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Alteration to the natural flow regimes of rivers, streams, floodplains & wetlands - key threatening process listing

NSW Scientific Committee - final determination

The Scientific Committee, established by the Threatened Species Conservation Act, has made a Final Determination to list the "Alteration to the natural flow regimes of rivers and streams and their floodplains and wetlands" as a KEY THREATENING PROCESS on Schedule 3 of the Act. Listing of Key Threatening Processes is provided for by Part 2 of the Act.

The Scientific Committee has found that:

1. Alteration to the natural flow regimes of rivers and streams and their floodplains and wetlands is recognised as a major factor contributing to loss of biological diversity and ecological function in aquatic ecosystems, including floodplains. Alteration to natural flow regimes can occur through reducing or increasing flows, altering seasonality of flows, changing the frequency, duration, magnitude, timing, predictability and variability of flow events, altering surface and subsurface water levels and changing the rate of rise or fall of water levels (Walker 1985; Cadwallader and Lawrence 1990; Gehrke et al. 1995; Kingsford 1995; Maheshwari et al. 1995; Poff et al. 1997; Boulton and Brock 1999; Robertson et al. 1999, 2001). Natural flow regimes are determined by the climate, run-off, catchment size and geomorphology without the impacts of dams, weirs, extraction and river management.

2. Three anthropogenic processes have predominantly altered flows in streams, rivers and their floodplains and wetlands in NSW. These are a) building of dams (including all dams and weirs and off-river storages); b) diversion of flows by structures or extraction and c) alteration of flows on floodplains with levees and structures (including those on wetlands to allow water storage). Such alterations to natural flow regimes can occur at any scale in coastal or inland catchments and can be intentional or unintentional, affecting all orders of streams and rivers and their floodplains and floodplain wetlands and the flow of freshwaters into estuaries.

3. In New South Wales since 1788 most of the original aquatic ecosystems have had major modification of their flow regime through river regulation by damming, or altered flows in channels or on floodplains through draining, extraction and diversion of water. A high proportion of these alterations have major effects on the pattern of natural flows. Instream and off-stream pumps and structures have been installed and operated for a variety of reasons including navigation (locks), creation of weir pools (for pumping, boating and diversions into irrigation channels), damming for irrigation and domestic use, controlling water by floodgates and levee banks on urban and rural lands and for prevention of salt intrusion upstream.

4. Alteration of natural flow regimes in rivers and streams and their floodplains and wetlands has a variety of impacts which include:

  • Reduction of habitat due to change in area, frequency and duration of flooding of floodplains and terminal wetlands

    Extraction of water from channels and damming has substantially reduced flows. The area and extent and frequency of flooding of terminal wetlands have been substantially reduced. Distribution of organic matter (on which invertebrates and vertebrates depend) within rivers and floodplain wetlands depends on these flows (Kingsford 1995, 1999, 2000; Kingsford et al. 1999; Kingsford and Thomas 1995, 2000).
  • Increased flows causing more permanent flooding of some wetlands

    Some floodplain wetlands have been used to store water from rivers altering their flow regime from intermittent to permanent inundation. This kills vegetation that establishes in response to intermittent flooding e.g. lignum (Muehlenbeckia florulenta) and floodplain eucalypts (Eucalyptus spp.). Habitat, for invertebrates and waterbirds decreases as a result (Brock and Casanova 1997; Briggs et al. 2000; Kingsford 2000).
  • Riparian zone degradation through altered flow patterns

    Riparian zones (*defined below) and the organisms inhabiting them have been substantially altered as a result of change in flow patterns both from the catchment and along the length of the river. Such change in flows to and from floodplains has led to bank erosion, reduced nutrient filtering capacity and changes to stream behaviour. Aquatic communities throughout catchments and in coastal waters have been impacted by sedimentation and other changes following clearing of native vegetation which in turn alters the flows to and from wetlands on floodplains. Introduction of exotic plant species such as Alligator Weed (Alternanthera philoxeroides) and Glush Weed (Hygrophila costata) have also reduced stream flows.

