We continue to progress the rehabilitation of Lagoon of Islands in Tasmania’s central highlands.

Rehabilitating Lagoon of Islands

We continue to progress the rehabilitation of Lagoon of Islands in Tasmania’s central highlands.

This unique wetland ecosystem was characterised by ‘islands’ of vegetation that floated on a mat of reeds until the natural system died after it was flooded in 1964 to deliver irrigation water to property owners along the Ouse River.

The demand for water increased rapidly and in 1984 Ripple Creek Canal was constructed to divert some small tributaries of the Shannon River into the lagoon. The canal substantially increased the water yield, but also contributed nutrient-rich water to the lagoon. Ultimately, nutrients from the decay of the reed mat and from Ripple Canal resulted in algal blooms and unacceptable water quality for downstream users.

We explored and implemented remedial actions to improve water quality and ecosystem health but none provided a long-term solution to the significant and ongoing problems with water quality.

During the 2006-8 drought, low inflows (rainfall) combined with existing poor environmental conditions at Lagoon of Islands prevented any water releases for irrigation purposes, and irrigation demand was instead met by water releases from yingina / Great Lake.

In response to the poor health of the Lagoon of Islands ecosystem, we established an ambitious project to decommission the dam and rehabilitate the lagoon to a natural, healthy and self-sustaining state. Ripple Creek Canal was decommissioned in May 2010. The four tributaries artificially flowing into the canal were redirected back to their natural course along the Shannon River. In 2013, the 320-metre-long, six-metre-high earth-wall dam and associated infrastructure were removed and the dam footprint was replanted with native vegetation.

 

A panel of scientific experts is advising on the rehabilitation, which involves active revegetation of the footprint of the dam and natural revegetation of the lagoon area. Seed that has lain dormant in sediment is being used to support revegetation.

Monitoring of water quality, vegetation, weeds and algae is conducted to track the progress of the lagoon’s recovery.

Water quality continues to improve as nutrient levels and turbidity reduce. Revegetation of the dam site and the lagoon bed are progressing beyond expectations.

In contrast to descriptions of the lagoon hydrology before damming, the lagoon bed dried in the summers following decommissioning. This may ultimately lead to a different type of wetland than existed prior to impoundment. In May 2016, the outflow was reconfigured to reduce the likelihood of the lagoon bed drying each summer.

Rehabilitation of Lagoon of Islands is a long-term undertaking. However, we have already seen significant improvements and we are confident that the lagoon is on the path to recovery. The site is closed to vehicles to protect the fragile vegetation that is recolonising the site.