The Murray–Darling Basin is a large geographical area in the interior of southeastern Australia, encompassing the drainage basin of the tributaries of the Murray River, Australia's longest river, and the Darling River, a right tributary of the Murray and Australia's third-longest river. The Basin, which includes six of Australia's seven longest rivers and covers around one-seventh of the Australian landmass,^[1] is one of the country's most significant agricultural areas providing one-third of Australia's food supply.^[2] Located west of the Great Dividing Range, it drains southwest into the Great Australian Bight and spans most of the states of New South Wales and Victoria, the Australian Capital Territory, and parts of the states of Queensland (the lower third) and South Australia (the southeastern corner).

The Basin is 3,375 kilometres (2,097 mi) in length, with the Murray River being 2,508 km (1,558 mi) long. Most of the 1,061,469 km² (409,835 sq mi) basin is flat, low-lying and far inland, and receives little direct rainfall. The many rivers it contains tend to be long and slow-flowing, and carry a volume of water that is large only by Australian standards.

The Snowy Mountains Scheme provides some security of water flows to the Murray–Darling Basin, providing approximately 2,100 gigalitres (7.4×10¹⁰ cu ft) of water a year to the Basin for use in Australia's irrigated agriculture industry, which is worth about A$3 billion per annum, representing more than 40% of the gross value of the nation's agricultural production.

The Basin was once home to a large number of Aboriginal people whose traditional lifestyle and cultures were gradually altered by the arrival of Europeans, while others were outright killed by the settlers. Although some tribes organised resistance, such as the Maraura, whose territory lay around the Rufus River above Renmark and the Tanganekald near The Coorong, they were eventually either killed, exiled, or succumbed to disease.^[3]

The Murray–Darling Basin is home to many native animal species. The true numbers may not be known, but a fairly confident estimate has been made of these animals and the current status of their population. The study found ^[4] that there were:

• 80 species of mammals, with 62 extinct and 10 endangered
• 55 species of frogs, with 18 endangered
• 46 species of snakes, with five endangered
• five species of turtles, with none endangered
• 34 species of fish, with up to half of them either threatened or of conservation significance^[5]

Historical records show that the previous abundances of fish provided a reliable food source.^[6] The bountiful fish became concentrated when the early stages of a flood left shallow water across the floodplain.^[6] Today, roughly 24 native freshwater fish and another 15-25 marine and estuarine species are existent in the Basin, a very low biodiversity.^[6]

Four varieties of carp were used to stock up fish dams. Since then they have made their way into the river systems, where they spread quite quickly. Human introduction, possibly by anglers using small carp illegally as live bait, has also increased their distribution.^[12] These fish are very mobile, breed rapidly and can survive in very shallow water and through long periods of very low dissolved oxygen content.

Carp are a problem because they feed by sucking gravel from the riverbed and taking all the edible material off it, before returning the rest to the water. This stirs up all the sediment, reducing the quality of the water. A project for developing daughterless carp shows promise for eliminating carp from the river system.

Cane toads have entered the upper reaches of the Darling Basin and there are several reports of individuals being found further down the system.^[13] Cane toads compete with native amphibians and are toxic to native carnivores.

Phyla canescens has invaded wetlands and floodplains with heavy clay soils in the Murray–Darling Basin, to the detriment of the native vegetation; the plant does best in habitats that are inundated occasionally, although it cannot compete with the grass Paspalum distichum and the sedge Eleocharis plana in more heavily inundated sites.^[14]

This area is one of the physiographic provinces of the larger East Australian Basins division, and encompasses the smaller Naracoorte Platform and Encounter Shelf physiographic sections.

Total water flow in the Murray–Darling Basin 1885 to the present has averaged around 24,000 gigalitres (24,000 hm³; 19,000,000 acre⋅ft) per year. This is the lowest rate of the world's major river systems.^[1] About 6.0 percent of Australia's total rainwater falls into the Basin.^[15] In most years only half of this quantity reaches the sea and in dry years much less. Estimated total annual flows for the Basin have ranged from 5,000 gigalitres (5,000 hm³; 4,100,000 acre⋅ft) in 1902 to 57,000 gigalitres (57,000 hm³; 46,000,000 acre⋅ft) in 1956.^{[citation needed]} Despite the magnitude of the Basin, the hydrology of the streams within it is quite varied.

