Thomson River Fishway Questions

This page answers many of the questions that have been asked over the life of this project so far.

The project has been independently assessed, has received all the necessary permits and approvals and is supported by DELWP and Parks Victoria.


Why is the Thomson River system so significant?

The Thomson River is one of the region and state’s most significant and ecologically important rivers, and the creation of a fishway to allow passage between the Gippsland Lakes to the Victorian alpine region is a state priority.

A connected river is a healthy river. The Thomson River re-connects the Gippsland Lakes to Victoria’s Alpine region, from Lake Wellington to the Aberfeldy River. The river has excellent water quality and river habitat, particularly in the upper reaches. An Environmental Water Reserve (EWR) provides the opportunity to reach environmental objectives, such as fish spawning, through managed releases.

The Thomson River above Cowwarr Weir and the Aberfeldy River within the Baw Baw National Park are both listed with heritage status in accordance with the Heritage Rivers Act 1992.

What are the ecological benefits to these two Rivers?

Waterway connectivity is critical for fish and aquatic life because it:

  • allows a species to thrive and succeed by providing genetic diversity, allowing fish populations to interbreed
  • provides greater access to food sources and habitat
  • reduces the risk of predator attack by providing more places to live
  • helps a species to return to an area after it has been through disaster (e.g. fire)
  • allows the fish to go through all the critical life cycle stages from spawning, maturing to breeding

The Thomson River is home to eight native fish species that migrate over their life cycle. Fish studies have shown that of these migratory species, only eels are able to populate the river upstream of Horseshoe Bend.

How will native fish benefit from the fishway at Horseshoe Bend?

The fishway unlocks vast reaches of the upper Thomson and Aberfeldy rivers to native fish.

The fishway allows fish to swim around Horseshoe Bend, benefiting seven species of native migratory fish and reconnecting Victoria’s Alpine Region with the Gippsland Lakes.

The idea of restoring the river flow at Horseshoe Bend was originally discussed in the 1990s. The final design approach was established in 2010 and reflects a new line of thinking, in response to community concerns.

The fishway at Horseshoe Bend enables fish to access an additional 22kms of the Thomson. It is a regular misconception that there was no point creating the fishway, as once the fish reach the Thomson Dam they will be blocked. However, restoring the flow at Horseshoe Bend also gives fish access to 64km of the Aberfeldy River, which is suitable habitat for vulnerable species such as the Australian Grayling.

Concerns have been raised that creating the fishway will allow carp to flourish. While carp can swim long distances, they generally only colonise in suitable water conditions. Carp are most abundant in still or slow flowing water, which is why they are virtually absent from the Thomson River above Cowwar Weir and pose no threat to the upper reaches.

Why a fishway?

The Australian grayling is classed as ‘vulnerable’ in ecological terms as stocks are dwindling. The fishway allows these fish to migrate upstream as part of their breeding cycle and help to halt the species decline.

There is conclusive evidence that the Horseshoe Bend tunnel stopped the passage of migratory native fish (e.g. Australian grayling, tupong, common galaxias) to the upper reaches of the Thomson and Aberfeldy rivers. There is also very strong evidence that these species will repopulate the upper reaches now they have been given access.

Here you can download the 2010 migratory fish study for more information.

What does the fishway look like?

Extensive consultation was carried out to ensure the fishway balances the significant recreational, heritage and cultural values of the site with environmental improvement objectives.

The fishway was created in the existing riverbed and does not consist of concrete structures. A range of options were considered before settling on the current fishway design. The fishway is 230 metres long and ranging in depth from a maximum of 2.7m deep upstream down to no excavation downstream. The average depth is 1.4m.

The fishway has an asymmetrical shape with a slightly receding slope (or batter) on one side and a series of four benches )or steps) on the other. As river levels increase due to increased flows these benches provide areas of shallow and slow moving water for fish to swim along.  This cross-section of the fishway design provides an illustrated explanation of the 7m design and battering.

In context, the valley floor at Horseshoe Bend varies from 20m to 50m wide.

Throughout the consultation period we have explored every option to minimise the size of the fishway. The result of this work is a low flow and a visually low impact fishway that will present itself as a meandering waterway – see the pictures on our website for more information.

