in by the enormous alluvial deposits of the rivers over a period of millennia, and it advances one hundred meters into the Gulf annually. The ancestral home of the ethnic Vietnamese, the delta accounted for almost 70% of the agriculture and 80% of the industry of North Vietnam before 1975. The Red River, rising in China's Yunnan Province, is about 1,200 kilometers long. Its two main tributaries, the Sông Lô (also called the Lo River, the Riviere Claire, or the Clear River) and the Sông Đà (also called the Black River or Riviere Noire), contribute to its high water volume, which averages 4,300 cubic meters per second. 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's, this ancient system has sustained a highly concentrated population and has made double-cropping wet-rice cultivation possible throughout about half the region. Mountainous The mountain plateaus in the north and northwest are inhabited mainly by tribal minority groups. The Dãy Trường Sơn (Annamite Range) originates in the Tibetan and Yunnan regions of southwest China and forms Vietnam's border with Laos. It terminates in the Mekong River Delta north of Hồ Chí Minh City (formerly Saigon). These central mountains, which have several high plateaus, are irregular in elevation and form. The northern section is narrow and very rugged; the country's highest peak, Fan Si Pan, rises to 3,142 meters in the extreme northwest. The southern portion has numerous spurs that divide the narrow coastal strip into a series of compartments. For centuries these topographical features not only rendered north–south communication difficult but also formed an effective natural barrier for the containment of the people living in the Mekong basin. Central Highlands Within the southern portion of Vietnam is a plateau known as the Central Highlands (Tây Nguyên), approximately 51,800 square kilometers of rugged mountain peaks, extensive forests, and rich soil. Comprising five relatively flat plateaus of basalt soil spread over