The Red River (Song Hong in Vietnamese), rising in China's Yunnan Province, is about 1,200 kilometers long. Its two main tributaries, the Song Lo (also called the Lo River, the Riviere Claire, or the Clear River) and the Song Da (also called the Black River or Riviere Noire), contribute to its high water volume, which averages 500 million cubic meters per second, but may increase by more than 60 times at the peak of the rainy season. The entire delta region, backed by the steep rises of the forested highlands, is no more than three meters above sea level, and much of it is one meter or less. The area is subject to frequent flooding; at some places the high-water mark of floods is fourteen meters above the surrounding countryside. For centuries flood control has been an integral part of the delta's culture and economy. An extensive system of dikes and canals has been built to contain the Red River and to irrigate the rich rice-growing delta. Modeled on that of China, this ancient system has sustained a highly concentrated population and has made double-cropping wet-rice cultivation possible throughout about half the region.

The Mekong, which is 4,220 kilometers long, is one of the 12 great rivers of the world. From its source in the Xizang plateau, it flows through the Xizang and Yunnan regions of China, forms the boundary between Laos and Burma as well as between Laos and Thailand, divides into two branches--the Song Han Giang and Song Tien Giang--below Phnom Penh, and continues through Cambodia and the Mekong basin before draining into the South China Sea through nine mouths or cuu long (nine dragons). The river is heavily silted and is navigable by seagoing craft of shallow draft as far as Kompong Cham in Cambodia. A tributary entering the river at Phnom Penh drains the Tonle Sap, a shallow fresh- water lake that acts as a natural reservoir to stabilize the flow of water through the lower Mekong. When the river is in flood stage, its silted delta outlets are unable to carry off the high volume of water. Floodwaters back up into the Tonle Sap, causing the lake to inundate as much as 10,000 square kilometers. As the flood subsides, the flow of water reverses and proceeds from the lake to the sea. The effect is to reduce significantly the danger of devastating floods in the Mekong delta, where the river floods the surrounding fields each year to a level of one to two meters.

The Mekong delta, covering about 40,000 square kilometers, is a low-level plain not more than three meters above sea level at any point and criss-crossed by a maze of canals and rivers. So much sediment is carried by the Mekong's various branches and tributaries that the delta advances sixty to eighty meters into the sea every year. An official Vietnamese source estimates the amount of sediment deposited annually to be about 1 billion cubic meters, or nearly 13 times the amount deposited by the Red River. About 10,000 square kilometers of the delta are under rice cultivation, making the area one of the major rice-growing regions of the world. The southern tip, known as the Ca Mau Peninsula (Mui Bai Bung), is covered by dense jungle and mangrove swamps.