Will the fishway work?

Advice on the fishway design has been provided by Arthur Rylah Institute, migratory fish experts from the private sector and confirmed by an independent Technical Assessment Panel. Over the last 20 years, Victoria has been recognised as world leaders in understanding fish passage and fishway construction. We are very confident that this design will be equally successful.

Annual fish monitoring in the Thomson River has been done since 2005 and will continue. It is expected to take 2-3 years for migratory fish species to repopulate the upper reaches and show up in monitoring data.

Why not a different fishway option?

Stakeholder engagement at the start of the project worked through a number of options including use of the use of environmental water, pipe and pool arrangement, a cofferdam at the tunnel entrance and a fishway. The fishway option was found to be the most appropriate based on the criteria of all stakeholders.

A community proposal to cut a small channel through a rockbar was the genesis of the current fishway design. The small channel option was abandoned because it would not have provided adequate flow conditions for fish passage. A larger channel was required but would have resulted in the excavation of the big natural rock bar, an option what was not supported by the community. This option was modified into the current design to use the natural features of the site to minimise the amount of excavation, provide minimal visual and structural impact and be a fully functional fishway.

Why is the fishway so big?

People have asked why the fishway has to be so large (up to 7 metres wide and 2.7 metres deep) if migratory fish like the endangered Australian grayling are so small.

To make it easier for these small fish to migrate to the upper reaches of the Thomson, the fishway needs to:

  • be deep enough to enable the water flow down into the old riverbed
  • be big enough to let enough water pass over the old riverbed so that fish can swim up it and enough water to flow pass the tunnel exit so fish can successfully navigate past the turbulent tunnel outlet flow
  • be wide enough to ensure the water flow through the fishway isn’t too fast for small fish to swim through.

The fishway is actually as small as it can be in order to create the right conditions for fish passage. The design and dimensions are based on the natural features of the river and ensuring there is enough water flowing from upstream to enable fish to swim past the outflows from the tunnel outlet.

As part of the design process, a smaller fishway option was investigated, but it didn’t meet the requirements for fish passage, and this was verified by the Independent Technical Advice panel.

Every fishway is slightly different but this is a truly unique design. Typically, fishways are constructed to overcome barriers such as weirs and dams. They are concrete structures consisting of small vertical slots for fish to pass through (eg. the fishway at Cowwarr Weir). The unique fish passage requirements listed above and the environment it is in (natural rugged mountainous waterway, heritage values) mean the Thomson River Fishway is unlike any other.

Does water still flow through the tunnel?

More than 60% of the river flow still passes through the tunnel, maintaining its visual and historical significance, while also allowing fish to swim freely upstream.

Based on consultation with the community and fish ecologists through the design phase, it was found that a minimum flow split of 60:40 between the tunnel and the fishway respectively was required.

After construction of the fishway, flows measurements at very low flows (135 ML/d) and high flows (600 ML/d) showed the split to be 66:34 between the tunnel and fishway respectively.

How have the heritage values of the tunnel been protected?

An important stage of this project is the Heritage Victoria application. In June 2018 Heritage Victoria granted a permit for the project, representing an endorsement of the project design and the balance we have struck between environmental and heritage values.

There have been no works done at or immediately near the tunnel. The fishway design was developed to ensure the heritage values of the tunnel, including the river flow through it and the surrounding area were maintained.

During construction, vibration monitoring and visual inspections of the tunnel were continuously undertaken to ensure not adverse impacts occurred. Exclusion zones were also marked out on site to protect surrounding historic mine sites. Works were overseen by an independent archaeologist and Heritage Victoria and were conducted in full accordance with the Heritage Permit and the Heritage Management Plan.

The design and construction controls for the fishway have preserved all the heritage values of the site for future generations.

What did the project cost?

Prior to construction, there was a rumour that the project would cost $20 million, however this was false and unfounded.

Conventional concrete fishways at barriers such as weirs and dams cost anywhere between $1M-$10M to build. The Thomson River Fishway, including the construction of a 2km walking track and visitor facilities, cost $1.06M.

