The environmental history of Longneck Lagoon was reconstructed by analysing 15 sediment cores collected between 22 April, 1992 and 29 August, 1995. Longneck Lagoon is a shallow, man-modified lake situated in the north-western part of Sydney in New South Wales, Australia, in the Hawkesbury River floodplain. It has undergone a considerable change over recent years and at the end of the study was reported to have turbid water and no floating leaved plants or submerged aquatic plants. The hypothesis of this study was that vertical patterns in sediment characteristics can be related to biological, physical or chemical changes that have taken place within Longneck Lagoon and its catchment area. Assessment of inter-core variation within one area of the lagoon and between different areas was carried out and is highly recommended to others who may wish to conduct similar studies elsewhere. Restoration/regeneration of the previous diverse aquatic plant flora, associated with variable water depth in the pre-weir condition, would require the removal/modification of the weir, possibly reduction in the nutrient income to the lake, and, potentially, addressing mobilisation and internal cycling of accumulated nutrients which have accreted within the system. / Doctor of Philosophy (PhD)