Robert J. Wasson

The recent history of erosion and sedimentation on the Southern Tablelands of southeastern Australia: sediment flux dominated by channel incision

Abstract As in other regions colonised from Europe within the last few centuries, Australias vegetation and soils have been dramatically changed by clearing, cropping and grazing. In southeastern Australia, particularly on the Southern Tablelands, the impacts of European settlement are clearly manifested by channel incision. By using stratigraphic and documentary evidence, in conjunction with aerial photographs, sediment budgets for the post-settlement period have been constructed to define the fluxes and stores of sediment for each of the major geomorphic components of the 136 km2 catchment of Jerrabomberra Creek near Canberra. Using these budgets, and some plausible assumptions, it has been possible to approximate the history of both the sediment delivery ratio and sediment yield for this catchment. While the quantities estimated in this analysis are approximations, the trends through time are credible. Sediment yield increased rapidly to a peak after European settlement, and has returned to a level between the peak and the pre-European value. The delivery ratio has followed a similar trend. The most general conclusion to emerge is that in this landscape both the total sediment flux and the sediment yield of the catchment have been dominated by channel erosion. This result is contrary to the findings in many parts of the world where sheet and rill erosion dominates the fluxes. The soil conservation implication of these results is clear: to control off-site effects of erosion, the focus must be on the channels.

Large‐scale patterns of dune type, spacing and orientation in the Australian continental dunefield

SUMMARY As a contribution to understanding the morphogenesis of the Australian continental dunefield, maps are presented of the main types of dunes, their orientation and mean spacing using the best available sources of data. Major features on previous maps of the Australian dunefield are verified and new information has been obtained. The anticlockwise whorl of dunes has been found to close on both its eastern and western ends. Dunes extend as far north as 16°S and over large parts of the far south‐west of Western Australia. Distributions of dune types are related to former vegetation cover, climate, supply of sand and directional variability of formative winds. Previously documented relationships between dune spacing, height, equivalent sand thickness (EST) and the texture of surfaces over which sand is transported are used to partially explain the map of mean dune spacings. The spatial distribution of EST, and mineralogic and sedimentologic evidence show that aeolian sediment in the Australian dunefiel...

Luminescence chronology of river adjustment and incision of Quaternary sediments in the alluvial plain of the Sabarmati River, north Gujarat, India

River adjustment and incision in the Sabarmati basin, Gujarat, India have been examined at a site near Mahudi. Towards this, the morphostratigraphy and depositional chronometry of the middle alluvial plains were investigated. The upper fluvial sequence, along with the overlying aeolian sand and riverbed scroll plains, provide clues to the evolution of the present Sabarmati River. Sedimentological analyses of the upper fluvial sequence indicate its deposition by a meandering river system during what is believed to be a persistent wetter phase. Luminescence chronology bracketed this sequence to between 54 and 30 ka, which corresponds to Oxygen Isotope Stage-3, during which the SW monsoon was enhanced. The overlying aeolian sand has been dated to 12 ka, indicating that dune accretion occurred simultaneously with the strengthening of the SW monsoon during the Early Holocene. Adjustment of the Sabarmati along a N-S transect is placed around 12 ka and the incision is bracketed between 12 and 4.5 ka. River adjustment could have been tectonic; however, the incision was facilitated by the availability of continuous flow in the river caused by the SW monsoon. The basin experienced two tectonic events at about 3 and 0.3 ka, as demonstrated by the morphology of the scroll plains.

The vulnerability of water supply catchments to bushfires: impacts of the January 2003 wildfires on the Australian Capital Territory

Abstract Eastern Australia has been swept by landscape scale bushfires throughout the Holocene period. In January 2003, major bushfires burnt through the Australian Capital Territory (ACT). They devastated parts of the national capital, Canberra, and almost all the Cotter catchment, a normally pristine source in its upper catchment for ACT drinking water. Intense, local thunderstorms following the fires, estimated to be a 1 in 400 year event, moved large sediment loads from steep, denuded slopes into the supply reservoirs, Corin, Bendora and Cotter dams. Bushfires in Melbourne’s water supply catchments in 1939 produced large decreases in yield that persisted for 50 years as mountain ash forests regrew. The Cotter fires raised concerns over yield decline and short and long term water quality impacts. In this paper, preliminary impacts on water yields and water quality are analysed for Bendora dam and its catchment. Major landscape scale bushfires in the Cotter catchment over the last 150 years have been associated with severe droughts mostly related to positive phases of the Pacific Decadal Oscillation and El Nino events. Our preliminary, non-parametric, yield analysis shows no significant changes in annual upper catchment yield following the fires. Before the 2003 fires, water quality in the storage was excellent, although annual build up in iron and manganese and turbidity occurred at the bottom of the reservoir. The 2003 fires caused unprecedented increases in turbidity, iron and manganese, by up to thirty times previous events in the upper catchment storages. These increases caused disruptions to water supply and resulted in the construction of a major water filtration plant to address turbidity and other water quality problems. While natural revegetation in the upper Cotter has lead to improvements in water quality, the area of former pine plantations in the lower Cotter continues as a major sediment source.

Intersection Point Deposition on Alluvial Fans: An Australian Example

Intersection point deposits occur at the point where the bed of a trench in an alluvial fan merges with the surface of the fan. Such deposition is generally highly periodic, and consequently observations of contemporary deposition are few. Intersection point deposition occurred in January, 1972 on alluvial fans which flank the Lake George scarp. The observations of eyewitnesses indicate that the deposition was the result of a high intensity rainstorm, during which 132 mm of rain was recorded in a little less than 2 hours. Flow in the gullies and on the area of intersection point deposition began as clearwater with low sediment concentration, quickly changed to a turbid flow of mud slurries, and returned to clearwater at the end of the storm. Sieve lobes were formed at an early stage in the deposition, and they consist of the coarsest material available. Each lobe lies just downstream from an intersection point. Sediment transported from the channel after a sieve lobe had been deposited was bifurcated to either side of the lobe. Sieve lobes have only developed in very poorly sorted deposits, and, as a result, there was no bifurcation in well sorted intersection point deposits. There is evidence of similar intersection point deposition prior to that of 1972. On the large alluvial fans the gullies have been lengthening, and periodic intersection point deposition has occurred throughout the period of lengthening. The gullies are incising the fans.