These waters are divided into four types:^[16]

• The Darling and Lachlan basins. These have extremely variable flows from year to year, with the smallest annual flow being typically as little as 1 percent of the long-term mean and the largest often more than ten times the mean. Periods of zero flow in most rivers can extend to months and in the drier parts (Warrego, Paroo and Lower Darling Basins) to years.^[16] Flows in these rivers are not strongly seasonal. In the northern regions the majority of floods occur in the summer from monsoonal penetration. For most of the Darling and Lachlan catchments it is typical to see high or low flows begin in winter and extend to the following autumn (see El Niño). High water extraction rates for irrigation and mining have heavily compromised these rivers.
• The southwestern basins (Campaspe, Loddon, Avoca, Wimmera). These have a marked winter rainfall maximum and relatively lower precipitation variability than the Lachlan or Darling. However, the age and infertility of the soils mean that run-off ratios are exceedingly low (for comparison, around a tenth that of a European or North American catchment with a similar climate^[17]). Thus, variability of runoff is very high and most of the terminal lakes found in these basins very frequently dry up. Almost all runoff occurs in the winter and spring and, in the absence of large dams for regulation, these rivers are often seasonally dry during summer and autumn.
• A number of small catchments in South Australia, of which the largest are the Angas River flowing through Strathalbyn and the Finniss River further west, are part of the Murray–Darling Basin. These catchments lead to Lake Alexandrina, one of the lakes at the end of the Murray system. The Angas River is often dry in summer because of high levels of water extraction. The Finniss River has permanent flow into Lake Alexandrina but was cut off by a weir for several years of drought in the early 21st century.
• The Murray, Murrumbidgee and Goulburn (except the Broken River which resembles the southwestern basins) basins. Because these catchments have headwaters in alpine country with relatively young peaty soils, the runoff ratios are much higher than in other parts of the Basin. Consequently, although gross precipitation variability is no lower than in the Lachlan or Darling basins, runoff variability is markedly lower than in other parts of the Basin. Typically these rivers never cease to flow and the smallest annual flow is around 30 percent of the long-term mean and the largest around three times the mean. In most cases the flow peaks very strongly with the spring snow melt and troughs in mid-autumn.

The two principal rivers of the Basin, the Murray and Darling, bring water from the high ranges of the east and carry it west then south through long flat and dry inland areas,^[18] often resulting in alluvial channel wetlands, such as The (Great) Cumbung Swamp, at the terminus of the Lachlan and Murrumbidgee Rivers. Nevertheless, these waters are subject to major diversions for municipal drinking supplies and irrigated agriculture that began in the 1890s.^[6] Currently, 4 major reservoirs, 14 lock and weir structures, and five coastal barrages interject the water flowing down the Murray–Darling.^[6] Of the approximately 13,000 gigalitres (13,000 hm³; 10,500,000 acre⋅ft) of flow in the Basin, which studies have shown to be divertible, 11,500 gigalitres (11,500 hm³; 9,320,000 acre⋅ft) are removed for irrigation, industrial use, and domestic supply. Agricultural irrigation accounts for about 95 percent of the water removed,^[1] including for the growing of rice and cotton. This extraction is highly controversial among scientists in Australia, regarding the agriculture industry's high water use in a region extremely short of water (as much due to exceptionally low run-off coefficients as to low rainfall).^[19] These extensive irrigation systems require a reliable supply of water, not the unpredictable flows that characterise the Murray–Darling.^[6] These structures and irrigation implements were ideal when there was a steady flow of water.^[20] However, during "the Big Dry", as the early 2000s drought came to be known, Australian farmers experienced a scarcity unlike ever before. The drought was so severe that numerous rivers and streams such as the Murray–Darling stopped flowing.^[20] The Basin contains more than 30,000 wetlands.^[21] Eleven of these are protected under the Ramsar Convention of Wetlands of International Importance.

• Climate change in Australia
• Drought in Australia
• Murray–Darling Cap
• List of drainage basins of Australia
• List of Darling River distances
• List of Murray River crossings
• List of Murray River distances
• List of Murrumbidgee River distances
• Murray Basin, the geological sedimentary basin