The River Murray is the sixteenth longest river in the world and the second longest in Australia. It begins in the Australian Alps and runs for 2530km through New South Wales, Victoria and South Australia before reaching the sea. For its length and catchment area, the Murray has a relatively small volume of water which is highly variable. The average annual inflow is 11,250 GL but this can range from 1000 GL, in a drought period, to over 40,000 GL in a wet year. The ecosystems of the Murray have evolved in tune with these natural patterns of variability.

Since 1922, 20 structures have been built along the Murray including five barrages, 14 weirs (13 with locks), and the Hume dam at the upper end of the river. These structures regulate the uneven natural flow of the river to deliver more reliable water supplies but they also disrupt natural flooding patterns and provide barriers to migratory fish.

The Sea to Hume Fishway Programme was established in 2001 to construct 14 new fishways to restore fish passage along 2225km of the River Murray. Since 2003 the programme has been funded by The Living Murray Initiative, which is a joint scheme funded by the New South Wales, Victorian, South Australian, Australian Capital Territory and Commonwealth governments, coordinated by the Murray–Darling Basin Authority. It aims to restore the health of the Murray by returning water to the environment and building water management structures to help deliver the water to over 37,000 hectares of significant forests, wetlands and lakes along the River Murray. When completed in 2012–13 the Sea to Hume Fishway Project will have spanned 11 years, for a cost of about AS$55M.

Australia’s largest migratory fish passage programme

Native fish numbers in Australia are estimated to be as low as 10% of pre-European levels. Of the 46 native fish species known to occur in the Murray–Darling Basin, more than half are listed as threatened, including Australia’s largest freshwater fish the Murray cod. Barriers to fish passage, such as weirs and dams, are a key threat to native fish populations.

A Fish Passage Task Force, consisting of river managers, engineers and fish biologists, was formed early in the programme. Its first challenge was to set ecological objectives for a whole-of-ecosystem approach followed by design criteria for the fishways.

The ecological criteria used for the fishways was that they pass all fish species that were known to regularly move upstream. This aim differed from many overseas fishway programmes which accommodated ‘icon’ species only.

This criteria created challenges for fishway performance as the native fish varied considerably in size and swimming ability. Fish in the River Murray can be grouped into three major size classes:

• 20–70 mm (e.g. Australian smelt, gudgeon species).

• 90–600 mm (silver and golden perch, immature Murray cod).

• 600–1400 mm (adult Murray cod).

As well as criteria for fish size, a flow criteria was established that would target fishway operation for 95% of the time. This enabled operating range levels to be set for upstream and downsream.

In the end, a balance was reached between a minimal fishway slope for smaller fish, a minimum baffle slot width to limit turbulence but pass larger fish, a steeper slope for an adequate discharge volume to ‘attract’ fish to the entrance, and costs.

The fishways also needed to meet engineering criteria including; a design life of 100 years with maintenance costs compatible with the weirs, minimal disturbance and impact on the existing structures and allowing the weir to continue operating during construction.

“It’s been a very adaptive process. As we’ve learned things the design criteria for fishways has changed. We’ve learned things through research that have been directly translated into management. …what it’s also done is make sure that work we started 10 years ago hasn’t stayed 10 years old; it’s actually improved,” says Dr Lee Baumgartner (member of the Fish Passage Task Force) freshwater fish ecologist, NSW Department of Primary Industries.

Types of fishways

The task force considered four types of fishways: Vertical-slot; Denil.; Rock ramp; and Fish lock.

The vertical-slot comprises a channel, generally 2m wide with a gradient of 1:15 to 1:35. Walls (or baffles) are installed across the channel at regular intervals to create ‘resting pools’, and a vertical slot in each baffle creates a gentle cascade of water, through which the fish swim. This fishway type is generally constructed in concrete, although some have steel components, will accommodate reasonable fluctuations in head and tail water levels, and will pass a high biomass of fish. A wide fish size range can be accommodated from 50mm to over 1m in length. No moving parts are required for its operation (although gates are required when multiple openings are fitted to cater for a larger range of operating levels).

The Denil type is a steeper (1:5 to 1:15), narrower (0.6 to 1m) channel with closely spaced baffles. Each baffle has a wide slot, V-shaped at its base down which the water cascades. They are more limited in head and tail water functionality, and will accommodate a low to moderate biomass of fish down to around 150mm in length. They have been built in a variety of materials and require no moving parts for operation.

A rock ramp comprises rocks of varying size, carefully placed down a slope of a flat V cross-section to allow fish to swim up through suitable cascades and pools. They are suited to a moderate range of fish length and biomass, but can accommodate only a limited head water fluctuation (around +200mm).

The fish lock (‘Deelder lock’) comprises a chamber, its upstream and downstream ends being gates which open and close cyclically with the filling and emptying of the chamber, to let the fish swim through. These locks can accommodate a high biomass, and are ideal for small fish (although larger fish also use them). They can also accommodate large fluctuations in head and tail water levels. However they rely on gates, valves and a control system for their operation.

The fishway type adopted for the first five weirs/locks involved a single vertical-slot. These weirs hold pools averaging 3.1m head drop and are between 86 and 169m wide. Each is founded on concrete at river bed level and comprise of three main parts; sluiceway, navigable pass and navigation lock.

