The flooding of Raakajlim Creek

Okay, I’ve finished sulking now and I’m ready to tell the story of the November/December 2022 flooding of Raakajlim Creek and what it might mean for our conservation property…

Chalka Creek and the Dry Lakes of Hattah Kulkyne National Park

The Hattah Lakes within Hattah Kulkyne National Park are watered semi-regularly from Chalka Creek, an anabranch of the Murray River. Environmental watering keeps these southern lakes topped up. But the north end is a different story. Lake Boolca and the Dry Lakes haven’t had a drink since the 1974/75 flood and for some areas, not since 1956! Water can flow into the Dry Lakes from the north, along Raakajlim Creek, or from the south, from Lake Bitterang.

The Bitterang levee on Eagles Nest Track was constructed to prevent environmental water from flowing to these northern Lakes from the southern Lakes system so the extent and duration of watering could be controlled. A proposal is to extend environmental watering north to Lake Boolca and beyond by installing a regulator in the Bitterang levee. The proposed watering includes a small section (about 40 hectares) of the Raakajlim conservation property. But what about flows along Raakajlim Creek from the Murray River?

Levees on Raakajlim Creek

When we purchased the property back in 2002, there were five levees that prevented the inundation of Raakajlim Creek during high flows of the Murray River. An investigation by Ecological Associates back in 2007 indicated that Raakajlim Creek would normally begin to receive water when the Murray River reached 42.5m AHD at Colignan. However, the big, boomerang-shaped levee at the east end of the Creek was constructed to a height of 44.1m AHD. This meant that really high flows in the Murray River were needed to flood the Raakajlim Creek (160-180,000 ML/d). The high rivers of 1993 and 2016 did not get past the boomerang levee.

The Raakajlim conservation property (north of Goosefoot Track). The darker vegetation is the Blackbox along the Raakajlim Creek. The red lines show the locations of levees in the Creek.

Why were the levees built? The previous owner of the land, Len Oatey, was a road builder and a true bush engineer. He also grazed sheep on Raakajlim and it was inconvenient to have the Creek and the shearing shed flooded on a regular basis. We think that the levees were constructed after the 1974 floods to prevent flooding in the 1975 floods.

Historical aerial photographs from 1948, 1953 and 1964 show no levees, but some levees appear to be in photographs from September 1975. All the levees are there by 1983. Incidentally, the 1975 photograph shows water lying in Raakajlim Creek, Lake Boolca and the Dry Lakes, indicating these areas were flooded in the 1974 flood. This was the last time Raakajlim Creek flooded.

Levees are generally not a great thing on floodplains because they prevent some areas from being flooded. Water can also get trapped behind levees and holding water on the floodplain for too long can be just as damaging to native vegetation. One of our first jobs after the property purchase was to breach two of the levees on Raakajlim Creek. We couldn’t do much about the boomerang-shaped levee though, because it is not on our property. Water was lapping near the crest of this levee in the 1993 and 2016 floods. We also left the levee in the west of the Creek because in 2007 there was some talk of using it to contain environmental water.

What happened in the flood of 2022?

Long story short, the Murray River at Colignan peaked at 44.1m AHD (8.96m at the gauge) and the Raakajlim Creek flooded all the way out to the Dry Lakes. The flood was higher than the 1974/75 flood, and just a little less than 1956.

On the 21st of November 2022, the boomerang-shaped levee failed and water flowed from east to west along the creek. Within 24 hours the Raakajlim Creek was full all the way to the western levee.
Around the 1st of December, floodwater overtopped the Bitterang levee in the south and started moving north towards Lake Boolca, Raakajlim and the Dry Lakes.
The floodwaters peaked in the river around the 13th of December, but floodwater kept moving northwards as the water held in the southern Hattah Lakes spread north across the floodplain.
On the 15th of December, water from the south arrived on Raakajlim. We breached the western levee and water flowed all the way out to the Dry Lakes, arriving around the 19th of December. Water came from both Raakajlim Creek in the north and from the south over the Bitterang levee.
As of 1st January 2023, floodwaters are now receding slowly. The Raakajlim Creek is now flowing west to east as the water drains off the floodplain, now that there are no levees to block the flow.

The satellite images below are from Sentinel Playground but you can also get a good idea of the flooding extent from the Loddon Avoca Mid-Murray flood modelling (turn on the 1% Annual Exceedance Probability layer).

