Dr Susan Lawler
First published on The Conversation on 14 May 2013, as part of a series on unknown natural wonders.
At first glance, the Barmah-Millewa forest may not seem all that impressive. If you’ve come in a dry year, via the main roads, you may find yourself on a river bank that does not look that much different to other parts of the Riverina.
But if you return on a wet year, when the forest is transformed into a swamp and teeming with birds, or if you could watch the landscape change over time through floods and droughts, you might see why the Barmah-Millewa forest is a Ramsar site (a wetland of International Importance), one of six icon sites in The Living Murray initiative, and has recently been declared a national park.
About 20,000 years ago, a geological event lifted a large area of land, blocking the Murray River. A huge lake was formed, and the river changed direction, flowing north and south around the uplifted land.
The narrow river channel at this point ensured regular flooding of the surrounding plains, creating a unique set of wetland habitats. Known as the “Barmah choke”, this is the narrowest part of the Murray River. When the Goulburn River is in flood, the Murray can actually run backwards at this point for up to 50km, all the way back to the Edwards river.
This unique geology supports a wide range of habitats. There are several lakes, swamps, and Moira grass plains. Red gum trees lie on the floodplains, Black Box trees on the high ground, and sand hills covered with Callitris pines. The ecological diversity supports an impressive array of biodiversity, including 553 native plant and 273 native animal species.
There are also plenty of fish in the area, including the iconic Murray cod. Three species of crayfish coexist by separating themselves according to habitats: Murray crayfish hunt in the river, yabbies crawl around the lakes, and Swamp yabbies dig in the floodplains.
Those numbers do not however include the insects and other invertebrates, which are legion. Being on the water at night during a wet year is a recipe for an invasion of insects and spiders that fall or jump into your boat looking for some place dry to build a web.
Those numbers also do not include the invasive weeds and animals, which, with the exception of cattle, are becoming more abundant every year. Wild horses are a problem, carp are common in the waterways, and foxes prey on the turtles which are culturally significant to the local Aboriginal community.
Human habitation of the area goes back 40,000 years. The abundance of wildlife and resources meant that indigenous people in the area did not have to be nomadic, but could stay in one place all year.
As early as 1860, the Yorta Yorta nation sought compensation from the government for the destruction of fishing areas by paddle steamers. They sought native title of the Barmah forest in 1984 and continue to have a strong connection to the land. Many members of the community actively contribute to environmental management of the area by helping control weeds by burning off and by supporting research on local turtle species.
European settlement in the mid 1800s was driven by excellent grazing, abundant Red gum timber, and easy transport of wool and wood using the river. Vast numbers of Red gum trees were harvested to supply railway sleepers for new railroads in the 1860s, and other industries thrived, such as beekeeping and leech collecting.
Threats to a thriving ecosystem
Best known as the largest River Red gum forest in the world, the Barmah-Millewa forest has some stands that are over 400 years old, providing important environmental support for breeding birds and other fauna through tree hollows and fallen timber.
Each ecosystem within the Barmah-Millewa forest is maintained by the frequency and depth of flooding. Red gums (pictured below) prefer more frequent flooding than Black box, while Moira grass plains prefer shallow annual floods. Sand hills remain dry most years.
Changes to the natural flood regime created by river regulation has affected the health of many ecosystems within the park. The Moira grass plains are suffering the most because they require more regular flooding. The most recent drought has led to the incursion of the grass plains by Giant rush and Red gum saplings. Both are native species, but are growing outside of their natural habitats. Red gums are starting to grow in dry creek beds, threatening to fundamentally alter the environment. This is an area that will benefit from the careful application of environmental water.
Water flows through the area are controlled by regulators and structures, such as canals and earthworks, that were initially designed to prevent the loss of water to the swamp. More recently, environmental water has been used to prolong and extend natural floods. Longer floods support the breeding of water birds such as Little egrets, White ibis, and Royal spoonbills.
Unfortunately, some floods in the area have turned into blackwater events. This occurs when a large amount of leaf litter accumulates over several years and is then inundated. The sudden decay of so much organic matter removes oxygen from the water. The water becomes dark and inhospitable to local fauna: fish die and crayfish crawl out to breathe. Managing these events requires careful planning to recreate a more natural flood regime.
Recent surveys in the Barmah-Millewa wetlands have detected significant and worrying declines in the abundance of birds, fish and some vegetation types. The fish community is only 10% of pre-European levels, and carp, which are an invasive species, are now common. Bird breeding can only occur if floods remain for several months, a situation that is not normally supported by the flood regimes dictated by irrigation schedules.
Fortunately, there are many projects underway to preserve this area, and as an icon site in The Living Murray Program, the Barmah forest will receive hundreds of Gigalitres of environmental water. Some people may think this is wasted water, but this is a wetland that is intricately woven into the web of life that supports all who depend on the Murray River: animals and plants as well as humans.
Dr Susan Lawler is the Head of Department of Environmental Management and Ecology at La Trobe University Albury-Wodonga. She writes a regular blog for The Conversation entitled This thing called life.
River Red Gum image credit: Barmah-Millewa Collective.