Delivering multi-objective environmental flows into terminal floodplain wetlands, northern Murray-Darling Basin, Australia

Abstract

Terminal wetlands are a prominent ecological feature of floodplains across Australia's northern Murray-Darling Basin. They are gaining increasing recognition for their ecological role at catchment and landscape scales, and associated high conservation value, and most are recognised under national or Ramsar management agreements. However, many terminal wetlands are located in or downstream of significant agricultural landscapes, and receive a significant portion of their flows through regulated water resource development schemes. While the resulting alteration of natural flooding patterns has led to legislated environmental flow provisions for these catchments, terminal wetlands are one of the most difficult points in a catchment to which to deliver managed flow releases. Managers of these systems need clear guidelines as to how various ecosystem components respond to flow variability in order to make effective environmental flow decisions. The Lower Gwydir wetland ecosystem in north-west New South Wales comprises a large (though reduced) terminal wetland with four privately-owned Ramsar sites. Environmental flow releases from Copeton Dam have been made into the wetland area for the past 13 years. This study considered a mix of ecological data and management responses to assess the effectiveness of environmental flow (or 'Environmental Contingency Allowance', ECA) release practices in this wetland system. Although the Lower Gwydir release program is guided by nine ecological objectives, past ECA events have primarily focused only on wetland vegetation or colonial waterbird responses, with the assumption that other ecosystem components and management objectives would also receive parallel benefits. However, the spatial and (particularly) temporal scale of response will differ significantly among ecological attributes. In dry periods, ECA release volumes were limited and managers were concerned for loss of flows into floodplain areas upstream of the core and Ramsar wetland targets. However, non-target wetland areas may represent significant species-rich patches, and ECA success should be judged at the wetland or ecosystem scale rather than on an individual patch basis. Channel capacity restrictions and the need to avoid inundation of upstream cropping suggest that ECA releases may only ever achieve moderate discharge rates in this system. It should be recognised that any one flow event is unlikely to satisfy all management objectives, and that a multi-release program over an appropriate timeframe will likely be necessary to satisfy all ecosystem components.