The ecological and hydraulic design criteria for the vertical-slot fishway recommended for each of these sites led to:

• A stilling pool size of 3m x 2m (to dissipate turbulence, aid maximum size fish, avoid impairing schooling fish and aid high biomass).

• Fishways of approximately 1:32 gradient between stilling pools (to accommodate a high biomass and wide range of fish sizes in a single fishway).

• A minimum depth of 1m (to aid maximum size fish, avoid impairing medium sized fish and aid high biomass).

• A slot width of 300mm.

It was expected that these fishways would pass a high biomass of native fish, from 70-90mm up to 1m in length.

The five barrages near the mouth of the Murray are long, low sluiceway structures, designed to accommodate low head differences. They were built to provide both regulated storage and a barrier to sea water inflows. The barrages total 7.6 km in length and are separated by a series of islands.

The fish ecology of this part of the Murray is complex, reflecting a complex estuarine-freshwater environment and the fish species differ from those found upstream. To date, four fishways have been built. A vertical slot fishway comprising a set of steel slotted baffles, constructed in modules to occupy a complete sluice bay and provide either part or full depth fish passage. A rock ramp has been installed where aggregations of small fish have been observed, followed by an adjacent small-fish vertical slot. A large-fish vertical slot fishway was installed in the middle of the largest barrage, again responding to past aggregations.

During testing and monitoring of the first fishways, it was discovered that small fish were attempting to migrate upstream in large numbers but couldn’t because they were below the target size. The revised fish passage design for locks 2-6 was a vertical-slot fishway with steeper 1:18 gradient and smaller footprint (to accommodate medium and larger fish), and an embedded fish lock (to accommodate small fish).

Euston Weir was built in the 1930s with a fishway, designed to the American salmon model. The steep (1:9 – 1:11.5) concrete chute was originally fitted with submerged orifice baffles which only large native fish could negotiate. With no weir immediately downstream the fishway is required to operate over a 5m height range. The existing chute made it economically attractive to retrofit fibreglass Denil inserts as a trial of a Denil fishway. The trial was successful with fish down to around 120-150 mm in size consistently negotiating the fishway.

For the significant populations of smaller native fish a fish lock has also been constructed. Its shorter, wider design enables it to be located securely in the navigable pass adjacent to the lock wall, where small fish aggregate.

The success of the Euston Weir Denil trials led to the construction of a similar fish passage at Mildura Weir.

Designing and constructing fishways

Each site had distinctive differences to challenge the designers. With no examples of a combined vertical-slot and fish lock in Australia, the operational elements of each were combined, with guidance from the Fish Passage Task Force, to produce the design. Flexibility was incorporated in the design where cost effective. An example was the downstream concrete entrance slots which were made wider and fitted with adjustable metal plates for fine tuning their operation.

Before construction began Aboriginal, heritage and environmental representatives were consulted to ensure that cultural and heritage places of significance would not be impacted.

Retrofitting any substantial structure to a 70 to 80 year old weir had risks. The work inevitably meant cutting into the structure to some degree. Care was taken during design to minimise this, and to analyse the possible effects under all river conditions. In addition, designs and construction methods were reviewed in order to minimise their impacts on the weirs.

Measuring success

A tri-state (New South Wales, Victoria and South Australia) monitoring team was formed to look at four key questions:

• To what extent has the Sea to Hume fishway construction programme contributed to positive changes in the abundance and diversity of native fish in the River Murray?

• Is the fishway allowing passage of the full range of fish size classes and species specified?

• Is the fishway reducing accumulations of fish downstream of the barrier?

• What lessons can be learned about optimising the location, design and operation of the fishway?

An early recommendation from the team, that passive integrated transponder (PIT) tag reader loops be installed in each new fishway, was implemented. Thousands of fish in the Murray have now been tagged, providing a simple and powerful tool for their ongoing tracking. Electro-fishing, radio tracking and sonar have also been used to establish the effectiveness of the individual fishways.

Initial monitoring has shown that in 40 days more than 50,000 fish used the fishways, comprising 13 species and a wide size-range (31mm to 1,040mm long). As the programme nears completion it is estimated that over one million fish migrating in the River Murray each year will use at least one of the new fishways.

With the completion of each fishway, its operation has been assessed on-site by the monitoring team with the designer. Detailed hydraulic measurements (such as headloss, velocity, flow distribution) have proved particularly valuable in fine-tuning the fishway’s performance, and providing information to improve subsequent designs.

The Sea to Hume Fishway Program spans more than a decade and is being implemented across three states. Such a program has required the commitment of consultants, contractors and staff from a wide range of government agencies. When completed, fishways costing A$55M will have been constructed at weirs and barrages currently valued at A$708M, that is 7.2% of the asset value. This was considered by the asset owner to be an acceptable cost to provide best practice fish passage.

It will be many years before the long term impact of the fishways on native fish numbers is known. Nonetheless, there is strong evidence that the fishways have already been highly successful.

“The most significant change has been the transformation of the River Murray from a series of isolated weir pools into a fully connected river where fish can migrate. And not just one or two species — it’s been designed for all native fish species between 20mm and 1m. That’s where we’re setting a worldwide standard,” says Dr Lee Baumgartner.

This article is based on a paper by John Prentice (Senior Assets Engineer, Murray-Darling Basin Authority), Jim Barrett (previously Director Native Fish Strategy, Murray-Darling Basin Authority) and Dr Martin Mallen-Cooper (Fishway Consulting Services, Sydney)