Tonle Thom – The Great River

From its headwaters in the Himalayas to its juncture with the South China Sea, the Mekong flows 2,703 miles. Those miles rank it as one of the world’s longest rivers.

The Mekong is longer than North America’s two longest rivers – the Missouri and Mississippi. But among Asian rivers, the Mekong ranks seventh in length, easily surpassed by China’s Yangtze and Yellow rivers at 3,917 miles and 3,395 miles respectively.

By volume the Mekong is dwarfed by the Amazon and Congo, but it still carries a lot of water. The river’s average annual discharge rate, which is the amount of water it spews from its mouth, is 565,008 cubic feet per second (cfs). This is almost the exact same amount of water dumped by the Mississippi each year into the Gulf of Mexico.

The amount of runoff in the Mekong fluctuates wildly. During Indochina’s dry season (from December through May), the Mekong’s average discharge rate is 64,975 cfs at Pakse, Laos. However, during the height of the wet season in September, the Mekong can haul over fourteen times that amount past Pakse. In comparison, the Missouri pushes on average only 30,000 cfs past Omaha, Nebraska. And even during the height of one of the Missouri’s greatest floods in 1952, the river only carried 396,000 cfs past Omaha – or less than half the volume carried by the Mekong at Pakse during its monsoon flood.

The Mekong gathers its waters from portions of six countries, including the People’s Republic of China, Myanmar, Laos, Thailand, Cambodia, and Vietnam. And although the Mekong isn’t a long river by world standards, and its drainage basin is relatively small, it still carries a huge amount of water.

The reason the river’s volume is so out of proportion to the size of its drainage basin is simple: so much rain falls during the monsoon season that Southeast Asia’s mountains, forests, and rice paddies cannot absorb it. By September each year, the land surrenders to the monsoon. In consequence, whatever water falls from the sky during and after that month strikes the earth and is immediately let loose to flow to the Mekong.

On its journey to the sea, the Mekong appears vastly different from one region to the next. The river rises in the Himalayas in a range known as the Tanghla Shan. The cold, fast, light-green colored river that begins in the Tibetan Plateau looks nothing like the warm, slow, silt-laden river at Phnom Penh.

The Mekong’s different traits have earned it different names. Near its headwaters in Tibet, the Chinese know the river as the Dza Chu – the River of Rocks – which refers to the predominate riverbed material through that reach. Another Chinese name for the stream is Lancang Jiang or Turbulent River, no doubt in reference to the river’s rapid descent from the Himalayan highlands. Because of the lower Mekong’s greater width and depth, the Cambodians know it as the Tonle Thom – which translates as Great River. The Vietnamese also show their deference to the Lower Mekong’s power and size by calling it Song Lon, which also means Great River.

Along its course through southern China, north-eastern Myanmar, and upper Laos, the Mekong is hemmed in by mountains and hills. At the former royal capital of Luang Prabang, Laos, the river flows through a tight slot bounded on the west by dark-green hills and on the east by a height of land topped by dozens of Buddhist temples. Here, the Mekong flows steady and deep.

Further south near Vientiane, the Mekong exits the mountains. No longer confined by rock walls on either side, the river spreads out, extending over a mile from bank to bank. Its color also changes. Gone is the chocolate brown water found further upstream. In its place is a river possessing a light, milky-brown hue.

During the dry season, a large sand flat stretches along the riverfront at Vientiane. The tan grains of sand are the ground-up remains of boulders that long ago fell from the mountains into the river. Not far from the drab buildings of the Laotian capital, the Mekong has laid the mountains to rest. Those grand peaks, once so imposing, hard, and high, now lie splayed out, lifeless, and pale.

For hundreds of miles, from the mouth of the Nam Heung to the mouth of the Mae Nam Mun, the Mekong serves as the border between Laos and Thailand. Along this reach, the Annamese Cordillera is visible on the eastern horizon. On the Thai side of the river, a level plain unfolds for as far as the eyes can see.

Before leaving Laos, the Mekong skirts to the west of the town of Pakse. The river here is simply beautiful. Its sprawling, mile-wide channel courses through a lush land of rice paddies and palm trees. To the east of the river, dark, timbered hills quickly climb to the Bolovens Plateau. And to the south, the river makes a grand, sweeping turn to the left, passes a tall, picturesque hill on its right, and then disappears into the wild lands of south-eastern Laos.

Soon after entering Cambodia, the Great River cascades over a series of waterfalls. North of Kratie, the Mekong thrashes and foams through the Samboc Rapids. Deep-draft river boats can travel all the way from the river’s mouth to Samboc Rapids, but can go no further. In the early 1860s, French businessmen in Saigon envisioned the Mekong as a grand water route, linking their colony in Cochinchina to Laos and southern China. But the rocks and narrow chutes at Samboc Rapids dashed their imperial dreams.

Samboc Rapids partly explains why south-eastern Laos and north-eastern Cambodia remained two of the most biologically-diverse regions in Southeast Asia up until the late twentieth century. The rapids made it difficult for water-borne profiteers to access the area, which saved the forests and wildlife from destruction. Unfortunately, modernity is currently reducing the rapids to rubble. The Chinese are blasting a channel through the rapids, which will open the region to international commerce and deliver a death sentence to what remains of its biological diversity.

Phnom Penh is Cambodia’s capital city. It is located at the convergence of four rivers – the Mekong, Prek Thnot, Bassac, and Tonle Sap. Situated in the heart of Cambodia, the Tonle Sap is the most interesting of Indochina’s rivers. It is more of a lake than a river. When the Mekong is at low stage, the Tonle Sap flows from northwest to southeast. However, when the Mekong goes into flood, the high waters of the bigger river act as a dam of sorts, blocking the southeast flow of the Tonle Sap and forcing it back upon itself. As the Mekong pushes higher, it pours its waters into the Tonle Sap, causing lake levels to rise from an average of 6.5 feet to thirty-two feet. The high inflows push the Tonle Sap far beyond its low-water boundaries. The lake normally covers an area of 1,042 square miles, but in the monsoon season it can grow to over 6,000 square miles. When the Mekong again drops below flood stage, the Tonle Sap reverses course and its waters drain back into the Mekong. Because of the lower Mekong’s increased depth, Phnom Penh is a deep-water port, accessible to ocean-going vessels that travel up the river from the South China Sea.

Just downstream from Phnom Penh, the Mekong makes a sharp turn to the east. Once it crosses the Cambodian-Vietnamese border, it enters its delta and spreads out into several narrow channels, known to the Vietnamese as the Song Cuu Long or Nine Dragons. After passing a string of towns in southern Vietnam, including My Tho, Can Tho, and Soc Trang, the branches of the mighty Mekong empty their waters into the South China Sea.


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