Broken River benefits flow from Mokoan decommissioning

The Broken River is less turbid (cloudy) and nutrient levels have dropped downstream of Casey’s Weir since Lake Mokoan was decommissioned in 2009, a recent report by The Murray-Darling Freshwater Research Centre (MDFRC) for the Goulburn Broken Catchment Management Authority (CMA) found.

Goulburn Broken CMA Strategic River Health Manager Simon Casanelia said before Lake Mokoan was decommissioned water from the lake was discharged into the Broken River below Casey’s Weir.

"This meant the river was more turbid downstream of Casey’s Weir,” Mr Casanelia said.

“Turbidity is caused by suspended material in the water, which means less light can pass through it, limiting plant growth. This in turn affects the fish and waterbugs that feed on the plants, and leads to lower levels of dissolved oxygen in the water.”

Discharge from Lake Mokoan had also resulted in high nitrogen and phosphorus levels downstream of Casey’s Weir . High levels of these nutrients cause algal blooms and lower levels of dissolved oxygen.

“The MDFRC researchers found that decommissioning of Lake Mokoan had significantly reduced turbidity and total nutrient levels in the Broken River, below Casey’s Weir,” Mr Casanelia said.

“This is a great outcome as it not only improves the ‘look’ of the Broken River, it means water quality has improved, supporting more aquatic plants and waterbugs that provide valuable habitat and food for native fish.”

Researchers also found that populations of spawning Murray cod above Casey’s Weir had increased.

“This is believed to be a result of a mixture of activities taken along the Broken River in recent years,” Mr Casanelia said.

“Previously flows above Casey’s Weir had been diverted into Lake Mokoan, meaning that flows were often very low, which was not ideal for native fish spawning.  

“Installation of a fish ladder at Casey’s Weir, allowing better fish movement along the Broken, fish stocking programs and the fact that flows are no longer being diverted into the lake since decommissioning have seen a marked improvement in the numbers of spawning and large bodied native fish in this section of the river.”

For more information, or to read the report The decommissioning of Lake Mokoan: Effects on water quality and fishes of the Broken River visit www.gbcma.vic.gov.au