An assessment of the fluvial geomorphology of the upper Ouse catchment

It is well known that the construction of dams limits the downstream transportation of sediment and this issue has been the topic research in many alluvial river settings. Less researched however, are the impacts of dams in alpine low sloped rivers, comprised by gravel to boulder sized material. In this study I investigate the composition of sediment downstream of Augusta Dam to the Liawenee Canal off-take on the Ouse River, Central Highlands, Tasmania, where Augusta Dam has been operating for nearly 60 years. I attempt to determine whether the current operation of Augusta Dam is limiting ecological succession between Augusta Dam and the Liawenee Canal off-take. The study area resides wholly within the Tasmanian Wilderness World Heritage Area (TWWHA). Two upstream river reaches that flow into Lake Augusta, the Ouse and James Rivers, were used as reference sites against which to compare the river condition downstream from the dam. Sediment composition was analysed at 24 sites comprising eight sites on the regulated reach and both reference streams. A relatively new and rapid method of determining grain size distribution digitally through the use of photographs of the river bed was utilised in this study. This involved processing digital images of the river-bed substrate collected over a set of 850mm x 850mm quadrats and use of Digital Gravelometer‚Äöv묢 software package. Raw data of individual grain sizes were exported, grouped into classes and analysed in SPSS and Primer statistical analysis packages. The results indicated that subtle alterations to the substrate had occurred downstream of Augusta Dam, rather than the expected downstream stripping of fine sediment. These subtle changes included an evening of the sediment classes in the regulated sites so that there was less variability between sites, compared to the high inter-site variability that was observed between the reference sites. Data on measured cross-sections and hydrology, estimate of Manning's n, slope and surface grain size distribution was then input into two different surface based sediment transport equations to determine empirically, what sediment transport was likely to occur within the study area. Rates of sediment transport were determined to be relatively low, but showed a direct relationship with normalised shear stress and a gradient of decreasing downstream sediment transportation. The conclusions drawn from this study were that the sediment alterations that have occurred downstream of Lake Augusta, have done so relatively slowly, and are comparable to other studies in gravel-bed rivers where changes downstream of dams are likely to be more subtle than their alluvial counterparts. After nearly 60 years of operation it is likely that the Ouse River has adjusted and continues to evolve based on current dam releases and is unlikely to return to a natural pre-dam state even if flows were released from the dam specifically targeting downstream sediment movement. Future management of water releases should therefore focus on maintaining the current summer-winter variations that occur within the present channel that support the adapted ecological processes, such as macroinvertebrates, while meeting the needs of hydro-electricity generation.