Читать книгу Introducing Large Rivers - Avijit Gupta - Страница 26

Meade (2007) described large rivers as massive conveyance systems for moving clastic sediment and dissolved matter over transcontinental distances. To illustrate, the Amazon and Orinoco are large rivers that transfer sediment for thousands of kilometres from the active margin of South America to its passive edge – from the Andes and its forelands in the west to the low floodplains and deltas of the two rivers on the east coast. The sediment is transferred between the source and the sink in steps, being alternatively transferred down-channel and stored in the valley for years in extensive alluvial plains, huge floodplains, or channel bars.

The mountains and forelands of the Andes are essentially the sources of the sediment of these rivers and such sediment is recycled as it travels in the downstream direction. The source of virtually all the modern sediments in the rivers is the Andes itself. The tributaries of the Amazon which do not originate in the Andes or its foothills, contribute very little sediment to the Amazon. This may also be true for other large rivers that originate in young fold mountains. In their study of global sediment, Milliman and Syvitiski have associated sediment sources with high mountains and stated ‘probably the entire tectonic milieu of fractured and brecciated rocks, oversteepened slopes, seismic, and volcanic activity, rather than simple elevation/relief …. promotes the large sediment yields from active orogenic belts.’ (Milliman and Syvitiski 1992, p. 540). Meade (2007) discussed (Figure 3.1) the example of the Orinoco River which collects water discharge from most of the basin irrespective of geology, but sediment only from the Andes and Llanos. Figure 3.1 also illustrates the progressive increase in discharges of water and suspended sediment downstream.

Examples of downstream sediment transfer are common. The Rio Madeira deposits a considerable amount of sediment as it emerges from the Andes Mountains to the lowlands of the Amazon. Some of this sediment is stored in the river but eventually moves downstream (Guyot et al. 1996; Dunne et al. 1998). About half of the sediment eroded from the Andes is deposited in the Andean foreland (Aalto et al. 2006) before removal. Before the Three Gorges Dam was closed, the Changjiang used to deposit nearly 100 million tonnes of sediment annually on floodplains and in lakes and stream channels between Yichang and Datong (Xu et al. 2007). The sediment was likely to have been derived from upstream mountains.

As the sediment emerges from the highlands, individual grains are as likely to be stored in the valleys of many large rivers as to travel downstream. When stored, they may remain at rest for a sufficiently long enough time to decompose in situ to a partly dissolved state which is removed by the river as solution load. The rest of the sediment remains in the solid state in the channel and is transferred downstream during high flows of the river as suspended load, or even as bed load if the grains are still big and the flow is powerful enough. Moving downstream via alternate storage and transfer, the sediment becomes mature in composition and over time may consist of more than 90% quartz, having lost rock fragments and feldspar by abrasion and decomposition. Of its maximum annual channel sediment load of about 1200 million tonnes at Óbidos, virtually all the suspended sediment of the Amazon comes from the Andes either via the main stem Amazon or one of its major tributaries, the Madeira, the headwaters of both starting deep in the Andes. The floods may take the sediment-laden water across the floodplain, deposit the sediment on the floodplain, and then during the falling-water season, return the clear water to the channel. The floodplain of the Amazon within Brazil measures about 90 000 km². The floodplains slowly grow vertically in floods and lose area by bank erosion associated with channel movement. These floodplains of Brazil (várzea) form a special environment with typical vegetation and animal life, strongly related to flood pulses. Junk et al. provide a detailed account of the physical and ecological characteristics of these floodplains in Chapter 5.

It is not uncommon for large rivers to transfer sediment both as downward flux and also sideways for floodplain-building (Goodbred and Kuehl 1998; Dietrich et al. 1999; Galy and France-Lanord 2001). The volume, however, is enormous for the Amazon River (Figure 3.3). If we consider the entire Amazon, more sediment moves laterally in and out of floodplains than as downstream flux (Meade 2007). Residence time for such sediment between the confluences of the Amazon with the Jutai and Madera has been estimated to be in the range of 1000–2000 years (Mertes et al. 1996). It could be longer, and thus sufficient time to change the proportion of sediment to nearly all quartz grains.

The lowermost gauging station on the Amazon is at Óbidos, about 1000 km above the sea. It is difficult to measure sediment deposition further downstream but apparently much sediment is deposited on floodplains and in partly filled floodplain lakes. Plumes of sediment are seen in satellite images entering the Atlantic from the Amazon mouth (Figure 3.4). Part of that sediment that passes through the sediment mouth settles on the ocean floor but the rest travels northeast along the coast to reach the Orinoco Delta. The outer Orinoco Delta includes more sediment from the Amazon than from the Orinoco. The Amazon probably provides the best example of large-scale sediment transfer but similar processes of sediment transfer and storage occur in other large rivers too.

Figure 3.3 Diagram showing average annual sediment movement between channels and floodplains of a 1500 km segment of the Amazon. (a) Schematic generalisation of average values for the entire 1500 km reach and map. (b) Individual sediment budgets for eight consecutive reaches of the main river between São Paulo de Olivença and São José do Amatari.

Source: Meade 2007 with details in Dunne et al. 1998.

Figure 3.4 Satellite image showing plumes of sediment entering the Atlantic from the Amazon mouth and then moving northeast along the coast to build part of the Orinoco Delta.

Source: NASA worldview application (https://worldview.earthdata.nasa.giv), part of the NASA Earth Observatory System Data and Information System (EOSDIS).

