Introducing Large Rivers. Avijit Gupta
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consumption of water which is not returned to the river
Of all the properties of a large river, a high discharge is the one most expected (Potter 1978). This implies that at least part of the river basin lies in an area of high precipitation, or the area of the river basin is so huge that the cumulative flow in the trunk stream reaches a large volume, or both. The Amazon, the river with the biggest discharge, has a huge basin. It rains and snows heavily over its headwater basins in the Andes Mountains. Large volumes of discharge are contributed by its many tributaries draining the enormous basin. Its neighbour, the Orinoco, exhibits a similar pattern. The Orinoco is fed by drainage from the Andes, the Andean foreland (Llanos), and the Guiana Shield, a craton (Figure 3.1). On average, every square kilometre of its drainage area carries more water than even the basin of the Amazon.
A large river is presumed to have a huge discharge, and large rivers commonly are found in areas of high precipitation, usually determined by the pattern of global climate, and often on the windward slopes of high mountains. In comparison, precipitation over the north-flowing rivers of Eurasia, the Ob, Yenisei, and Lena, are not high but these still have become large rivers because of the collective precipitation falling over their huge catchment areas. The evapotranspiration is low due to prevailing low temperature.
There are variations from such simple explanations. The basin of the Indus is mostly arid, but the water collected in the headwater-mountains from local high annual precipitation maintains its large channel across the dry lower part of the drainage basin. In brief, discharge of a river is derived from both climate and size. By plotting basin area against mean annual discharge for 1100 rivers, Milliman and Farnsworth (2011) demonstrated that 68% of the variance in discharge for the same drainage area can be explained by their climatic characteristics.
We should note that the discharge given for a river refers to the discharge at a specific measurement station. The last gauging station of a large river is located not at its mouth but usually several hundred kilometres upstream, near the end of its tidal limit. The last discharge station on the Amazon is at Óbidos, about 1000 km above its mouth. The last station on the Changjiang is at Datong, 600 km from the sea. Our knowledge about the discharge of water and sediment over the last few hundred kilometres on large rivers is limited.
3.3 Global Pattern of Precipitation
The primary source of pre-precipitation moisture is the atmosphere. Precipitation requires cooling of moist air by upward convection or mixing between two air masses of different temperature. The moist air becomes saturated by cooling and condensation. With further cooling, the moisture falls as rain or snow, depending on the ambient temperature.
Most of the atmospheric water is stored in the troposphere, especially in the warm air of tropical latitudes (Hayden 1988). Evaporation from the seas happens efficiently in the warm climate in the tropics, adding moisture to the atmosphere. Evaporation of more than 60% of water takes places between 30° north and south latitudes. In contrast, only about 5% of total evaporation takes place beyond the 50th parallels. More than 60% of evaporation is from the oceans (Lamb 1972; Hayden 1988). This implies a higher presence of moisture in the tropical air and on the windward side of continents. The stored atmospheric water is condensed before precipitation in two broad ways: barotropic and baroclinic.
Barotropic conditions prevail in the low latitudes. In a barotropic atmosphere, the horizontal thermal gradients are small. The condensation is carried out by vertical lifting of the heated air, and where vertical wind shear is low, may give rise to huge convective clouds. Lifting of humid air is accompanied by a continuous production of latent heat by condensation which uplifts air in the barotropic atmosphere in several ways. The uplift commonly happens by:
Figure 3.1 Average discharges of (a) suspended sediment and (b) water in the Orinoco River and its tributaries.
Source: Meade 2007 and references therein.
Convergence of northeast and southeast trade winds along the Intertropical Convergence Zone (ITCZ).
Circulation of air giving rise to tropical storms (which may reach even the rotating velocity of tropical cyclones) and easterly waves.
Orographic uplift of air when moist airstreams reach the windward slopes of mountain regions.
A high amount of rain may fall where such conditions are fulfilled, increasing river discharge. Where such conditions are weak or absent in low latitudes, arid conditions prevail, as in North Africa or Central Australia, and large rivers are either absent or survive only by importing a high discharge from the upstream basin area. The ITCZ and its associated belt of rainfall moves north and south annually, giving rise to a pronounced seasonality in rainfall. A pattern of rainy summer and dry winter is known as the monsoon system which brings copious rainfall to many parts of the tropical world, especially where the incoming moist summer air is lifted against an orographic zone. The southern slopes of the Himalaya and the eastern slopes of the Andes are excellent examples. Both regions nurture a set of major rivers.
Episodic rainfall occurs from large-scale cyclonic circulations in the lower latitudes, some of which may develop into tropical cyclones producing destructive and heavy rainfall (for details, see Gupta 2011). Tropical cyclones generally do not form near the Equator or over the South Atlantic but are found in other parts of the tropics. These storms tend to give rise to immense volumes of rainfall while moving west within the belt of trade winds. Significant rainfall in the tropics also occurs from the converging meteorological phenomenon known as the easterly waves. A number of large rivers thus exist in the tropics.
A baroclinic atmosphere is typical of extratropical latitudes with sharp horizontal thermal contrasts. The contact between two converging air masses with different level of properties, such as pressure, temperature, and moisture, is known as a front. For example, in the northern hemisphere, a front could be a meeting of dry cold polar air coming from the north and wet and warmer air coming from the south. The horizontal contrast in pressure and temperature is followed by a vertical movement of air. The warmer air rises above the colder one which leads to cooling, condensation, and precipitation. A jet stream, if present at a level high above the front, increases its intensity.
The frontal storms of the baroclinic atmosphere are large but variable in size. Diameters range from several hundred to a thousand kilometres. Hayden (1988) has described the areas of precipitation from such storms as matching the size of large river basins of the middle latitudes. Usually, along a frontal area, multiple storms occur, following one another, filling the channels and flooding the rivers. Although compared with the deep convection pattern of the barotropical atmosphere, rainfall rates are much less, the compensating longevity of baroclinic systems leads to a substantial amount of rainfall.
Flooding may also occur from melting of snow and ice, accumulated earlier, from a number of storms in the winter season. Flooding in the middle latitudes therefore often happens in spring or early summer. A second source of river discharge therefore is the accumulated snow and ice on the land surface of river basins which melts into annual floods as the climate turns warmer.
We can therefore have two classes: a low-latitude barotropic and a higher-latitude baroclinic section. This pattern controls the rise and fall of the river hydrographs and floods. Large floods in big rivers occur under specific circumstances. For example, in the tropics, cyclonic circulations give rise to large rain-bearing storms which may develop up to the strength of tropical cyclones. Heavy, intensive, and episodic rainfall from such storms commonly arrives in the middle of the wet season when the river is high and the ground is wet, giving rise to flood discharges, extensive erosion, and sediment transfer (Gabet et