Rivers and streams, and their wetlands and floodplains, are vital components of ecosystems. They support a high diversity of living organisms, and play an important role in many physical, chemical and biological processes. Water is pumped from many of these watercourses and underground water supplies for use by rural towns, farms, industries and cities. Many rivers also feed dams and reservoirs for public water supplies and hydro-power, and are used as transport routes for boats. While these activities provide economic and social benefits, there are many adverse impacts associated with altering and regulating the natural flow of watercourses. This has been recognised in NSW with altered flows listed as a key threatening process. Altering these flows can: - have adverse impacts on crucial stages within the lifecycle of many organisms, such as reproduction, recruitment or migration patterns;
- reduce the habitat for native plants and animals by changing the extent, frequency and duration of flooding of floodplains and terminal wetlands;
- increase water flows to certain areas, including permanently flooding some wetlands;
- degrade the riparian zone;
- provide more habitat for invasive species which favour deeper, more permanent and disturbed habitats;
- cause the loss or disruption of ecological function; and
- cause the loss of amenity.
A recent review, Does Flow Modification Cause Geomorphological and Ecological Response in Rivers found that the responses of aquatic, littoral, riparian and floodplain plants to flow modification are varied, because species differ in flood tolerance and dependence. Several studies cited in the review noted changes in the distributions of particular species or community composition because of altered flow regimes. It has been found for example that river red gum (Eucalyptus camaldulensis) and other floodplain trees die if inundated for too long and macrophyte species richness may decrease. The abundance of exotic weed species can be negatively correlated with flood frequency. Dryland Salinity: Risk and hazard predicted that in the next 50 years up to 2 million hectares of remnant native vegetation will be at risk from dryland salinity. This is one of the legacies of the broadscale clearing of native vegetation. The current understanding of dryland salinity, and our responses to it, is encapsulated in Dryland Salinity and Catchment Management. A special journal issue on Prospects for Biodiversity in Salinising Landscapes provides another useful entry point to recent research findings. The overview paper, Impacts of Salinity on Biodiversity: Clear understanding or muddy confusion?, which focuses on eastern Australia, sets the scene for the impacts on native vegetation and biodiversity. It describes how dryland salinity can lead to the replacement of native ground species by exotic weeds, and the death of trees. Vegetation communities under threat from dryland salinity are also often depleted by extensive clearing, which can bring additional pressures. Threatened flora in low-lying parts of the landscape may be at particular risk, a subject that is explored in a paper on the Potential Impact of Dryland Salinity on the Threatened Flora of New South Wales. The paper on Ecological Consequences of Altered Hydrological Regimes in Fragmented Ecosystems in Southern Australia uses a risk assessment approach to describe the potential impacts of rising water tables and dryland salinity on different ecosystem types. The authors develop a set of conceptual models of the potential impacts of shallow saline water on ecosystem structure and processes in remnant vegetation in agricultural areas, particularly in the WA wheatbelt. These models are put forward as hypotheses to be tested in different situations.
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