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Discharge of large rivers is derived from (i) the nature of precipitation falling on their basins, (ii) melting of ice and snow in spring and early summer, if present within the basin, and (iii) stepwise contribution of the tributaries to the main streams.

Figure 3.2 demonstrates the annual hydrograph of several major rivers from different climates as examples of basins operating in different climatic zones, starting with the Amazon whose huge basin of nearly 7 million km² extends to both sides of the Equator. The precipitation over the basin is dominated by the annual pattern of movement of ITCZ. Precipitation over the Andean regions is affected also by the South Atlantic Convergence Zone (SACZ) which has a seasonal variation. Precipitation is thus generally uniformly distributed over the basin throughout the year, with orographic enhancement in the west. The average annual precipitation is around 2000–2500 mm of rain, geographically ranging from less than 2000 mm over the northeastern and southern basin to 7000–8000 mm over the lower slopes of the Peruvian Andes (Mertes and Dunne 2007 and references therein).

Figure 3.2 Average annual hydrograph of selected large rivers as examples: the Amazon, Congo, Ganga, Brahmaputra, Mekong, Changjiang, Mackenzie, and Nile.

Source: Wohl 2007.

The rainy season starts in November and December in the southern basin but two months later in the northern part, the pattern followed by the discharge of the Amazon. The peak discharge is attained approximately in May–June, the date depending on the location of the gauging station. Mertes and Dunne (2007) described the annual pattern of discharge as unimodal and damped. The arrival of the rainwater in the Amazon is delayed by the length and size of the drainage network in the basin, and especially by temporary storage in more than 100 000 km² of the huge floodplain of the Amazon (see Chapter 5 for a detailed discussion). Temporal offsets also occur between the northern and southern tributaries of the Amazon (Mertes and Dunne 2007; Richey et al. 1989). The ENSO system controls the size of the annual discharge. The El Niño years tend to be drier in the basin, especially over the northern part. In contrast, higher rainfall and greater discharges happen in the La Niña years. Such climatic fluctuations not only affect discharge and inundation along the river but also erosive activities in the headwaters and subsequent sediment load of the Amazon.

The discharge pattern of the Congo, whose huge 3.74 million km² drainage basin stretches across the Equatorial region and beyond, illustrates the combined effect of (i) the annual movement of the ITCZ across the basin with (ii) the seasonal wet and dry regime of the headwater tributaries of the Congo coming from the north and south edges of the basin. Flow regimes are different at different locations of the main river. The Congo River receives 1800–2400 mm of rainfall over its middle part with no dry season. A distinct wet and dry distribution, however, characterises the discharge of the streams in the southern regions, as in the Shaba Highlands, which are sub-humid and located in the southern hemisphere. There a peak in discharge occurs between March and May and a conspicuous dryness between September and November. In comparison, a marked high discharge is reached between September and November and the lowest between February and April in the major tributary, the Oubangui River (also known as Ubangi), which drains part of the basin north of the Equator and joins the Congo below its middle section. This derivation from both equatorial and seasonal climates near the source and downstream makes the annual hydrograph of the Congo River complicated, as shown for Kinshasa with a double peaked hydrograph (Figure 3.2). The average annual discharge of the river is 46 200 m³ s⁻¹ in Kinshasa and the highest recorded discharge is 64 900 m³ s⁻¹. The variability of flow is swamped by the heavy annual rainfall that regularly falls over the central basin. As expected, variability in flow increases towards the headwaters, away from the Equator (Runge 2007 and references therein).

Moving away from the Equatorial basins, the Ganga, Brahmaputra, Irrawaddy, and Mekong provide excellent examples of monsoon-supported seasonal large rivers in South and Southeast Asia. Most of the annual rainfall arrives in the southwest monsoon with the rain-bearing system moving upstream from the coast. Both the Ganga and Brahmaputra also demonstrate (i) the effect of orographic lifting against the southern slopes of the Himalaya Mountains and (ii) a limited addition of meltwater in summer with melting of snow and ice on slopes.

In the Ganga Basin, the wet southwest monsoon is responsible for 70–80% of the annual rainfall (a figure that rises to 90% in places) between July and September. The average annual rainfall decreases westward over the basin, from about 1000 mm in Bengal to 500 mm in Uttar Pradesh and Haryana, then rising to about 2300 mm on the Himalayan slopes. The rain is seasonal and can be episodic and intense. The Ganga receives its flow from rainfall, subsurface flow, and meltwater from the glacier at the head of the river. Large contributions are added stepwise from various tributaries coming from both the Himalaya Mountain in the north and the peninsular craton to the south. Each square kilometre of the Ganga Basin annually receives 1 million m³ of water as rainfall. Of this 30% is lost through evaporation, 20% seeps to the subsurface, and the remaining 50% is available as surface runoff (Das Gupta 1984). Annual flooding is typical of all rivers in the plain of the Ganga. Flooding is commonly caused by the arrival of episodic rain-bearing circulations arriving in the wet season when the ground is already wet and the channel full.

The drainage basin of the Brahmaputra is more complicated, being an assemblage of varying climatic and hydrologic zones (Singh 2007). It is cold and arid in Tibet in the rain shadow of the Himalaya, but humid tropical or subtropical in the rest of the basin. Outside Tibet, the main source of the discharge of the river is the wet southwestern monsoon, operating from June to September. The annual rainfall varies from a low of 300 mm in Tibet to about 5000 mm in the eastern basin where the river crosses 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.