What did high flows mean for the Lower Murray?

By Luciana Bucater | South Australian Reasearch and Development Institute (SARDI)

Three consecutive years of La Niña (also known as triple-dip event) have been responsible for heavy rainfall in Queensland, New South Wales, and Victoria. This caused sustained high in-channel flows in 2021-22 and, in 2022-23, exceptionally high flows along the Murray River, including the lower part in South Australia.

South Australia, being a semi-arid territory, has historically recorded sparse floods and high flow events.

In 2022, Riverland communities watched flows increase steadily in late May, reaching bank-full flow (45 gigalitres per day) at the South Australian border in late July. Following predictions of flows reaching more than 200 GL/day, riverbank levies were built in many Riverland towns and community members and businesses prepared for the high water levels as best they could.

This flooding caused significant damage to homes, businesses and public infrastructure, and a lengthy and costly recovery process lies ahead for our river communities.

A street is rendered almost invisible under floodwater in Renmark, with trees visible in the background and a grey, overcast sky.

Twentyseventh Street in Renmark underwater during the recent floods. Photo credit: SARDI.

For a few months, many images of picnic tables, streets and bridges under water appeared on the news and social media channels. Floodplains were inundated and tree trunks and electricity poles were submerged.

A floodplain near Renmark is inundated with water under a grey, overcast sky. A single tree can be seen standing in the centre of the floodplain.

An inundated floodplain near Renmark. Photo credit: SARDI.

However, the high flows that damaged buildings and roads also delivered several environmental benefits. Water flow is the life of all rivers, including the mighty Murray River, and periods of high flow are an integral part of a natural flow regime. High flows and floods represent the river’s strong heartbeat.

Positive environmental outcomes from high flows include improving connection between rivers and floodplains, replenishing groundwater, filling up wetlands and moving salt, nutrients and biological matter from floodplains.

High flows are particularly important to support the breeding cycle of many waterbirds and fish. Some native fish such as golden perch rely on high water flows to trigger spawning, while others like Murray cod generally show improved recruitment when fast flowing habitats are present. The Lower Murray team have recorded improved recruitment of both species, at different times, since 2015.

Juvenile golden perch collected near Lock 6 in April 2022, after a high flow period.

Juvenile golden perch collected near Lock 6 in April 2022, after a high flow period. Photo credit: SARDI.

Juvenile Murray cod collected in a fast-flowing area of the river near Lock 4 in February 2021.

Juvenile Murray cod collected in a fast-flowing area of the river near Lock 4 in February 2021. Photo credit: SARDI.

High flows also contributed to the regeneration and maintenance of floodplain vegetation such as river red gum and black box. Temporary high water levels can wet usually dry areas of the riverbank (littoral zones) and assist the germination of previously deposited seeds (seedbank).

Wetting the riverbank can also increase the diversity of zooplankton (water bugs) by transporting zooplankton eggs (egg bank) from these zones. High flows also help with the dispersion of aquatic plants and animals.

River red gum sapling found after riverbank wetting near Lock 1.

River red gum sapling found after riverbank wetting near Lock 1. Photo credit: SARDI.

Starting seed germination trials using seedbank soil collected downstream of locks 1, 4 and 6.

Starting seed germination trials using seedbank soil collected downstream of locks 1, 4 and 6. Photo credit: SARDI.

Finally, high river flows assist greatly in flushing salt out of the Murray-Darling Basin. Preliminary analysis showed that about 1.2 million tonnes of salt were exported out of the system through the Murray Mouth, between July and mid-December 2022. The total salt exported for the entire year and the amount that was impeded from entering the Murray Mouth (imported) will be calculated later in the year.

It should be noted that some negative impacts are observed during flood events, including the proliferation of invasive species like carp and negative impacts on native fish due to Hypoxic blackwater in some upstream valleys. Over the long-term, the positive benefits of high flows to the health and functioning of the river are believed to far outweigh these impacts.

In stark contrast to their impact on the man-made environment, high flows are generally very beneficial to plants, animals, and ecosystems in the Lower Murray River.

Our work in the Lower Murray River Selected Area

The Lower Murray River is at the end of the Murray Darling Basin (MDB) system and includes the only estuary of the MDB, which connects to the Southern Ocean. Annual flow is variable, being influenced by inputs from the southern and northern basin, and rainfall and water extraction experienced in these regions. The Lower Murray River is complex, and includes the main river channel, anabranches, floodplain / wetlands, billabongs and stream tributaries. Being towards the end of the system, the Lower Murray River is wide and deep relative to upstream reaches, and significant flow is required for floodplain inundation. Our study area covers four different zones: floodplain, gorge, swamplands and the Lower Lakes and Coorong.

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