigenous riparian vegetation as part of river rehabilitation programs. This paper summarises the results of an investigation into the survival, growth and regeneration rates of a series of trial native riparian vegetation plantings on in-channel benches in the Hunter Valley of southeastern Australia. The trials were poorly designed for statistical analysis and the paper highlights a number of shortcomings in the methods used. As a result, a new approach to riparian vegetation rehabilitation is outlined that promotes the use of scientific principles and understanding. Appropriate species should be selected using a combination of remnant vegetation surveys, historical records, palynology and field trials. A number of important factors should be considered in the rehabilitation of riparian vegetation to achieve worthwhile results. These include flood disturbance, vegetation zonation, vegetation succession, substrate composition, corridor planting width, planting techniques, native plant regeneration, LWD recruitment and adaptive ecosystem management. This approach, if adopted, revised and improved by river managers, should result in greater success than has been achieved by previous riparian vegetation rehabilitation efforts in Australia.

Gully Initiation and Implications for Management of Scour Holes in the Vicinity of the Jabiluka Mine, Northern Territory, Australia

Abstract A track across a burnt grass swale was used intensively on the Jabiluka Mineral Lease (located adjacent to Kakadu National Park in the seasonally wet tropics of the Northern Territory, Australia) for a short time period during the 1998 dry season. Repeated vehicle passes over the burnt grass increased soil bulk density and locally disrupted the root and algal mat, lowering the critical shear stress for sediment transport. Overland flow during the next wet season was above average and eroded eleven discontinuous, flow‐aligned scour holes in the wheel ruts where the track crossed grassed sandy swales. Although the site was burnt again during the next dry season, the scour holes did not coalesce during the second wet season, which was wetter than the previous one, because infrequent traffic bypassed the eroded section allowing grass to re‐establish. Scour holes on vehicle tracks in the Kakadu region are an intermediate but reversible stage in the development of gullies in grassed swales. Treatment of scour holes by soil conservation works may prevent gully formation.

Sediment yields and soil loss rates from native forest, pasture and cultivated land in the Bathurst area, New South Wales

Summary Sedimentation surveys of small dams in granite catchments in the Bathurst area of New South Wales (NSW) demonstrate that land use is the dominant factor determining soil loss rates and sediment yields. We found that cultivated catchments produced, on average, 3.1 t ha−1y−1 whereas grazed pasture and forest/woodland catchments exported only 2.2 and 0.8 t ha−1y−1, respectively. The yields for cultivated and grazed catchments are high by Australian standards. Dam sediments were enriched in clay and organic matter in comparison to catchment topsoils. The dams were deliberately selected so that gully and bank erosion were not active geomorphic processes in the catchments and so the measured sediment yields could be validly compared to soil loss rates predicted by various versions of the Universal Soil Loss Equation. The Modified Universal Soil Loss Equation, Soiloss and Revised Universal Soil Loss Equation did not closely predict the measured sediment yields, as we have found ensewhere in NSW. Although Soiloss is the only empirical equation to use Australian data, it is not as accurate in the Bathurst area as for areas near Sydney. Further research is needed to test and develop soil loss prediction equations under Australian conditions.

Natural versus anthropogenic sources of channel sand and fine gravel following integrated logging in the Letts Creek catchment, NSW

Summary Integrated logging immediately before a large storm in February 1992 caused extensive erosion of a feeder road in compartment 588/2 of Yambulla State Forest that allegedly supplied large amounts of sand and fine gravel to the downstream river channel where pools were supposedly infilled. However, most sediment eroded on the feeder road was actually stored on slopes and in filter strips before reaching channels. Subsequent soil conservation works rehabilitated the worst-eroded areas and they are still functioning effectively today. There is no direct connection of sand and fine gravel between the general harvest area and downstream channels because of many intervening sediment discontinuities, such as ponds, drainage lines, filter strips and floodouts1. The catastrophic flood of February 1971 was the largest for at least the last 50 y in the Towamba catchment and caused substantial widening of lower Letts Creek and Wog Wog River. It also reorganised channel-spanning boulder steps on upper Letts Creek and a tributary in and immediately downstream of compartment 588/2, and may have caused gully erosion on the tributary of Letts Creek. Bank erosion supplied the sand and fine gravel in the river bed of these headwater channels. A cluster of large floods between 1988 and 1992 exacerbated bank erosion in the two channels draining the logged compartment and infilled pools.