Fish killing blackwater event surfaces along a big stretch of the Murray River

There have been reports of fish kills and Murray crayfish leaving the river from Echuca downstream to Swan Hill.

Blackwater floods are said to be natural events which often happen in flood time which removes oxygen from the water.

Murray-Darling Basin Authority senior director, environmental management, Dr Janet Pritchard said widespread flooding in the Murray-Darling Basin would see water extending to some floodplains which have not had water on them for several years.

"Hypoxic blackwater events occur as leaf litter and other carbon-based debris is swept into waterways.

"As this material is consumed and broken down by bacteria, oxygen can be sucked out of the water, making it difficult for fish and other aquatic animals to survive," Dr Pritchard said.

There is very little river authorities can do to manage these blackwater events which seem to be happening more frequently, after also happening in 2016 and 2011.

Authorities are also expected blue-green algae outbreaks may happen when the weather warms up.

In Swan Hill, fisheries officers have been collecting the weak Murray crays from river banks and relocating them to holding tanks in Narrandera.

Commonwealth Environmental Water Holder Simon Banks said state and federal environmental water managers were looking to divert small volumes of good-quality water from the main river channel, via irrigation channels, and then diverting it back into creeks further downstream where there was poor water quality.

"These small flows can create localised pockets of better water quality for fish and crayfish to move into. This worked well last year and is being used again in the Edward Kolety Wakool region, where natural flooding has already resulted in low-oxygen blackwater," Dr Banks said.

"There are already isolated reports of crayfish exiting floodwaters to escape poor water quality," Dr Pritchard said.

"With the onset of warmer weather in the coming weeks, water temperatures will start to climb and the possibility of hypoxic blackwater will increase and unfortunately, we could see fish deaths like we did during the 2011 and 2016 floods.

MORE READING: No forecast end to the big wet just yet.

"In the past 20 years, large-scale hypoxic blackwater events have been associated with late spring or summer floods, particularly in the Murray, Edward-Wakool, Lower Darling and Murrumbidgee rivers.

"This season the risk will apply to several other systems across the Basin in New South Wales (NSW), Victoria and South Australia as well.

"The high flows and widespread flooding will also add nutrients to waterways, which will favour the growth of blue-green algae with warmer conditions. As floodwaters recede back to normal levels, we will also be on the look-out for elevated salinity levels in some locations."

The current watchpoints for potential blackwater events in NSW include - Barwon River between Walgett and Brewarrina, Murrumbidgee River at Balranald, Wakool River, Murray River, Murray River, Little Merran Creek, Thule Creek, Barber Creek.

Dissolved oxygen levels in these areas had been steadily declining over recent weeks.

Meanwhile, the main water storage for the Wimmera-Mallee pipeline is full for the first time in 26 years.

Lake Bellfield in the Grampians has not spilled since November 1996.

Lake Bellfield, which holds about 80,000 megalitres, has received more than 7300ML of inflow over the past two weeks.

As of this morning, the spillage, which has been triggered by the inflows, is about 160 ML/d.

MORE READING: Silver lining in the west from state's floods.

Lake Bellfield is located in the eastern Grampians near Halls Gap and also supplies water to Halls Gap and Pomonal.

GMWater's biggest storage, Rocklands Reservoir, is also rising fast and is about half full. Rocklands holds about 296,000ML when full.

As a result of the recent rains, reservoir levels have risen steadily, with Grampians catchments receiving almost 57,000 ML of inflow (equivalent to approximately 10 per cent of total capacity) over the past fortnight.