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Lowest-flow great river system
The Mississippi Basin has an average annual flow of nearly 530,000 gigaliters, more than 22 times as much. The Murray-Darling waters flow southwest and empty out on the south coast, and most of the basin lies beyond the Great Dividing Range, where little rain falls to replenish the rivers.
It is still the best the continent has. The basin provides drinking water for around 3 million Australians, and the area around it has developed into the agricultural heartland of the nation, producing nearly all the food that Australia's 26 million people eat, with enough left over to export to millions more across Asia.
As climate change worsens drought, the water feeding that heartland is getting harder to come by with every passing decade. Higher temperatures are rapidly increasing evaporation, and some sections of the basin at times remain completely dry for months on end.
How does evaporation shrink Australian rivers?
Inland rivers run slowly across very flat land, spreading into wide shallow channels, floodplains, and wetlands where sun and wind pull water into the air. Some water also soaks into the ground along the way. Across most of the basin, the amount of water that could evaporate in a year exceeds the rain that falls. The Murray-Darling Basin covers about one seventh of Australia's land area.
How is the river's water shared among users?
Since 1995, a cap has limited how much water can be diverted from the basin for farms and towns. The Water Act 2007 then ordered a basin-wide plan, and the Murray-Darling Basin Plan became law in 2012. It sets limits on extraction for each river valley and aims to return some water to the environment. The Murray-Darling Basin Authority was set up in 2008 to oversee it.
Why does the Mississippi carry so much more water?
The Mississippi drains a far larger area, and much of it receives reliable rain from humid air moving up from the Gulf of Mexico. Its tributaries include the Missouri, which carries snowmelt from the Rocky Mountains, and the Ohio, which drains the wet Appalachian side of the country. Each of those adds water the Murray-Darling has no equivalent for. The Mississippi's drainage basin covers roughly 41 percent of the contiguous United States.
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