Introducing Large Rivers. Avijit Gupta
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the Himalaya in a 5075 m deep gorge around the Namcha Barwa Peak. Downstream the annual rainfall varies between 1000 mm and 2000 mm on the southern slopes of the Himalaya, and rises to 3000 mm in the Mishmi Hills of the eastern basin. Apart from the high annual precipitation and discharge with a marked seasonality, floods are very commonly caused in the Brahmaputra. Floods arrive with rainstorm systems in the middle of the wet monsoon season, and also are caused by tectonic disturbances. The enormous earthquakes of 1897 and 1950 (both with magnitude 8.7) partially blocked the river, giving rise to subsequent huge floods (Goswami 1985). Anthropogenic activities such as deforestation on upper slopes and poorly planned floodplain encroachments may also have aggravated the flood situation.
The Mekong, with a basin area of 795 000 km2, is an example of a large river with seasonal discharge in Southeast Asia. It is a seasonal monsoon river which episodically floods in the rainy season. This 4880 km long river runs on rock through narrow mountainous valleys for the first 3000 km, and flows freely on alluvium only for the last 600 km in a wide lowland that converges to a major delta. The Mekong therefore illustrates the contrasting nature and behaviour of a seasonal large river, on both rock and alluvium. It used to be a natural river but currently is being modified with dams, reservoirs and various other engineering structures in its basin. Such changes are discussed in Chapter 10.
The 6300 km long Changjiang (Yangtze) rises at 6000 m on the snow-covered Tibetan Plateau and flows eastward to the East China Sea. A number of rainfall and gauging stations have recorded the monsoon-driven rainfall and seasonal discharge for this 1.80 million km2 basin for years. Such information has been important for water management, especially concerning the recent construction of the Three Gorges Dam. A large proportion of rainfall over the basin is due to monsoon-driven precipitation from the warm air from the Pacific and Indian Oceans travelling up the valley between June and October.
Rain over the upper basin plus the snow and glacial melt on the Tibetan Plateau produce about half of the discharge of the river. The rest arrives mainly from the overflow of two lakes (Dongting and Poyang) in the middle Yangtze. Annual rainfall gradually increases downstream, from 400 mm in the upper basin to 1600 mm in the lower. The annual discharge of the river increases downstream: 1.4 × 104 m3 s−1 at Yichang (4300 km from the source); 2.3 × 104 m3 s−1 at Hankou near Wuhan (about 1000 km from Yichang); and 2.8 × 104 m3 s−1 at Datong (about 700 km further east) (Chen et al. 2001 and references therein). The discharge follows the seasonal precipitation but is slightly damped. The wet season floods in the upper Changjiang are caused by the steep rivers of Sichuan. The common sources of floodwater in the middle Changjiang below the three gorges are the Han River from the north joining the Changjiang at Wuhan, and the overflow from the Dongting and Poyang Lakes downstream.
Over the middle latitudes baroclinic conditions prevail. Over the Mississippi Basin this gives rise to frontal storms which result in snowfall in winter. Convective storms also occur, mostly in summer. The huge 3.2 million km2 Mississippi Basin is located generally in temperate climate but its eastern half is comparatively humid whereas the western part is relatively semiarid. Large differences occur in the basin between the winter and summer temperatures. Annual precipitation decreases east to west and also towards the north. The average annual totals vary: about 1400 mm near the mouth of the river in the south, more than 1000 mm in the east over most of the Ohio River basin, and less than 600 mm to the west in the basins of the Missouri and Arkansas Rivers. This results in a marked east–west reduction in runoff in the west of the Mississippi until the Rocky Mountains are reached. Higher runoff occurs to the east over most of the tributary basin of the Ohio River, even higher in the southern Appalachian Mountains. It is reduced towards the north, even lower towards the northwest basin. The runoff is less on the western Great Plains, but like precipitation, increases abruptly as the Rocky Mountains on the western boundary of the basin are approached. This disproportional distribution of precipitation and runoff is also reflected in episodic flood runoffs of the river (Knox 2007). Other large rivers of the middle latitudes are also maintained by combined flows of frontal rainfall, convectional summer rain, and the melting of glaciers.
The waters of five large arctic rivers (the Ob, Yenisei, and Lena in Eurasia and the Mackenzie and Yukon in North America) flow to the Arctic Ocean. Their runoff ranges between 250 mm and 500 mm and their drainage basins extend over a range of physiographic and bioclimatic zones. Apart from the mountains that occur within their catchment areas, their basins drain variations of tundra, taiga, mid-latitude forests, dry steppe, and semi-desert areas. All these rivers display a highly uneven seasonal pattern of runoff, primarily marked with melting of snow and ice giving rise to large spring floods. In general, the rivers are high from April to June, and low in summer and winter. The large rivers continue to flow through winter but not the smaller tributaries. Some of their discharge comes from rainfall but most of the runoff comes from snowmelt and melting of permafrost. Both the headwaters region in the mountains and the deltaic lowlands in the north, at the two ends of a river, remain frozen for several months. With higher temperature in summer, melting of permafrost, groundwater movement and landslides or bank failures occur in sequence. Permafrost is widespread in the Lena and the northern part of the Yukon Basin, but less in the other three. The annual range of Lena's runoff therefore is impressive, from a minimum of 366 m3 s−1 to a maximum of 241 000 m3 s−1 with a mean value of 16 530 m3 s−1 (see Chapter 11). The range of discharge in these large arctic rivers, however, is commonly reduced by the presence of many lakes and reservoirs in their lower courses.
Significant future increases in discharge are expected on account of current climate change and warming of the temperature in the arctic region. This is happening in the Eurasian rivers despite the construction of a number of dams and reservoirs.
Several large rivers flow through arid landscapes but manage to sustain their flow because of the high discharge arriving from the upper non-arid parts of their drainage basins. The Indus, for example, maintains its long lower course through the arid area of Pakistan by seasonal discharge from snowmelt and orographic monsoon rain that falls in the mountains of its upper course. Other large rivers with a significant part of their drainage basins arid include the Nile, Colorado, Niger, and Murray-Darling. Commonly, water in these rivers is utilised by construction of dams and reservoirs, and as such requires careful management (Chapter 9).
The 6500 km long Nile is a well-known example. It rises as the White Nile in wet Central Africa from Lake Victoria and flows north for about 2700 km through the Sahara Desert without any significant water input. On the other hand, a high rate of evapotranspiration occurs in the wetlands of the Sudd. The annual rainfall decreases from near 2000 mm in the Lake Victoria area to about 175 mm at Khartoum. The White Nile is joined at Khartoum by the Blue Nile and further downstream by the Atbara. Both streams are seasonal and monsoon-fed from the mountains of Ethiopia. About 85% of the annual flow of the Blue Nile is concentrated between July and October. The Atbara is even more seasonal (Woodward et al. 2007). A seasonally flood prone Nile then flows through Egypt to build a fertile delta.
Milliman and Farnsworth (2011) estimated that the rivers of the world altogether discharge about 36 000 km3 of water to the oceans annually. Given the pattern of global precipitation, rivers of northern South America and South, Southeast and East Asia contribute about half of this amount. Table 1.1 shows that large rivers of these regions provide most of this discharge. The discharge of the Amazon is particularly high, being 6300 km3 per year, a figure comparable with the total annual discharge of the next eight large rivers.
3.5 Sediment in Large Rivers
Meade (2007) described large rivers as massive conveyance systems for moving clastic sediment and dissolved matter over transcontinental distances. To illustrate, the Amazon and Orinoco are large rivers that transfer sediment for thousands of kilometres from