Solving an erosion problem – the fate of Trapper’s Dam

Thursday 20 March, 2008

A success story from Carnarvon Station Reserve Manager Darren Larcombe.

Erosion gully caused by Trapper’s Dam spillway. Soil from the dam wall is used to fill the gully. Bush Heritage ecologist Murray Haseler casts the first handful of native grass seed. Seed harvester cutting seed from the reserve. Photo Darren Larcombe and Glen Norris.

Erosion gully caused by Trapper’s Dam spillway. Photo Darren Larcombe and Glen Norris. 

It's usually years, or even decades, before the results of conservation work can be seen in real improvements on the ground.

As reserve managers, we're used to being patient and watching and monitoring the land for years before we see the fruits of our labours. So achieving a major conservation goal at Carnarvon Station Reserve in the space of a few weeks was really inspiring.

We resolved a major soil erosion problem and restored the natural flow of water from Gorge Gully to the Channin Creek.

Carnarvon Station Reserve in central Queensland was purchased by Bush Heritage and its supporters in 2001. This 59 000 hectare reserve protects endangered bluegrass grasslands and formerly unreserved poplar box woodlands.

The reserve management plan identified soil erosion caused by historical pastoral activities, a poorly constructed spillway on the main dam, and siting of roads, as one of the key threats to these ecosystems.

We began managing soil erosion as soon as we took possession of the reserve. Removing the cattle significantly reduced the grazing pressure, which had been a major contributor to the problem.

Soil from the dam wall is used to fill the gully. Photo Darren Larcombe and Glen Norris.

Soil from the dam wall is used to fill the gully. Photo Darren Larcombe and Glen Norris.

Trapper’s Dam in Gorge Gully was a significant problem. Built during the mid-1980s as a stock watering point, it not only held a large volume of water that rightly belonged in the Channin Creek system but also had an incomplete spillway that had caused any overflow of water to cut an erosion scar 22 metres wide along the drainage line. The scar stretched for 280 metres and ranged in depth from two to ten metres; it was a very big hole!

As a conservative estimate, that hole equated to the loss of about 37 000 m3 of soil. With no remedial action, this landscape with its endangered vegetation community would continue to deteriorate as the erosion proceeded to undermine the adjacent alluvial plain.

Our goal was to prevent further soil loss and halt the destabilisation of the fringing areas.

Planning for the rehabilitation of the site began in 2005. We commissioned an environmental engineer from the University of Melbourne to do a site survey, and sought the advice of local earth-moving contractors. Getting advice about removing the dam proved difficult. We kept getting the same response:

‘You want to do what?!!’

Far more seemed to be known about building dams than about removing them.

A number of options were explored but the best and permanent solution involved breaching the dam wall and returning the water to the creek. That involved moving approximately 100 000 tonnes of earth.

Giant water-holding frog. Photo Sandy Walters.

Giant water-holding frog. Photo Sandy Walters. 

The Queensland Government helped with funding from the Biodiversity Incentives Tender, and the Southwest Natural Resource Management contributed through the Nature Assist Program.

Expert staff and volunteers were recruited and a D8 bulldozer was booked for the month of October 2007.

The dam had dried out due to the prolonged drought and we hoped that the parched conditions would continue for just a bit longer. Our National Operations Officer Glen Norris used his dozer skills to remove a 240 metre section of dam wall. He used the soil to fill and repair the eroded spillway and construct new contour banks.

Expert volunteer John Hansen then took over and used the remaining soil to reshape the surrounding country. Using the tractor, the reserve staff then cut furrows into the contour banks to prevent further erosion and in preparation for sowing.

Bush Heritage ecologist Murray Haseler casts the first handful of native grass seed. Photo Darren Larcombe and Glen Norris.

Bush Heritage ecologist Murray Haseler casts the first handful of native grass seed. Photo Darren Larcombe and Glen Norris.

We spread native grass seed harvested earlier in the season from other areas of the property.

The recent heavy rains were the ultimate test of the success of the project. A torrential 900 mm of rain has fallen on the site in the three months since the work was completed.

The old dam site now channels water into Channin Creek, which forms the headwaters of the Warrego River, a major feeder system for the Murray–Darling catchment. Our newly formed and vegetated contour banks held up under the strain and have significantly lessened the silt load flowing into an already stressed river system.

This has been the first soil conservation project of this type undertaken by Bush Heritage on any of its reserves. It required 620 hours of staff and volunteer time.

We extend special thanks to Rob Argent for his technical advice, to Glen Norris for his work on the dozer and for imparting his practical knowledge of earthworks, and to John Hansen for his extraordinary skills in earth-moving.

Our new native grassland on the old dam site. Photo Darren Larcombe.

Our new native grassland on the old dam site. Photo Darren Larcombe. 

Peter and Margaret Calder did a wonderful job collecting the grass seed required for the revegetation work, using the grass seed harvester generously donated by the Andyinc Foundation.

Finally, ‘thank you’ to our team of ‘chook-feeders’ for spending a hot afternoon spreading seed. Without your efforts, the contour banks would have washed away in the first heavy rains.

Now we have a newly constructed native grassland that's green and lush and doing its job of holding the soil and helping to further stabilise this once-degraded area.

Rain pays dividends

Following the recent rain, the Carnarvon grasslands are producing a bumper crop of native seed and teeming with invertebrates.

As we set out to do the next monitoring round we wondered whether this might produce some interesting animals. It did!

So far, our pitfall traps have revealed two new species for the reserve, the pale field rat and the giant water-holding frog. It’s great to see these species making a comeback.

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