4th December 2022 satellite imagery showing water in Raakajlim Creek (top right) all the way to the western levee. Water is just starting to creep northwards after overtopping the Bitterang levee (bottom right).

29th December 2022 satellite imagery showing the maximum extent of the 2022 flood, all the way out to the Dry Lakes (top left). Water came from the north along Raakajlim Creek, and from the south, over the Bitterang levee.

Flooding is a good thing, right? Not always.

There’s nothing natural about the way the Murray River currently operates since the construction of dams, weirs and locks from the mid-1930s onwards. The regulation of the Murray River was undertaken largely to ensure a consistent supply of water for irrigation and urban use. The locks are also important for navigation. The result of river regulation is a reduction in the frequency of flooding from overbank flows. Environmental watering seeks to return water to some of our floodplains using pumps, levees and regulators.

But in the absence of flooding (50 years in our case), the landscape has become terrestrialised to some extent. Plants had established in the Raakajlim Creek that have now been killed by flooding, like native Pine, Hopbush and pretty much all of the Saltbush understory. Over the past few weeks, I’ve been sad to see these plants die. Some of the Pine trees were 15 years old.

Flooding in the Raakajlim Creek. Whilst the Blackbox have a flush of green leaves, all the Hopbush has turned yellow, killed by the floodwater.

Flooding and the Arid Bronze Azure butterfly

But really, I’m sulking because of our special Arid Bronze Azure butterfly. This is another species that appear to have colonised the floodplain in the 50 years since the last flood. You might be wondering why we didn’t immediately breach the western levee and let water flow along Raakajlim Creek all the way to the Dry Lakes. We actually sand-bagged the levee, an ultimately unsuccessful attempt to protect a dense patch of nesting sites used by the butterfly. We only breached the levee when it became clear that water would reach the levee from the south as well. Then it became critical to remove the levee so that receding floodwaters can drain freely off the floodplain.

The western levee on Raakajlim Creek holding back water from the north (left). We sandbagged it in an unsuccessful attempt to protect the habitat of the Arid Bronze Azure butterfly. Floodwater eventually came from the south (right) as well.

The Arid Bronze Azure butterfly lays its eggs at ground level on a tree or shrub at the entrance to a Sugar Ant nest. They seem to particularly like Blackbox trees (Eucalyptus largiflorens) that grow on the higher parts of the floodplain. There are around 21 locations where butterfly eggs have been recorded in Hattah-Kulkyne National Park and on Raakajlim (R8 2022b), and sixteen of these locations have been partly or fully inundated. Sadly, this flood occurred at a time of year when there were no butterflies above ground. All the eggs, larvae and pupae that were underground in the flooded areas have been drowned.

As the flood waters rose, I spent some time watching how the Sugar Ants responded to the flooding of their nest. I was hoping they would evacuate and move up the tree, onto higher ground, or perhaps raft to safety like Fire Ants do. But they didn’t. They simply retreated into the nest as the floodwaters came. Ants in the flooded colonies have also drowned. Although they will hopefully return after the flood, I’m not sure how many years it might take for the ant colonies to grow big enough to host the butterfly again.

But this blog is about good news, so I’ll leave it there, hoping that there are enough butterflies and ants on high ground to sustain what was the largest and most secure population of the Arid Bronze Azure butterfly.

References

Ecological Associates (2007) “Investigation of options for the delivery of environmental flows to Dry Lakes, Lake Boolca and adjacent floodplains”. Report prepared for the Mallee Catchment Management Authority, October 2007.

R8 (2022a) “Victorian Murray Floodplain Restoration Project - Specialist assessment C Surface Water” Report prepared as part of the Environment Effects Statement, EES Central: Belsar-Yungera and Hattah Lakes North, August 2022. Accessed 01/11/2022 from https://caportal.com.au/vmfrp/projects/hattah-belsar/ees

R8 (2022b) “Victorian Murray Floodplain Restoration Project - Technical Note 02 - Ogyris subterrestris subterrestris surveys” Report prepared as part of the Environment Effects Statement, EES Central: Belsar-Yungera and Hattah Lakes North, December 2022. Accessed 1/01/2023 from https://engage.vic.gov.au/project/VMFRP-SIAC-EES-Central/page/tabled-documents

VMFRP (2022) “Hattah Lakes North story and FAQ” Factsheet prepared as part of the Victorian Murray Floodplains Restoration Project.