Introducing Large Rivers. Avijit Gupta
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decreases from the east to west, from 1600 to 500 mm. The southwestern basin is relatively dry. As expected, rainfall increases on the Himalayan slopes, reaching 1500–2300 mm (Singh 2007). The upper basin also receives some summer snowmelt from the Himalaya. More than 70% of the annual rainfall, rising to 80% in certain locations, arrives between July and early October in the wet monsoon system. The rain often falls intensely and in episodic tropical storms, some reaching cyclonic status, over the lower basin leading to floods.
The flow of the Ganga reflects both seasonality of rainfall and stepwise increment in discharge where the major tributaries, such as the Yamuna, Gomati, Ghaghara, Gandak, Son, and Kosi join the trunk steam. Half of the annual rainfall enters the river as surface runoff, 30% is lost by evaporation, and 20% seeps to the subsurface. During the dry season, part of the subsurface water flows through the high alluvial banks of the Ganga to its channel as baseflow. The mean discharge of the Ganga at Farakka, before it divides into its deltaic distributaries, is 70 547 m3 s−1. About 60% of this arrives from the Himalaya and the northern plains (Das Gupta 1984).
4.4.3 Sediment Load
The sediment load of the Ganga comes mostly from the tectonic Himalaya Mountains. Chemical weathering is not significant in the mountains and the solution load is low, being diluted even further when the discharge is high in the wet monsoon. The suspended and bed load of the Ganga is very high, the suspended load the second highest in the world, superseded only by the Amazon. The annual suspended load of the Ganga has been estimated by Milliman and Syvitski (1992) as 520 million tonnes. About 90% of the sediment travels during the wet monsoon (Singh 2007). The bed load of a large river is difficult to measure but Wasson has estimated that 600–2500 million tonnes of bed load reaches the delta of the river each year. Most of the sediment arrives in the Ganga from the Himalayas along the large tributaries that originate in the mountains and the foothills (Sinha and Friend 1994). The tributaries that come from the south drain the old cratonic rocks of Peninsular India and contribute a high proportion of coarse sediment.
The change in bed material of the Ganga from Haridwar at the foot of the Himalaya Mountains to Ganga Sagar where one of the major distributary channels, the Hugli, enters the Bay of Bengal is plotted in Figure 3.5. Measured from bar samples, it indicates the general downstream fining characteristic of the Ganga, interrupted by periodic coarsening of the bed by contributions from large tributaries (Singh 1996). The bar sediment of the Ganga is essentially sand, the mineralogy of which is primarily quartz with minor amounts of feldspars, micas, and rock fragments. Material from the weathered source-rocks undergoes further alteration when grains are stored as part of floodplain alluvium between their transportation in high flows (Wasson 2003).
4.4.4 Morphology
In the Himalaya Mountains, Ganga and its headwater tributaries flow in narrow, gorge-like valleys flanked by small discontinuous patches of floodplains and terraces. After emerging from the mountains, in the wide alluvial plain between the Himalaya and the cratonic Peninsular India, the rivers are entrenched below the surface of the plain. The channel of the Ganga is confined within a 10–25 km wide elongated lowland, bounded by alluvial cliffs, several metres high. The cliffs are eroded by gullies and small ravines. The lowland bounded by the cliffs comprise: (i) the channel of the Ganga commonly displaying braid bars and meander scars, (ii) the floodplain, (iii) terrace-like features which are higher than the floodplain, and (iv) miscellaneous wetlands. The river channel and the floodplain together extend up to 3 km in width. Large sand bars, kilometres in length, are common in the channel. The Ganga remains confined within this cliff-bounded valley and rarely overtops the cliffs, even in large floods. The floodplain and terrace-like features, however, are inundated periodically, the frequency depending on their height above the channel. The channel pattern of the Ganga changes from place to place (Figure 4.8), and the wide valley between alluvial cliffs is modified by various agencies: the main channel of the Ganga; the smaller channels; the miscellaneous water bodies on the floodplain; and the alteration of the cliff slopes.
Being a seasonal river, the channel of the Ganga is full of bars and multiple channels in the dry season. Huge kilometre-scale bars consisting of braid bars, lateral bars, and point bars are visible at low flow. The low flow effect is enhanced by the large-scale transfer of water into irrigation canals or to meet other demands. In places, the river displays a meandering pattern with point bars and local narrowing of the channel due to extensions of peninsular lineaments under alluvium. The river may briefly change its direction in such locations. The meandering pattern using the entire channel commonly appears in high flows when nearly the entire channel is under water and mid-channel bars are submerged and removed. During the dry season, however, a braided pattern may re-emerge between the cliffs, as has been described for the Narmada (Gupta et al. 1999), for rivers of monsoon areas.
Figure 4.8 The Ganga from satellite imagery in alluvium, bars and bends.
Source: NASA Worldview application (https://worldview.earthdata.nasa.gov), part of the NASA EOSDIS.
The bars occur at several levels related to the frequency of inundation. The higher ones are under vegetation and usually farmed. Sediment transfer varies between seasons. During the dry period, it is confined to deeper sub-channels. During the wet monsoon, sediment travels across the entire channel and occasionally even over the floodplain. Several metres of sediment are scoured from temporary storage on top of the floodplain in high flows (Shukla et al. 1999). The general channel pattern remains the same but the location and geometry of the bars vary over time. The river currently tends to shift only several kilometres within the high cliffs.
The huge Ganga-Brahmaputra Delta is discussed in Chapters 6 and 7.
4.5 Morphology of Large Rivers: Commonality and Variations
A review of the morphology and behaviour of the Amazon and Ganga highlights characteristics common to many large rivers. The origin, geographic extension and physical characteristics for many depend primarily on plate tectonics. Certain large rivers have existed for a long time, and most of them reflect repeated changes they have undergone during the Quaternary, concerning the geography of their basins and nature of their course. Regional structural features such as an arching bedrock underneath the channel alluvium or a network of faults modify the general characteristics for the river flowing over such structural features. Large rivers commonly consist of a number of reaches of variable morphology longitudinally assembled to form a big river. Smaller rivers, in contrast, tend to be monotonic in nature (Lewin and Ashworth 2014).
All large rivers are maintained by a large volume of precipitation falling on their basins, at least over a significant part of them. The precipitation can be uniform or seasonal. Given the large size of their basins, large-scale climatic variations such as the ENSO are related to fluctuations in discharge. Such fluctuations may bring in both dry and wet periods. Floods tend to occur in the wet years and from cyclonic disturbances.
Most of the sediment load comes from the high mountains and travels downstream in stages, interrupted by periods of storage in floodplains, on bars, and on bed. As a result, the sediment of a long river becomes progressively enriched in quartz grains and demonstrates textural sorting along the river, the modal class being medium and fine quartz sand. Where tributaries carrying coarse sediment join the main river, local coarsening of the bed material happens for a short distance immediately downstream of the confluence.
Morphologically a large river includes a channel, floodplain, and probably terrace fragments. The channel pattern depends