The particular configuration of Viet Nam is usually indicated with the image of “two baskets of rice supported by a stick” (the Annamite Chain). The two plains of Tonkin and Cochinchina are actually two areas of equal geographical weight: they are the two large plains of the country, both formed by the alluvial deposits of two large rivers. The first, the Tonkin, opens into a tectonic depression between the Annamite mountain alignment and the ancient reliefs of southern China. The valleys that branch off from the Yunnan reliefs converge into it, initially narrow, then with large alluvial bottoms; that of the Red River, which has a straight course and a total length of 1183 km (of which less than 500 in Vietnamese territory), begins to open after the passage called Lao Cai (the “door of China”) and approx. 200 km from the coast it flows into the great plain of Tonkin. Overall, this region looks like a large amphitheater-shaped basin facing the sea (Gulf of Tonkin). The flat, alluvial part, all settled by man, is very populated; the rivers are hanging and from them derives a whole dense network of canals. The coast is low, flat, centered on the Red River delta, to the E fragmented into low island formations. All around the plain is bordered by reliefs made up of archaeozoic rocks subjected to formations of the Paleozoic and Mesozoic, it was during which the birth of the depression occurred, coeval with the emergence of the Annamite Range. This is in fact part of the great arc of corrugations that preceded the Cenozoic orogeny and which extends from Kunlun, through Yunnan, to Indochina; however, it is essentially the result of fractures and compact uplifts of ancient granite and schist massifs, with overlapping Mesozoic sedimentary blocks: hence the absence of harsh mountain forms. However, the range reaches quite pronounced altitudes, especially to the W of Tonkin, in the massifs that rise between the valley of the Red River and that of the Black River., where the Fan Si Pan reaches 3143 m. The chain, which in the central section falls for the most part in Laotian territory, widens towards the S in an area affected by large fractures that have given rise to intense magmatic activity, responsible for the basaltic formations of the central plateaus (of Kon Tum) and of the southern ones (or of the Moi), high on average 1000-1500 m, which overlook the Cochinchina. A delta land where the debris flows of the largest Indochinese river are deposited, the Cochinchina also corresponds to a depression once occupied by marine waters; it has a much larger area than Tonkin and is largely occupied by the delta area of the Mekong. Towards N it is bordered by low escarpments and morphologically alternates slight intumescences sandy areas in low-lying areas subject to periodic flooding. However, unlike Tonkin, whose plains are all intensely shaped and controlled by man (given the pensility of rivers), Cochinchina is more naturally stabilized, lacks pensiveness, and floods are less threatening than in Tonkin, as they find vent in the Tonle Sap, the great lake of the Cambodian plains. All along the coastal edge that connects the Nambo to Bacbo (these are the real Vietnamese terms, while the corresponding Cochinchina and Tonkin are of French origin) has a varied morphology: the rocky promontories, appendages of the hilly belt that dominates the coast, alternate with small alluvial plains originating from the rivers that descend from the Chain Annamitica.
Hydrographically, the Vietnamese territory is very fragmented. Its belonging to the basins of its two major rivers, the Red River (Song Coi or Song Hong) and the Mekong, is very marginal as they both run most of their course outside Viet Nam; especially as regards the Cochinchina, it falls within the Mekong basin only with the extreme terminal and deltaic part. The great Indochinese river, entering Viet Nam (where its course extends for only 220 km, out of a total of 4500), divides into two main branches, the Tien Giang and the Hau Giang, in turn divided into minor branches: the The set of mouths is called Cuu Long (the “nine dragons”). Typical river with a rain-monsoon regime, the Mekong presents very impressive variations of flow; the floods occur starting from the summer months (when the monsoon rains begin) and reach their maximum in October-December, however the increase is quite gradual and the waters vent, as well as in the Tonle Sap, in some amphibious areas of the delta. Also for the Red River the floods are often vigorous (the range can vary from 1000 m3 / s in the dry season to 55,000 m 3 / s in the rainy one) and often break the banks, flooding the Tonkinian plains. The Red River has as its major tributaries the Black River (Song Da) and, a little further upstream, the Clear River (Song Lo). The whole large territorial section dominated by the Annamite chain is hydrographically fragmented into numerous and small basins; the widest are those of the southern rivers, including the Song Be which flows towards the plains of the Mekong and the Song Ba which draws water from the Kon Tum plateau and flows into the southeast coast. Visit ask4beauty for Asia hydrography.