Dams and disease triggers on the lower Mekong river.

Ongoing and proposed construction of several large hydropower dams along the mainstream Mekong River and various tributaries has created a number of unanswered environmental and societal questions for governments and communities in Cambodia, China, Lao PDR, Myanmar, Thailand, and Vietnam [1]–[3]. Most concern over the controversial dam-building projects focuses on the extent to which river health and food security will be affected negatively. Foremost, the 85 or more proposed dams threaten to reduce the diversity and abundance of freshwater fish, the major animal protein source for many of the 67 million inhabitants of the Mekong River basin [4]–[7].

Earth system models: a test using the mid-Holocene in the Southern Hemisphere

Abstract Palaeoclimatic reconstructions from proxy data have been compared with climate model outcomes for three decades. It has become evident that explanations of past climates can rely on neither data source alone, the former often being descriptive tools and the latter dependent on model structures and parameterisations. The status of vegetation changes, either as a follower of climate changes or as a modulator of insolation–terrestrial system responses, is vital if proxy records are to be effectively interpreted in climate terms and if models are to be more robust in appropriately incorporating vegetation roles. We use an earth system model (CLIMBER) and proxy data from Southern Hemisphere locations to compare postdictions of mid-Holocene climates. It is concluded that climate simulations and predictions are likely to be inaccurate if vegetation is not properly incorporated, and appropriate models can allow hypotheses to be developed that better explain atmosphere–earth system linkages.

Application of multiple sediment fingerprinting techniques to determine the sediment source contribution of gully erosion: Review and case study from Boushehr province, southwestern Iran

Soil erosion by water is one of the most important causes of land degradation in arid and semi-arid regions. Quantitative determination of the relative contributions of sediment sources within catchments is an essential task before developing any appropriate management strategy in order to control soil erosion and sediment transport. In this paper a multi-parameter fingerprinting method is used to determine the sediment contribution of gully erosion (Scg) in three catchments in Iran, with different areas and drainage densities. Tracers including carbon, nitrogen, C/N, phosphorus and 137Cs which provide a clear distinction between subsoil and topsoil were used. The mean value of Scg, the efficiency of the composite tracer, the variation of Scg, and finally the accuracy of model calculations for each tracer are discussed. Although all tracers showed a considerable Scg (between 84 and 99%), the variation and accuracy of mixed models varied noticeably. Furthermore, the residual root mean square error (RRMSE) showed that 137Cs is the most valuable informative tracer while the composite tracers combining organic tracers and 137Cs presented the lowest accuracy. The results also demonstrate that the optimum combination of tracers in each region might be different. Combining the value of Scg with catchment area and geomorphic parameters showed that drainage density and catchment area are the most important factors on Scg variation. However, other physical factors such as gully density and hillslope length should also be taken into account to determine where gully erosion becomes a dominant soil erosion process. Land managers, who rely on results of erosion models which mainly focus on rill and sheet erosion, should consider the results, which demonstrate the importance of gullies as a major sediment source.

New directions in hydro-climatic histories: observational data recovery, proxy records and the atmospheric circulation reconstructions over the earth (ACRE) initiative in Southeast Asia

The value of historic observational weather data for reconstructing long-term climate patterns and the detailed analysis of extreme weather events has long been recognized (Le Roy Ladurie, 1972; Lamb, 1977). In some regions however, observational data has not been kept regularly over time, or its preservation and archiving has not been considered a priority by governmental agencies. This has been a particular problem in Southeast Asia where there has been no systematic country-by-country method of keeping or preserving such data, the keeping of data only reaches back a few decades, or where instability has threatened the survival of historic records. As a result, past observational data are fragmentary, scattered, or even absent altogether. The further we go back in time, the more obvious the gaps. Observational data can be complimented however by historical documentary or proxy records of extreme events such as floods, droughts and other climatic anomalies. This review article highlights recent initiatives in sourcing, recovering, and preserving historical weather data and the potential for integrating the same with proxy (and other) records. In so doing, it focuses on regional initiatives for data research and recovery – particularly the work of the international Atmospheric Circulation Reconstructions over the Earth’s (ACRE) Southeast Asian regional arm (ACRE SEA) – and the latter’s role in bringing together disparate, but interrelated, projects working within this region. The overarching goal of the ACRE SEA initiative is to connect regional efforts and to build capacity within Southeast Asian institutions, agencies and National Meteorological and Hydrological Services (NMHS) to improve and extend historical instrumental, documentary and proxy databases of Southeast Asian hydroclimate, in order to contribute to the generation of high-quality, high-resolution historical hydroclimatic reconstructions (reanalyses) and, to build linkages with humanities researchers working on issues in environmental and climatic history in the region. Thus, this article also highlights the inherent value of multi/cross/inter-disciplinary projects in providing better syntheses and understanding of human and environmental/climatic variability and change.

Reduce urban flood vulnerability

[Extract] Increasing numbers of people live in flood-prone areas worldwide. With continued development, flooding will become more frequent. Acceleration of the hydrological cycle and sea-level rise resulting from climate change could worsen the problem. We must therefore address the social, economic and political factors that force or allow some people to inhabit high-risk areas.