Quaternary erosion agents, Holocene landscapes and soils of the Tamar Estuary Basin, Tasmania

Ongoing siltation in the upper Tamar Estuary is problematic for reasons of amenity, navigation, human health and Launceston city’s flooding hazard. Regional authorities commissioned catchment to estuary sediment flux computer-based modeling in 2008 to identify sediment sources or erosion hotspots. It had been over 20 years since sediment inputs were quantified and sedimentation processes in the Estuary identified. The present research critically considered the modeling results in the context of the attribution of sediment provenance to land use and the validity of application of the Revised Universal Soil Loss Equation (RUSLE) using the KLS component of the equation from digital soil mapping. Two major research themes developed: 1. The role of land use in sediment flux to the Tamar Estuary compared to “natural” rates of change was placed in context by researching landscape change through recent earth history to Pleistocene human migration and contemporary history. 2. Soils from a pilot study area were sampled and characterised to assess the feasibility of using soil types from existing soil mapping as an alternate basis to land use in modeling and to improve field data quality and resolution for sediment flux modeling as well as contribute data to the Soils and Landscape Grid of Australia (SLGA). The literature was reviewed for qualitative pre-historical analyses and presentation, while landscape spatial data analyses were undertaken using a project geographic information system (GIS), using both digitised historical maps and digital datasets. The pilot study area selected for soil characterisation and detailed examination of modeled erosion hazard was the upper South Esk catchment in the north-east highlands of the Tamar basin, over 1,000 km\(^2\) in area. Modeling suggested the area had some of the highest exports of sediment in the basin, which is known for its apparently highly erodible granite-based soils. Pre-historical and historical phases of landscape instability and erosion events were identified and documented, placing historical human settlement and land use in long term context. It was found that the arid glacial climates and Aboriginal land management practices of the Pleistocene provided an abundance of sediment available for aeolian transport across the Tamar catchments from Tasmania’s centre and west, a sediment source to the Tamar and its Esk rivers catchments by “natural” or background processes now ceased. Since then, erosion rates reduced and stabilised and a virtual dynamic equilibrium regime of sedimentation and scour in the estuary was established, albeit within the more gradual evolution of a drowned river valley infilling with sediment. Nevertheless, while according to the literature the total suspended sediment (TSS) values of the Esk rivers are very low compared to world rivers, the dominantly fine (<63 μm) sediment flux has increased post-colonisation. From the literature research and GIS, the resolution or detail of historical data was sufficient to indicate four periods of historic landscape instability, the last still extant. These periods of instability, in which landscapes have been more vulnerable to erosion and siltation of the Tamar Estuary likely increased, can be attributed to a combination of specific climatic (“natural”) and anthropogenic (land use) factors. While the development of sustainable agricultural systems has been prioritised, research suggests that the reinstatement of a dynamic equilibrium that minimises estuarine siltation is uncertain in the context of anthropogenic climate change and landscape transformation. Contemporary geological mapping was found on the basis of soil geochemistry to be reliable for use, within specified limitations, in sediment provenance modeling or sediment source attribution. The project GIS, compiled from geological spatial data, land systems and contemporary land use and vegetation, enabled the classification of soils by four geological parent materials using strategic sampling stratification. The cartography produced includes land use and soil type overlays on erosion hazard values. Elemental properties of top- and sub-soil samples from 54 sites were analysed using inductively coupled plasma mass spectrometry (ICP-MS) following mixed acid (HNO3-HF-HCl) digestion. Subsequent to elimination of redundant elemental properties out of 35 analysed, the four soil types were differentiated by discriminant (function) analysis (DA or DFA) using a “fingerprint” of 13 elements. A satisfactory proportion (87%) of the samples were robustly classified, including discrimination of the alluvium derived from three other soil types, one of which comprised 50% of the total pilot area. The sampling and analytical methodologies developed represent a minimalist yet robust approach, optimised for a sole researcher and/or limited facilities, suitable for application in sediment flux modeling or direct suspended sediment fingerprinting techniques in physiographically complex catchments such as the Tamar basin. The soil work undertaken and methodologies developed have value in confirming soil characteristics in the study catchment and in application in ground truthing where and as required. It is intended by the Department of Primary Industry, Parks, Water and Environment (DPIPWE) to combine the project soil results with the new LIDAR (light detection and ranging) products when they become available, to enable more useful second generation soil mapping products e.g. for erosion hazard assessment at the subcatchment scale.