Physical geography of Mekong River
The upper reaches of Mekong River is a long and narrow river valley, accounting for about 1/4 of the total basin area, and crossing the mountains and plateaus of China. The lower Mekong River is 2390 kilometers long below where it began to form the border between Myanmar and Laos. It takes the Hora Plateau in Thailand, the western slope of Annan Mountains in Laos and Vietnam, and most parts of Cambodia as basins, and then reaches the sea through the delta branch of Vietnam.
Mekong River is one of a group of big rivers that originated in the plateau between salween and the upper reaches of the Yangtze River. The river bed goes deep into the rugged area where it flows. Between Myanmar and Laos, about 20720 "? The territory of Myanmar is the Mekong River Basin, which is a rugged area. The relatively gentle lower Mekong River constitutes a long border between Laos and Thailand, which is the main reason for friction and cooperation among Cambodia, Laos, Thailand and Vietnam. The topography of the Mekong River can be divided into five regions: the northern plateau, the Annan Mountains (Changshan Mountains), the southern highlands, the Nakhon Ratchasima Plateau and the Mekong Plain. The northern plateau includes mountainous areas in northern Laos, Loy and Chiang Rai provinces in Thailand, with mountains everywhere, with an altitude of 65,438+0,500-2,800 meters, and only a few highland plains and river valley alluvial platforms. The Annan Mountains extend from northwest to southeast for more than 800 kilometers, with steep slopes in the north and middle and hilly areas in the south. The southern and western slopes are affected by the southwest monsoon, with heavy rainfall, while the central valley is dry. The southern highlands include Doukou Mountain in Cambodia, with rolling mountains in the east and hilly areas in the southwest. Nakhon Ratchasima Plateau, including parts of northeast Thailand and Laos, is a butterfly-shaped intermountain basin with a length of about 500 kilometers and a width of about 500 kilometers, through which tributaries of Menghe River and Xihe River pass. Mekong plain is a large lowland, including delta area.
Known locally as the upper source of Angqu and Zaqu, it originated in the Tanggula Mountain Highland at an altitude of more than 4,900 meters on the southern border of Qinghai Province. They flow southeast through Qamdo, Tibet. The main river, called Lancang River, flows southward, passes through Yunnan Plateau, which has been eroded and cut into hills and valleys, and reaches a point south of Jinghong, becoming the boundary river between China and Myanmar. The river then flows southwest; This river is 20 1 km long and is the border between Myanmar and Laos. The lower reaches of Myanmar can be divided into six main parts-northern highlands, Nakhon Ratchasima Plateau, eastern highlands, southern lowlands, southern highlands and deltas. Most of the vegetation in the downstream basin belongs to tropical broad-leaved tree species, but with the difference of latitude and topography, different individual tree species also appear.
There are high-folded mountains in the northern highlands, about 2743 meters above sea level, and many mountains have steep slopes. To the south of the east-west channel in the lower reaches of Yongzheng River is the Nakhon Ratchasima Plateau, which covers almost all areas of the Mekong River Basin in Thailand and the lower reaches of the Mekong River tributaries in Laos. This is a gently undulating hill, criss-crossing with the flat alluvial plain. The eastern highlands are part of the Annan Mountains, and rivers in the mountains flow into the Mekong River from the west. In most areas between Ky Son(Muong Sen) in Vietnam and Kracheh in Cambodia, this watershed forms the border between eastern Vietnam and Laos and western Cambodia.
The southern lowlands are adjacent to the banks of the Mekong River in the lower reaches of Pakse, Laos. The Mekong River empties into Cambodia at Koen Falls. Between Kongnan Waterfall and Orange Well, rapids and alluvial plains crisscross. Below Kampong Cham, the riverbed slopes slowly, and the river flows through the vast alluvial belt on its flood plain. Near Phnom Penh, the Mekong River joins tonle sap river and connects with tonle sap river through the latter. The flow in tonle sap river changes with the seasons. At the peak of flood season, when the water level of Mekong River is high, water flows into the lake through Tonle Sap River, and the lake surface changes from 2590 ″? I added about 10360㎞? The maximum area of. In the dry season, the flood recedes and tonle sap river flows into the Mekong River in the southeast. Tonle Sap Lake is a high-yield fishing ground.
The Xiangshan Mountains and Cravan Mountains in southwest Cambodia constitute the southern highlands. Several tributaries of tonle sap river originate from these highlands. The Mekong River is divided into two tributaries below Phnom Penh: the Mekong River itself and the Basse River. Since then, the delta has been extended to the seaside, with a total area of about 64,750 "? , divided into three parts.
