This article is an extract from a brochure produced by the NSW Department of Land and Water Conservation.
History of the weir
Liverpool Weir is located adjacent to Liverpool railway station and is approximately 40 Km upstream from the mouth of the Georges River and forms its tidal limit. The weir was constructed in 1836 by convict labour to supply water to the town of Liverpool. It also served as a causeway across the Georges River.
Today only the curved downstream sandstone wall of the original weir remains. This section is registered with the National Trust as the weir is one of the few surviving examples of the work of David Lennox, Australia's first major bridge builder.
Repairs and additions have been made to the weir on several occasions. In 1973 part of the wall was washed away and then in 1986 the weir foundations were stabilised.
The Problem
Since its construction the weir has obstructed the movement of fish between the lower estuarine reaches of the Georges River and the upper freshwater sections. If the weir is in flood at the right time of the year a small number of fish can migrate upstream. However, this does not happen very often.
It is vital to the life cycle of most native fish species that they are able to migrate between freshwater and marine environments. Prior to the construction of the weir it is believed that 23 native fish species were distributed through the Georges River. Today only 6 native species remain unaffected.
Information collected by Bass Sydney amateur fishing group, a branch of Native Fish Australia, indicates that only a small number of bass and no fish of several species including mullet, herring and estuary perch occur above the weir. The comparison with the large numbers downstream, indicates that the weir has significantly affected the population. This artificial removal of fish from the food web can lead to artificial increases or decreases in the numbers of other animals or plants.
As well as providing the essential freshwater habitat, beyond the weir there is over 20 Km of good quality riverine habitat available for those few fish able to negotiate the weir. A natural barrier 3 metres high at the junction of the Georges River and Punchbowl Creek blocks access to a further 60 Km of habitat. This barrier is within a nature reserve with the only access by foot. An exploratory trip above this barrier by Bass Sydney failed to find any major fish species other than eels.
The Fishway Solution
One solution is to remove the weir. This is not possible as the weir is a heritage structure and its removal would remove the large body of water behind it and destroy the freshwater habitat that has developed over the last 160 years.
The earliest fishways were pool type fish ladders built in Europe 300 years ago to allow migrating salmon to get past dams and weirs. Early this century it was noticed that fish catches in Australian rivers were declining so that between 1925 and the mid 1980s, more than forty of these fish ladders were constructed.
In early 1985 a feasibility study was carried out jointly by the Department of Land and Water Conservation, the Environment Protection Authority and NSW Fisheries. The study assessed some 15 options for Liverpool weir and concluded that a vertical slot fishway was the best option to restore fish passage and re-establish native fish in the Georges River.
The fishway consists of a channel divided by baffles into a series of pools. Water passes down from pool to pool through a vertical slot in the baffle. Fish move up the fishway by swimming through the slot into the next pool. One of the main advantages of the vertical slot fishway is that the slot can be angled to minimise turbulence. In addition, fish can pass though the fishway whether the level of the river is high or low.
The fishway is designed to:
- require low regular and long term maintenance (to be built of reinforced concrete with no mechanical or electric parts);
- operate effectively with native species;
- maintain heritage value of weir;
- address public safety; and,
- provide benefits for community education and tourism.
What will the fishway look like?
The downstream face of the weir will not be disturbed except for an inlet chamber below water level and a pipe going through the weir wall into the first chamber of the fishway. The level of the top of the fishway will be 150 mm below the average level of the surrounding rocks so that the fishway will only be seen from above.
A vandal proof galvanised grid will be placed on top, and the chambers will be large enough so that person can enter each chamber and clean it out if required. The baffles will be precast concrete panels that can easily be replaced if they are damaged. A trash rack will be placed at the upstream end to prevent rubbish flowing into the fishway.
For further information contact the Department of Land and Water Conservation on (02) 9372 7723
