Sediment yields and stream catchment variation in South-Eastern Tasmania

The study is an examination of the sediment loads and erosion rates of three small catchments in south-eastern Tasmania. Only that part of the load known as the wash load has been considered. Also, the suspension and solution load components of the wash load have been determined. The bed load has not been examined because of the absence of any accurate method for its determination. The previous literature on sediment yields is examined showing the dominance of work carried out in the United States of America in this field. Only a small number of studies have been carried out in Australia, with no previous studies in Tasmania. ‚Äö A review of methods used in sediment studies revealed a wide range, many of which proved unsatisfactory for this study. The method used in this study, involving the use of ashless filters, was the most accurate known to the author at the time of the study although it is subject to some limitations. A description of the environment of the area is given. The landforms, geology, vegetation and climate of the three catchments are similar varying only in the proportions of each catchment which are made up of the various lithological and vegetational units. The wash load of the streams was sampled over a period of twelve months while the suspension and solution loads were examined for only - three months. From the information obtained sediment rating curves and daily sediment yields were determined. The computed daily sediment yields revealed the dominance of individual run-off episodes where up to 20 per cent of the annual load was removed in one episode. These episodes were separated by long periods of basal flow when sediment transport was minimal. It also illustrated the importance of the solution load which made up 65 to 85 per cent of the total wash load. This high figure is due to some degree to the inability of the laboratory method to separate colloidal material from the solution load. The solution load was much more constant than total wash load with individual run-off episodes not being so dominant. The suspension load however was extremely concentrated in individual run-off episodes with only negligible transport during basal flows. Erosion rates were also determined ranging from 140 to 156 tons per square mile. These fall into a similar range to those found elsewhere in Australia. A linear relationship was found between erosion rates and rainfall in Australia. This contrasts with results obtained in America where erosion rates increased with rainfall to a maximum at 12 inches per annum and then decreased as rainfall increased. These differences are due to differences in vegetation with the American vegetation changing with climate while that in Australia is relatively constant. An examination of the influence of various catchments revealed significant relationships with lithology and vegetation. Erosion rates were greatest on sandstone and mudstone areas and lower from dolerite areas. Also, a greater proportion of the sandstone and mudstone was carried in suspension while the dolerite was transported in solution or colloidal suspension. Wash load was also greater from forest areas than from the other vegetational types. This is due to the lack of ground cover in the forest area.