Mekong River

The river carries lots of mud. At the sea, the mud settles down.

Little by little, the mud has made new land. This land is called a delta.

The Mekong carries mud and sand toward the sea: in the 1990s, an estimated 110 million tons a year. Where it reaches the coast in southern Vietnam, this sediment has piled up into a delta of about 50,000 square kilometers.

Scientists drilled into the delta and found that its coast has moved about 220 kilometers out to sea in the past 7,500 years. Satellite images since 1973 show that this growth slowed down, and since about 2005 parts of the shore have started to wear away instead.

Liu and colleagues (Oceanography, 2017) describe a subaerial delta plain of about 50,000 km², which they cite as the third largest tide-dominated delta after the Amazon and Ganges-Brahmaputra. Cores show it has prograded about 220 km southeastward within the past 7,500 years, historically at a mean rate of more than 30 m a year. Offshore, the river builds a sigmoidal clinoform up to 15 m thick, and Mekong mud extends about 300 km along the shelf to the tip of the Ca Mau Peninsula.

Landsat images from 1973 to 2015 show seaward shoreline growth slowing from 7.8 to 2.8 m a year, then turning to a retreat of about 1.4 m a year after 2005: the delta shifted from building to eroding.

In the rainy season, the Mekong gets very full. It pushes into a smaller river.

That smaller river flows backward and fills a big lake.

From June to November, monsoon rain swells the Mekong so much that, in low-lying Cambodia, one of its rivers is forced to flow backward. The Tonle Sap River, about 100 kilometers long, turns around and fills Tonle Sap, the Great Lake.

In the dry season, the lake covers a few thousand square kilometers and is about 2 meters deep. During the floods it can grow to as much as 12,000 square kilometers and 10 meters deep. When the dry season returns, the lake drains back into the Mekong.

NASA describes the monsoon flood pulse: beginning in June, the rising Mekong inundates the roughly 100 km Tonle Sap River, which reverses and fills Tonle Sap Lake. The lake grows from a dry-season low of a few thousand square kilometers and about 2 m deep to as much as 12,000 km² and 10 m deep; another NASA page puts the change as roughly tripling, from about 3,000 to about 10,000 km².

When the dry season returns, the lake drains back toward the delta, where its fresh water helps balance the intrusion of seawater into farmland. The floods also lay down silt that renews forests and fields, and the lake and its wetlands support a diverse freshwater fish ecosystem.

Very big fish live in the Mekong. One giant stingray weighed 661 pounds!

Scientists measured it and let it swim away.

The Mekong is famous for giant fish. In June 2022, a fisherman in northern Cambodia accidentally hooked a giant freshwater stingray more than 13 feet long that weighed 661 pounds. Scientists from the Wonders of the Mekong project measured her, a tracking tag was fitted, and she was released alive. Guinness World Records named her the largest freshwater fish on record.

Before her, the biggest scientifically documented catch that fish biologist Zeb Hogan had found was a 646-pound Mekong giant catfish, caught in northern Thailand in 2005. The World Wildlife Fund has counted more than 2,400 new species found in the Greater Mekong between 1997 and 2015.

National Geographic reports that a female giant freshwater stingray (Urogymnus polylepis), caught by accident near Koh Preah island in northern Cambodia on the night of 13 June 2022, measured more than 13 feet from snout to tail and weighed 661 pounds. Fish biologist Zeb Hogan's Wonders of the Mekong team measured her, she was fitted with an acoustic tag and released alive, and Guinness World Records certified her as the largest freshwater fish on record. The benchmark in Hogan's long search had been a 646-pound Mekong giant catfish caught in 2005.

This stretch of the river, lined with seasonally flooded forest, is believed to spawn up to 200 billion fish a year and is thought to be a dry-season refuge for megafish. National Geographic lists overfishing, invasive species, water pollution and dams that block migrating fish among the threats to river giants. Liu and colleagues cite a WWF count of more than 2,400 new species in the Greater Mekong from 1997 to 2015.

In the rainy season, the river can rise fast. Never walk into flood water.

Some fishers who catch giant fish let them go back to the river.

The Mekong floods every rainy season. The US National Weather Service warns that just 6 inches (15 centimeters) of fast-moving flood water can knock over an adult, so it is never safe to walk or drive into flood water.

The Wonders of the Mekong scientists have worked with local fishers, who report catches of stingrays and other endangered fish and then release them. PlayOwl's advice for visitors: enjoy river animals from a distance and leave them in the water.

The US National Weather Service notes that people underestimate the force of moving water: about 6 inches (15 cm) of fast-moving flood water can knock over an adult, and about 12 inches can carry away most cars, so it is never safe to walk or drive into flood water. That matters on a river whose monsoon floods arrive every year from about June.

On the conservation side, the Wonders of the Mekong team has built a network of fishers who report catches of stingrays and other endangered fish before releasing them; Hogan calls such fishers great allies. PlayOwl's own advice is to watch wildlife without disturbing it.