Is the walking track to the inlet and outlet of the tunnel be accessible all year round?

Access to the tunnel outlet is available all year round except during times of flood.

The walk to the tunnel inlet includes two river crossings (stepping-stones). During high river flows these crossings will become inundated and access unavailable. A section of the walking track has been built to inform walkers of high river levels. This track section includes a warning sign and is designed to take on water so walkers do not try to cross the river further up the track.

Where did the excavated rock go?

Based on the final detailed design, it was estimated up to 3500m3 of rock may need to be relocated. The actual volume of rock relocated during construction was significant less than this – approximately 2200m3. The difference being due to the conservative over estimates in the design phase, and the use of natural features within the river bed to reduce the volume of excavation required.

The rock to construct the fishway is still onsite. Options to remove the rock off-site were explored however the impact to the area would have been significant. It would have required extensive widening of the access track, particularly at the sharp bends, to allow haulage trucks to access and leave the site.

Instead, the excavated rock has been located near the northern section of the walking track. It has been blended in with the natural features of the inner bend in the river and provides a stable base for that section of the walking track. Natural processes will also ensure that it will regenerate over time. Based on the natural regeneration in other project areas, we estimate this will take approximately five years.

Why was the track widened and what about tree removal?

In order to get machinery in to the site, there needed to be some impacts to the surrounding area. These were kept to an absolute minimum and full rehabilitation of the site post construction is a priority.

The access track was temporarily widened to a maximum of four metres, and then narrowed back to two metres at the end of the project. Vegetation disturbance was conducted under an approved planning permit, and flora and fauna permit. Permanent native vegetation offsets were secured to offset the temporary loss of vegetation due to the works.

A three year site rehabilitation and revegetation agreement has been signed between the WGCMA and land managers DELWP and Parks Victoria.

Our staff have previously conducted photo point monitoring over a two year period to watch how the bush at this site regenerates and changes. You can view some of the photos taken from this monitoring. We expect the area to fully regenerate within the next 2-5 years.

What if the fishway needs maintenance or repair?

We are committed to ensuring the fishway is a success and will be undertaking remediation work in the unlikely event it is negatively impacted during floods. When repair is needed it will be done by hand or using small scale equipment.

Will the fishway fill with sediment?

During times of high flows rivers become more active and sediment is transported downstream. In a flood, the speed of the flowing water picks up sediments and when water movement slows, sediments deposit again. In the upper Thomson River, sediments typically move from one bend in the river to the next; in other words, sediment deposits chase each other downstream.

We expect some sediment to be deposited on the fishway, but it has been designed to mimic the natural system. During high flow events sediment deposits will be picked up and transported downstream just as they do in the rest of the river system.  If large flooding does impact the fishway, we will complete maintenance by hand or using small scale equipment.

Has the "heritage river" been be damaged by this work?

The Thomson River has heritage status under the Heritage Rivers Act. The vast array of environmental and heritage issues associated with the fishway were worked through during the planning phase of the project.

The act identifies “Heritage Rivers Areas” and “Natural Catchment Areas” and outlines activities that are not permitted within heritage river areas (section 10). The Thomson River at Horsehoe Bend is included in the Thomson River Heritage Area. The activites associated, nor the structures associated with the fishway breach any conditions listed in the Act.

What about the EBPC Act?

Section 18(4) of the act says that “a person must not take an action that has, will, or is likely to have a significant impact on a listed threatened species.” As part of the project, we sought legal advice and advice from the Department of Environment and Energy (the statutory authority for EPBC) was provided.

As the construction of a fishway at Horseshoe Bend will have a positive impact on EPBC listed species such as Australian Grayling by increasing the amount of habitat available, and there are no other apparent risks to EPBC species, a referral is not required.

Did you consider the possibility of mercury becoming mobile when the fishway was created?

Yes – we did. Independent testing and risk assessment of mercury at the site was undertaken during the planning and construction phases of the project. These tests found that mercury levels detected were well below the human or environmental health risk threshold. In reality the levels were consistent with natural background levels, meaning no added risk of gold mining related mercury deposits were found.