Читать книгу Introducing Large Rivers - Avijit Gupta - Страница 12

A number of individual large rivers have been studied and such studies published discretely. A collection of advanced essays on the general characteristics of large rivers, their selected case studies, and their utilisation and management is also available (Gupta 2007). In comparison, this volume is primarily an integrated textbook on large rivers and introduces the reader to the morphology and management of these huge conduits on which both the general physiography of the basins and utilisation of the resources of the rivers depend.

The discussion on large rivers starts with an account of their geological framework (Chapter 2) that determines where they can be located and also what their physical characteristics would be. The geological framework of a large river is based primarily on large-scale tectonics commonly driven by plate movements. An uplifted zone and the adjoining subcontinental-scale water catchment area are necessary requirements for a big river. Smaller tectonic movements may further modify the basin and the channel and explain their detailed morphological characteristics.

Figure 1.2 The Mekong. (a) On rock, downstream of Chiang Saen, northern Thailand. (b) On alluvium near Savanakhet, Lao PDR, photographed from the air. Note the difference in form and behaviour between the two reaches. Large rivers commonly are a combination of a number of similar variations.

Source: A. Gupta.

The regional geology should create a drainage basin large enough to accumulate enough precipitation to support and maintain the big river. Chapter 3 discusses the nature of water and sediment in a large river. The discharge in a large river is determined by various climatic criteria depending on its location: annual rainfall, seasonality in rainfall, and high episodic rain from synoptic disturbances such as tropical cyclones. The supply of water to large rivers could be from almost all parts of the watershed but the sediment supply generally is associated selectively with high mountains. For example, the discharge of the Orinoco is collected from most of the basin, irrespective of geology or relief, but its sediment supply is only from the Andes Mountains and the alluvial Llanos plains formed near the Andean foothills. In certain cases, several large rivers flow through arid landscapes without identifiable addition to their discharge but manage to sustain their flow because of the high discharge arriving from the upper non-arid parts of their drainage basins. Sediment in flood moves in large rivers both in downstream and lateral directions if large floodplains are present. The sediment grains travel a long distance to reach the sea and, in the process, become mature and sorted.

Large rivers have been aptly described as massive conveyance systems that move detrital sediment and dissolved matter over transcontinental distances (Meade 2007). Their morphology is dependent on regional geology, discharge and sediment flux, and may change several times between the headwaters and the sea (Chapter 4). Morphologically a large river usually has a channel flanked by bars, floodplain, and terrace fragments. The channel pattern depends on the gradient of the river and the nature of water and sediment it transports, and the pattern varies among different rivers as they adjust to the local physical environment. Floodplains of large rivers are important not only for their origin and age but also for their ecology which supports a wide variety of species, and their economic utilisation by people. The role of flood pulses in the maintenance of the floodplains and its ecology is crucial. This is discussed in detail by Junk et al., in one of the two invited chapters in this book (Chapter 5). The huge discharge of water and sediment that is deposited by a big river in the sea may create a large delta. Deltas are morphologically fragile and change over time (Chapter 6). Deltas of many large rivers support a large population, and hence are of importance.

Large rivers could be geologically long-lived rivers such as the Mississippi or the Nile. A river that exists for a long time has a history. Tectonic processes commonly influence the origin, geographical location, and modification of major rivers. Understanding of such rivers requires knowledge of their history as rivers have changed episodically through tectonic movements, and especially through climate and sea-level changes in the Quaternary (Chapter 7).

Large rivers are a useful resource to people. A proper utilisation (Chapter 8) and management (Chapter 9) of large rivers is important. The land use of their basins and the use of their water have modified the environment over years of human civilisation. This has led to alteration of large rivers and their basins at various levels, especially over the last hundred years. The form and behaviour of many of the present large rivers have been modified mainly due to construction of large dams and reservoirs. The present state of a large river is conditioned by both the original physical environment of the basin and anthropogenic alterations imposed on the channel.

This requires proper management of the rivers so that basinal economic development and environmental degradation can be balanced in a sustainable way. A management procedure which simultaneously allows both economic development and environmental sustenance needs to be chosen. As a large river usually flows across multiple countries, each with different expectations and varying ability of resource utilisation, there is also a political aspect of large river management.

Chapter 10 deals exclusively with the Mekong River as a case study to illustrate the techniques and problems of managing a multistate river in a complex physical environment. It illustrates the reality of river management which involves dealing with the complexity of the physical characteristics of a big river, meeting the different expectancies of multiple stakeholders of the river basin, and maintaining the quality of the river for future generations, all at the same time.

Chapter 11 is on the special case of major rivers in the arctic. It deals mainly with the Lena, Yenisei and Ob in Siberia and the Mackenzie and Yukon in North America. These rivers flow through a unique environment and are expected to go through large changes in the near future due to global warming. This discussion on arctic rivers by Slaymaker is the second invited contribution in this book.

The last chapter deals with the possible modifications of large rivers in the near future. They may undergo significant changes following climate change and construction of large-scale engineering structures. The general tenets of climate change are known and accepted, but we have limited knowledge regarding its impact on large rivers. We, however, need to consider the future for understanding and management of present large rivers, as such changes would impact the lifestyles of a very large number of people, as the rivers of the future are likely to be different.