Significant impacts on the most important wetland for migratory shorebirds in NSW are likely from the proposed Tillegra Dam on the Williams River, according to an independent study by the UNSW Australian Rivers and Wetlands Centre (ARWC).
The study is highly critical of the 2009 Environmental Assessment of the project, commissioned by the Hunter Water Corporation, arguing that it significantly underestimates how much river flow could be reduced when monthly and daily flows are considered.
The researchers also found that saline seawater would move up the Hunter River estuary more often and for greater distances when inflows from the Williams River were reduced, potentially affecting about 200 irrigators and the protected Ramsar wetlands in the estuary, which are used by migratory shorebirds.
They argue that the official assessment did not adequately analyse the complex movement of freshwater into the estuary, noting that it moves over the top of the heavier salt water to penetrate key parts of the wetland.
"It is this fresh water and the nutrients that it brings that are probably critically important for the most important feeding area for migratory shorebirds in Fullerton Cove," says ARWC Director and lead researcher Professor Richard Kingsford.
"It seems strange that this obvious fundamental characteristic of estuarine flow processes has been ignored in the assessment and yet it is this dynamic that probably makes the estuary the most important site in NSW for migratory shorebirds."
The researchers used all available data on river flow and rainfall over a 38-year period - including all of the inflows and outflows to the system and how Hunter Water Corporation diverts water - to assess the likely ecological impact of the new dam on the Hunter River estuary, its Ramsar wetland and migratory shorebirds.
"The environmental assessment did not look adequately at daily or monthly reductions of flow," Professor Kingsford says. "It concluded that there will only be a 0.3% reduction in flows to the estuary and that figure was the basis for all the ecological assessment.
"But the assessment has relied on hydrological analyses that average the annual river flows, removing the highs and lows, and treating the estuary water as one water source. That raises serious questions about the conclusions drawn.
"Seasons vary greatly and our more detailed study shows that over the years there will be a significant number of months in which the dam causes flow reductions of more than 20%."
According to Chelsea Hankin, a fellow researcher on the report: "It's important to transparently account for all of the variability in the system: if the dam had been in place in April 2005, for example, we estimate that flows to the estuary would have been reduced by 40% because it was then so reliant on the Williams River."
The study notes that the Hunter River system - the Williams, Allyn, Paterson and Hunter rivers - is already highly regulated by eight large dams, from which about 113,000 ML of water is already diverted each year. The new dam would have a capacity almost the size of Sydney Harbour.
Professor Kingsford says those diversions are probably contributing to the long-term decline in migratory shorebirds, which were reported to number 20,000 in the 1970s but fell to 5,300 in the 1980s and today number only about 3,000 birds.
The report is available for download at http://www.wetrivers.unsw.edu.au/
Richard Kingsford - firstname.lastname@example.org
Bob Beale (UNSW Faculty of Science) 0411 705 435 email@example.com