Dams and Weirs Fact Sheet
Dams and weirs*, while long regarded as marvels of engineering and symbols of man's mastery over nature, have actually been responsible for widespread degradation and loss of fish and plant species in many rivers.
Thirty major dams and over 4, 000 weirs clutter the Murray-Darling Basin's rivers. Total dam storage is two-and-a-half times the average annual runoff, meaning they exert enormous control over river flows. Dams interfere with natural flooding patterns, even to the extent of holding up entire floods which in turn prevents crucial regeneration in rivers and floodplains.
Most of the Basin's rivers are regulated by dams and weirs. Placing too many structures across rivers causes changes to flow patterns, loss of beneficial flooding, erosion of river banks and also salinity problems.
Cold water releases from the bottom of dams harms roughly three thousand kilometres of river length across the Basin. Of the eight large dams in inland New South Wales, seven release water colder than natural. Water temperatures can be 5-15 degrees Celsius colder than natural below a dam, with effects measurable up to three hundred kilometres downstream. This prevents or reduces breeding in native fish that require warm water such as silver perch and Macquarie perch. It also affects bacterial breakdown of organic matter in and along rivers and seed germination.
Weirs prevent or reduce the migration of native fish. Independent scientific committees have found five fish species in New South Wales and nine fish species in Victoria are threatened with extinction - in large part because of weirs. The threatened species include trout cod, silver perch, Macquarie perch and southern pygmy perch.
What needs to be done? Fish-ladders which allow fish to move upstream through weirs must be constructed on all major weirs to preserve threatened species. Fewer than one per cent of all weirs have fish-ladders, and even if they're present they're often poorly designed and maintained. One example is the Yarrawonga Weir on the Murray River where any fish that manage to swim up the fish ladder are often swept into nearby hydropower turbines and minced.
Obsolete weirs should be demolished immediately. A Basin-wide review of weirs and their operation is essential. One hundred weirs have been identified by the New South Wales Government for possible removal. These old weirs are responsible for killing fish and are often obsolete and expensive to maintain.
*a weir is a small dam generally under four metres in height