The Mekong is the twelfth longest river in the world - about 4,900 kilometres from end to end. That is longer than the whole of Europe from top to bottom. Along the way it passes through six countries and is the lifeline for around 60 million people.
In Cambodia, the Mekong flows through the capital city, Phnom Penh, where it meets the Tonle Sap river in a famous four-way junction called the 'Chaktomuk' - the Four Faces. Fishermen have been casting nets here for thousands of years.
The Mekong is home to some of the most unusual freshwater animals anywhere. The Mekong giant catfish (one of the largest freshwater fish in the world), the irrawaddy dolphin and the giant freshwater stingray all live here. Scientists are still discovering new species in its waters.
The river floods every year during the monsoon season, leaving rich mud on the banks when it retreats. Cambodian farmers have used this fertile mud to grow rice for thousands of years. Rice grown near the Mekong feeds millions of people across the region.