Читать книгу Introducing Large Rivers - Avijit Gupta - Страница 11

We have an intuitive recognition of large rivers although a proper definition is elusive. Even though it is difficult to define a large river, we would probably select the same 15 or 20 rivers as the biggest in the world. Potter identified four characteristic properties of large rivers: they drain big basins; they are very long; they carry a large volume of water; and they transfer a considerable amount of sediment (Potter 1978). It is, however, difficult to attribute quantitative thresholds to these, and not all big rivers exhibit these four characteristics. We associate large rivers with high discharge and sediment transfer, but both water and sediment vary over time and space and their data are difficult to acquire. It is easier to identify large rivers by the size of their drainage basins and their lengths; both are easier to measure.

Based on the areal extent of their drainage basin, Potter (1978) examined 50 of the world's largest rivers, ranked by Inman and Nordstrom (1971), starting with the Amazon. All but one of these rivers are more than 10³ km long, and the smallest drainage basin is about 10⁵ km². These 50 rivers collectively drain about 47% of the land mass, excluding Greenland and Antarctica. The Amazon alone drains about 5% of the continental area. These rivers also have modified the physiography of a large part of the world. Table 1.1 lists the top 24 large rivers (Figure 1.1), ranked according to their average annual water discharge. Their ranks would change if the rivers were listed according to any of the other three properties.

There are other lists. Hovius (1998) tabulated the morphometric, climatic, hydrologic, transport, and denudation data for 97 river basins, all of which measured above 2.5 × 10⁴ km². Meade (1996) ranked the top 25 rivers twice: first, according to their discharge; and second, according to their suspended sediment load. The two lists do not match well. For example, large rivers such as the Zambezi or Lena carry a large water discharge but a low sediment load. Impoundments too have drastically reduced the once high sediment load of many rivers such as the Mississippi-Missouri. Over approximately the last 100 years, many rivers have been modified by engineering structures such as dams and reservoirs. The Colorado or the Huanghe (Yellow River) at present may not flow to the sea round the year. Such changes have also reduced the amount of sediment that passes from the land to the coastal waters. Large rivers such as the Nile or Indus have been associated with human civilisation for thousands of years and show expected modifications.

Table 1.1 Selected characteristics of 24 large rivers.

River	Average annual water discharge (106 m3)	Length (km)	Drainage basin area (km2)	Current average annual suspended sediment discharge (106 t)
1. Amazon	6300	6000	5.9	1000–1300
2. Congo	1250	4370	3.75	43
3. Orinoco	1200	770	1.1	150
4. Ganga-Brahmaputra	970	B-2900 G-2525	1.06 (B-0.63)	900–1200
5. Changjiang	900	6300	1.9	480
6. Yenisey	630	5940	2.62	5
7. Mississippi	530	6000	3.22	210
8. Lena	510	4300	2.49	11
9. Mekong	470	4880	0.79	150–170
10. Paranẚ-Uruguay	470	3965	2.6	100
11. St. Lawrence	450	3100	1.02	3
12. Irrawaddy	430	2010	0.41	260
13. Ob	400	>5570	2.77	16
14. Amur	325	4060	2.05	52
15. MacKenzie	310	4200	2.00	100
16. Zhujiang	300	2197	0.41	80
17. Salween	300	2820	0.27	About 100
18. Columbia	250	2200	0.66	8
19. Indus	240	3000	0.97	50
20. Magdalena	240	1540	0.26	220
21. Zambezi	220	2575	1.32	20
22. Danube	210	2860	0.82	40
23. Yukon	195	3200	0.83	60
24. Niger	190	4100	2.27	40

These figures vary between sources, although perhaps given the dimensions, such variations are proportionally negligible. Discharge and sediment figures are from Meade (1996) and Gupta (2007) and references therein. Drainage areas are rounded off to 10⁶ km to reduce discrepancies between various sources. The Nile is not listed, even though it is 6500 km long. It does not qualify for this table as its water and sediment discharges are relatively low.

The great lengths of these rivers allow them to flow across a range of environments. The Mekong, for example, flows on both rock and alluvium, looking different (Figure 1.2). The end part of the river needs to adjust to all such environmental variations plus the Quaternary changes in sea level.

Fluvial geomorphology generally is based on small and logistically manageable streams. A study of large rivers is necessary, although difficult, for multiple reasons. Large rivers form and modify subcontinental-scale landforms and geomorphological processes. A high number of them convey and discharge a large volume of water and sediment to the coastal seas. An understanding of modern large rivers helps us to explain past sedimentary deposits. Large rivers, such as the Amazon (Mertes and Dunne 2007), and their deposits may reveal basinal and regional tectonics, past and present climate, and sea-level fluctuations. Management of the water resources of a large river is often an essential step toward the supply of water and power to a large number of people. We need to study large rivers for many such reasons.

Figure 1.1 A sketch map showing the location of 24 large rivers in the world: 1, Amazon; 2, Congo; 3, Orinoco; 4, Ganga-Brahmaputra; 5, Changjiang; 6, Yenisei; 7, Mississippi; 8, Lena; 9, Mekong; 10, Parana-Uruguay; 11, St. Lawrence; 12, Irrawaddy; 13, Ob; 14, Amur; 15, Mackenzie; 16, Zhujiang; 17, Salween; 18, Columbia; 19, Indus; 20, Magdalena; 21, Zambezi; 22, Danube; 23, Yukon; 24, Niger.