The world's big rivers are running out

The water level of some of the most important rivers on the planet has dropped significantly over the past 50 years, according to a large-scale study by hydrometeorists in the US.

The water level of the Mekong River is decreasing.Photo: internationalrivers.org.

The American Meteorological and Hydrological Association studies the flow variation of 925 major rivers in the world and collects data during the period 1948-2004. The results show that the amount of water flowing into the ocean decreases as a general trend of most rivers, from the Yellow River (China), the Ganges (India), the Colorado River (USA), the Nile River (Egypt), Mekong River (Southeast Asia) to the Amazon River (South America). Particularly, the Mississippi River water level (USA) decreased by 22% during the study period.

Therefore the amount of water the oceans receive from the rivers also decreases. The average annual reduction of the Pacific is 6%, equivalent to 526 billion cubic meters. The Indian Ocean loses 140 billion cubic meters of water every year. However, the Arctic Ocean's water volume increased by 10%, equivalent to 460 billion cubic meters.

The main cause of falling water levels is human activities, such as the construction of dams and irrigation of fields. Climate change is also a cause, because the rise in temperature increases the rate of water evaporation and reduces the annual rainfall.

Many large rivers provide fresh water to tens of millions of people, such as the Yangtze River (China) and the Niger River (West Africa). The decline in freshwater supply to rivers occurs in the context of an increasing world population, which worries scientists.

However, the water level of some Arctic rivers and rivers taking water from the Himalayas - like the Yangtze River (China) and Brahmaputra (South Asia) - increased again. Experts say melting ice is the reason why water levels rise. In the future, when the Himalayas runs out of ice, the water level of the rivers taking water from it will decrease drastically.