Vientiane, from China, Myanmar and Laos to Laos, is upstream, with a length of 1053 km. Most of the areas flowing through are at an altitude of 200- 1500m, and the terrain fluctuates greatly. Along the way, blocked by the mountain, the river bends several times, the valley width alternates repeatedly, the riverbed slope is steep and there are many dangerous beaches. Vientiane to Pakse is the middle reaches, with a total length of 724 kilometers. It flows through the foothills of the Nakhon Ratchasima Plateau and the Fuliang Mountains, and the elevation of most areas is100-200m, with little topographic relief. Among them, the upper valley is wide and the water flow is calm. From savannakhet to Bashe, the riverbed is steep, with many rocks, shoals and rapids. Pakse to Phnom Penh, Cambodia is downstream, 559 kilometers long. The flowing area is a flat and slightly undulating quasi-plain, with an altitude of less than 100 meters. The riverbed is wide and there are many tributaries, but some sections of the river are tightly bound by hills or cross the river, forming rapids and rapids, and the largest dangerous water cave waterfall in the whole river is here. The delta extends from Phnom Penh to the estuary, with a total length of 332 kilometers. After accepting tonle sap river near Phnom Penh, the Mekong River is divided into Qianjiang River and Houjiang River. Qianjiang River and Houjiang River enter Vietnam, then divide into six tributaries and enter the sea through nine estuaries, so their reach into the sea is also called Jiulong River. The average elevation of the delta is less than 2 meters, covering an area of 44,000 square kilometers, with low terrain, dense water network and fertile soil. It is one of the important rice producing areas in Southeast Asia. Mekong River Basin is located in the center of Asian tropical monsoon region. It is affected by southwest monsoon from the sea at the end of May-September, and it is wet and rainy. The rainy season is May-65438+10. 165438+ 10 was affected by the northeast monsoon from the mainland to the middle of March the following year, which was dry and rainy. 165438+ 10 to April of the following year is the dry season.
Thunderstorms in the whole rainy season are strong in intensity, short in duration, small in influence range and frequent; Long-lasting and wide-ranging rainfall is the most frequent in September, which can cause serious floods, but its impact is mostly confined to the delta and the western part of the basin, and occasionally crossing the mainland makes a wider area suffer from long-term heavy rain. Due to the uneven seasonal distribution of rainfall, all parts of the basin experience droughts of different intensities and durations every year. The normal annual rainfall in Mekong River Basin has increased from below 65,438+0,000 mm in northeastern Thailand to over 4,000 mm in southern Laos, Cambodia and Vietnam, with an average of 2,000 mm in Cambodia. The average annual rainfall is unevenly distributed, and about 88% of the annual rainfall is concentrated in May-June, 438+October.
The temperature in Mekong River basin has little change, with the highest average temperature of 30℃ in Vietnam and 33.5℃ in Thailand. The lowest average temperature is 65438 05℃ in Laos and 22.7℃ in Cambodia. The relative humidity is 50%-98%.
Runoff in Mekong River Basin comes from rainfall. Due to the influence of the annual monsoon, the main water level hydrograph from the last hydrological year to the next hydrological year is almost unchanged, and the gap between abundant water and low water is not big. If it is stipulated that the annual flow in wet year is greater than 1 10% of the average flow for many years, and the annual flow in dry year is less than 90% of the average flow for many years, then the occurrence probabilities of wet year, normal year and dry year are about 25%, 50% and 25% in Vientiane Station and 20%, 60% and 20% in Kratie Station.
The water quantity of Mekong River mainly comes from the rainfall in its downstream basin, and changes with the seasonal rainfall difference caused by monsoon. Water is usually at its lowest point in April. In May or June, with the monsoon moving in from the south, the water volume began to increase, and the eastern and northern highlands changed the fastest. The upper reaches can reach the highest water level in August or September at the earliest, and the highest water level in the southern reaches can not be reached until 10. The northeast monsoon in southern China usually starts in June 5438+065438+ 10, which brings dry weather until May of the following year.
The temperature in the lower Mekong River basin is warm all year round. The average daily maximum temperature in Phnom Penh is 32℃, and the average minimum temperature is 23℃. The temperature in the upper reaches of the basin is somewhat regulated by altitude, generally lower than that in the south, and shows more seasonal changes.
The average annual discharge of Mekong River in Kratie, Cambodia is about 14000 m3/s, which is about twice that of Columbia River in North America. The lowest flow recorded by Kratie is about112 of the average flow, while the annual maximum flow is about 4 times of the average flow.
According to records, Barcelona has the largest amount of sediment every year, about 65438+87 million tons; The border of Myanmar is only about half of this amount, and Phnom Penh is about two-thirds of this amount.