Intensive livestock farming is a major source of organic pollution. Discharge of farm effluents, rich in organic pollutants, reduces biodiversity in rivers and disrupts aquatic ecosystems by depleting oxygen levels. Over the last 50 years, meat production has increased rapidly, and the upward trend is only likely to continue due to population growth, urbanization and increased income. So what kind of future do the world's rivers face, what impact is globalization having, and which rivers are already at breaking point?

To answer these questions, Yingrong Wen, from Delft University of Technology in The Netherlands, and her colleagues used global livestock trade figures to calculate the biological oxygen demand associated with pig, chicken and cattle meat for each country. Livestock farm maps identified which river catchments pollution would run-off into, and a hydrological model estimated the amount of organic pollution that each river would receive, taking into account local weather conditions, river flow rates and the level of water treatment in each country.

Immediately the team saw that some countries, including Russia, Japan, Saudi Arabia, Mexico, Hong Kong, Italy and the UK, are heavily dependent on imported meat products, and would suffer a significant increase in river pollution if forced to produce meat locally. Meanwhile, major exporting countries like the US, Brazil, The Netherlands, Australia, Belgium and France must process more organic pollution than their fair share, due to their large meat export markets.

Surprisingly, more livestock farming didn't always mean dirtier rivers. "Increases in organic pollutant loading do not always translate into more pollution," said Wen, whose findings are published in Environmental Research Letters (ERL). "We found that rivers in eastern Australia, New Zealand and the Philippines had high enough cleaning capacities - dilution and natural degradation - to assimilate the increased loading."

But some countries, like the UK, are already at capacity when it comes to river pollution. The results show that despite advanced wastewater treatment techniques, UK rivers would be overwhelmed if livestock farming was to increase. Instead the UK benefits by importing meat, offloading the environmental degradation to countries such as The Netherlands.

Meanwhile, nations that export a large quantity of meat, like The Netherlands, manage their pollution by concentrating livestock farming along particular river segments. This makes it easier to capture the pollution before it reaches the river. But for some countries this form of intensive farming isn't an option. "Although Russia possesses extensive natural resources, the climate means that livestock farming is limited to the west," said Wen. Farms must be widely distributed here as the quality of land is poor and can’t support large numbers of animals.

Studies like this show how important it is to quantify the impact of livestock farming on a river by river basis. "It is the actual pollution level in the river that matters, rather than how much pollution enters the river," said Wen. "The same pollutant load will result in different pollutant levels depending on local conditions – river discharge and natural degradation rates, for example – which may vary significantly within a country."