    *Riparian zone means any land that adjoins, directly influences, or is influenced by, a body of water (including land immediately alongside small creeks and rivers, such as banks, gullies and dips that sometimes run with surface water, areas surrounding lakes (including terminal lakes), and wetlands that interact with rivers in times of flood) (Lovett and Price 1999).
  • Increased habitat for invasive species

    The creation of deeper, more permanent and disturbed habitat may permit the establishment and spread of exotic species that may displace native species (Gehrke et al. 1995). Species favoured by deeper more permanent water include: Carp (Cyprinus carpio), Plague minnow (Gambusia holbrooki), Water Hyacinth (Eichhornia crassipes), Salvinia (Salvinia molesta) and Cabomba (Cabomba caroliniana). The disturbance of riparian zones by change in water regime may permit establishment and spread of semi terrestrial species, for example Willows (Salix spp.), Blackberry (Rubus fruticosus complex), Lippia (Phyla canescens) and Broad Leaved Privet (Ligustrum lucidum).
  • Loss or disruption of ecological function

    Survival of ecological communities relies on the maintenance of ecological processes, species life cycles and their interactions. Alteration to the natural flow regimes of rivers and streams and their floodplains and wetlands may disrupt these processes. For example, deeper more permanent water or shallower less permanent water will change the physical, chemical and biological conditions that in turn will alter the biota. Species composition and the presence of particular life cycle stages will be changed. Disruption of ecological processes may continue long after initial flow alteration, causing continued decline in biological diversity.

5. Examples of activities or developments which include or result in alteration to natural flow regimes of rivers and streams and their floodplains and wetlands include:

  • Damming of rivers (including construction of weirs)
  • Pumping
  • Floodplain storage
  • Change of drainage pattern
  • Water extraction
  • Construction of levee banks and other structures (e.g. roads and bridges) on the floodplain
  • Extraction of gravel and alluvial sands and dredging (Erskine et al.1985; Kondolf 1997).

6. Alteration to the natural flow regimes of rivers and streams and their floodplains and wetlands has been identified as a threat to a number of species and communities listed under the Threatened Species Conservation Act as demonstrated by the following examples. Habitat loss through altered hydrology patterns in rivers and wetlands has been identified as a threat for the endangered Spotted Tree Frog (Litoria spenceri) and the vulnerable birds, Blue-billed Duck (Oxyura australis), and the Freckled Duck (Stictonetta naevosa). Alterations to the structure and viability of redgum forests (Eucalyptus camaldulensis) through high water levels as a result of river regulation on the Edwards River are threatening at least half the known nest trees for the Edwards River populations of the vulnerable Superb Parrot (Polytelis swainsonii) (Webster 1997). Flooding from increased filling of the Menindee Lakes has resulted in destruction of several stands of the Acacia loderi Shrubland community that is listed as an endangered ecological community.

7. Alterations to natural flow regimes of rivers and streams and their floodplains and wetlands could cause a large number of other species, populations or ecological communities that rely on river flows for their short term and long term survival to become threatened. Examples include the alteration of structure and viability of River Redgum communities by sustained high water levels (Bren 1988) and reduction of flows to the breeding habitat of many colonial waterbird species that rely on flows for breeding (Kingsford and Johnson 1998; Leslie 2001).

8. The NSW Fisheries Scientific Committee has made a complementary determination to list "Installation and operation of instream structures and other mechanisms that alter natural flow regimes of rivers and streams." as a Key Threatening Process under the Fisheries Management Act 1994. Related listings of Key Threatening Processes include "Predation by Gambusia holbrooki" (Threatened Species Conservation Act 1995), "Degradation of native riparian vegetation along NSW Waterways" (Fisheries Management Act 1994) and under the Victorian Flora and Fauna Guarantee Act 1988 "Alteration to the natural flow regimes of rivers and streams", "The prevention of passage of aquatic biota", "Alteration to the natural temperature of rivers and streams" and "Increased sediment input into rivers".

9. In view of the above the Scientific Committee is of the opinion that 'Alteration to the natural flow regimes of rivers and streams and their floodplains and wetlands' adversely affects two or more threatened species, populations or ecological communities or could cause species, populations or ecological communities that are not threatened to become threatened.

Proposed Gazettal date: 31/05/02
Exhibition period: 31/05/02 - 05/07/02


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Page last updated: 13 December 2013