Blog | 12th Jan, 2011

A lack of environmental water during the dry years.

Dead metre-long Murray Cod are floating down the Murray River, their deaths caused by a lack of dissolved oxygen in the water.

Over recent years, blackwater events have not been uncommon in Victoria’s rivers. Since December, the Loddon, Goulburn-Broken and Murray rivers systems have all experienced oxygen-poor flows and wildlife deaths.

The incidence of blackwater increases in spring and summer as seasonal flood events wash built-up organic material – leaves, twigs, bark – into waterways. There this matter decomposes, leeching excess carbon into the water.

This makes it hard for aquatic life – including native fish, shrimps and yabbies – to breathe. Crayfish, usually dormant in burrows at this time of the year, have even been sighted climbing river banks gasping for oxygen.

So what might have prevented extent and severity of these problems?

Because it has been so dry, for so long, organic matter has built up for years and years.

More environmental flows during the drought period would have flushed organic matter through river systems more frequently, preventing a build up.

Fewer leaves and twigs to leach less carbon means more dissolved oxygen in the water for aquatic critters to breathe.

In December, the Murray-Darling Basin Authority posted an update on the blackwater events and fish kills being experienced across the basin. They wrote that the build up of leaf litter over the drought years led to the current blackwater events.

Just last week, as the blackwater and fish kills made their way along the Murray to Swan Hill, the MDBA again cited flooding after years of drought, as their cause.

The Environmental Farmers Network say that “had environmental flows been available to recreate even a minor flood during the past drought, the most extreme effects of the blackwater events would have been reduced.”

And a recreational fisher website reports that opportunities to mitigate blackwater and fish deaths through environmental or dilution flows will be investigated and implemented.

Clearly more could have been done during the drought years to support these riverine ecosystems.

Now, the MDBA Plan is our best chance to put measures and plans in place to restore these rivers to health, and prevent excessive blackwater events and fish kills in the future.

As shown in the ACF’s submission to the Guide to the Plan, a return of 3,000GL per year to the environment will mean almost all of Victoria’s rivers will still be in a poor to moderate condition. Without adequate environmental water – especially during drought periods – we can expect more of the same blackwater events and fish kills.

These events show that a robust Murray-Darling Plan is needed: a Plan that recommends a return of enough water to rivers (not less than 4,000GL) so that proper environmental watering is carried out in dry periods.

Image credit: Mallee Catchment Management Authority