Читать книгу Introducing Large Rivers - Avijit Gupta - Страница 38

Rising in the Himalaya, a convergent fold mountain range, the Ganga passes along a foredeep, joins with two other large rivers, the Brahmaputra and Meghna, and flows into the Bay of Bengal through the largest delta on Earth. The three rivers together drain over a million square kilometres, discharging the fourth highest water volume (970 × 10 km³) and the second highest annual suspended sediment load (about 1000 million tonnes) to the sea. Such figures are reached because the river drains a high tectonic mountain range with intense monsoon rain falling over the catchment area. Unlike the Amazon, the Ganga has a long history of anthropogenic utilisation of the water of its channel and the land of its basin. The two major diversions of the Ganga happen via the upper Ganga Canal at Haridwar and Lower Ganga Canal at Narora. Part of its water is diverted down the Hugli distributary channel at the Farakka Barrage. The falling quality of the river water, transferring more and more polluted material, is probably the biggest concern in river management.

The Bhagirathi, the main headwater of the Ganga, rises at 3800 m from the Gangotri Glacier. The river is known as the Ganga from Devprayag where it is joined by the Alaknanda. After another 300 km of river distance, the Ganga descends to 290 MSL at Haridwar. Following the steep descent, the river flows parallel to the mountains along the extensive low-gradient alluvial plains of the Himalayan foreland, a subsiding continental interior setting.

The Ganga turns east and flows along a wide alluvial plain built on the foreland by the main river and its tributaries (Figure 4.7). The river receives both water and sediment from the Himalayas to the north and the cratons of the Peninsular India to the south. All along its course, the river is joined from the north by a number of large Himalayan tributaries at intervals of hundreds of kilometres. The Ganga and these tributaries have built huge alluvial fans (megafans) at the Himalayan highland–lowland contact. Some of these megafans have been studied in detail, e.g. the Kosi (Box 4.1).

The tributaries from the south drain the old rocks of the northern edge of the Indian Peninsula, contributing a smaller amount of water and coarser sediment. Several of these tributary streams flow into the Yamuna, the biggest tributary of the Ganga, instead of directly joining the Ganga. The Yamuna is a Himalayan river and at its confluence with the Ganga at Allahabad, it contributes about 59% of a combined discharge of 130 × 10 m³ (Das Gupta 1984).

The Ganga turns south at the eastern margin of its large alluvial plain through a gap in the basaltic low hills of Rajmahal, a short distance above the apex of its delta. One of its two main distributaries (the Bhagirathi or Hugli) flows south within India, collecting drainage from the northeastern corner of the Indian Peninsula. The other major distributary, the Padma, carries most of the water into Bangladesh where it meets with the Brahmaputra and then the Meghna. These three rivers combine to build and maintain the large Ganga-Brahmaputra Delta.

Figure 4.7 The Ganga from satellite imagery.

Source: NASA Worldview application (https://worldview.earthdata.nasa.gov), part of the NASA EOSDIS.