Depending on texture, fluvial sediment can be transported as dissolved load in solution, in suspension through the water column, and as bed load moving in traction along the river bottom. Dissolved load is finer than 0.62 μm. Suspended load, which is coarser than 0.62 μm, is often described in two parts: wash load (the finer fraction of suspended load that nearly always remains in suspension) and bed material load (the material which is carried by eddies only in high flow). The coarser part of the suspended load can be picked up from the bed and transported in suspension when the power of the river increases. Sand for example can be transported both as suspended and bed load. The coarsest material is usually moved discretely along the bed in very high flows, coming to rest on the bed at other times. Pebbles are dragged and rolled downstream as bed load but have been noted to travel in suspension in very big floods. Very coarse material such as cobbles and boulders are carried as bed load, except in extreme floods. Such catastrophic floods occurred when glacial lakes burst in overflows towards the end of the Pleistocene (Baker 1981, 2007). Very large floods, for example those caused by tropical cyclones, have been known to suspend pebbles and cobbles for short distances. This rarely happens, and boulder, cobbles, and pebbles are carried usually as bed load. Sand usually travels as bed load but in floods is often carried suspended by eddies. Silt and clay are transported as suspended load unless in aggregates. When separated, grains of such fine material may be transported either as suspended or dissolved load.

Data for clastic river sediment often only include total suspended load. Bed load is very difficult to measure. For many meandering rivers it is only a few per cent of the total load and considered to be within the margin of error for sampling (Milliman and Meade 1983 citing C. Nordin). This assumption probably does not work for sediment supplied by small mountainous streams or where large rivers cut through mountain ranges. The Brahmaputra, for example, is considered to be a river which carries a higher percentage (probably 30%) of bed load (Goswami 1985; Best et al. 2007).

The removal (erosion) of particles is a function of stream velocity. The critical erosion velocity of a particle is the velocity at which it starts to move. Once sediment grains are entrained, they can be transported with a velocity lower than that for entrainment. A sediment grain being carried either on bed or in water comes to rest (deposition) when the velocity of the river falls below the value needed to carry it. This is the fall velocity, related directly to grain size. Sand is the easiest to erode from the channel perimeter and bars. Pebbles, cobbles, and boulders require a higher critical erosion velocity but as they are bigger and heavier, requiring high fall velocity, they can be transported only for a short time or distance. Sand is carried longer. The critical erosion velocity of silt and clay, sticky and forming aggregates, is higher than sand, but once suspended in water they are transported for a long time and distance (Hjulström 1939).

This pattern of river transport leads to a sorting of material downstream, finer material travelling longer. In large rivers with room for deposition inside the valley, such sorting also happens across large bars and floodplains (Figure 3.5). The channel of a long river therefore displays pebble, cobbles, and boulders near the mountains and silt and clay near the sea. Sand is ubiquitous, and because of sorting and weathering on bars and floodplains increases in proportion along the channel. The modal size of sand grain, however, decreases downstream. This general pattern persists except where major rivers cross erodible fills or are joined by short tributaries bringing coarse sediment. The anomaly is corrected over a stretch below the confluence with the tributary downstream along the main river. The formation and development of floodplains and sorting of sediment grains across them is discussed by Junk et al. in Chapter 5.

Figure 3.5 The Ganga River. Changes in the grain size of bar material from Hardwar in the Himalayan foothills to Ganga Sagar on the delta. The coarsening of the bars in the middle reach is due to the contribution of southern rivers draining the Indian Peninsular.

Source: Singh 2007.

The total volume of sediment per unit time is considered as sediment load or sediment discharge. Sediment yield is the total load of the river divided by the upstream basin area. This assumption implies uniform load shedding from all parts of the basin which is incorrect as a very large part of the sediment on large rivers may come from the headwaters with high relief. Furthermore, the eroded sediment in the basin is not always transferred efficiently. Only about 10% of the total eroded sediment in the conterminous United States may reach the ocean (Milliman and Farnsworth 2011, referring to Holeman). Wasson et al. (1996) indicated that only about 1% of the entire eroded soil mass reaches the sea in Australia. Sediment discharge also varies with time, changes in vegetation cover, and anthropogenic alterations of the environment. The question of reliability is more relevant for sediment than water discharge. Milliman and Farnsworth (2011) opined that rounded figures are safer to use, attempted precise measurements are likely to be less accurate.

It is difficult to prioritise all the factors behind erosion and sediment supply to large rivers. Certain factors have been discussed by geomorphologists, such as relief, intensity and amount of rainfall, water discharge, the weathered nature of country rock, etc. (Milliman and Farnsworth 2011 and references therein). Numerical models have been proposed to compare the relative importance of such environmental factors for sediment discharge. For example, Syvitski and Milliman (2007) opined that geological factors explain 65% of the variation in sediment load, whereas climate and anthropogenic factors account for another 30%. The importance of these factors, however, vary among rivers, and anthropogenic modifications can significantly modify the natural pattern. For example, a series of dams have considerably reduced the volume of sediment that used to flow into the Mississippi River from the basin of its west bank tributary, the Missouri (as discussed in Chapter 8). Hovius and Leeder (1998) discussed the difficulty of establishing a reliable universal relationship between certain characteristics of drainage basins and sediment production. The difficulty arises mainly because of the varying importance of a series of tectonic, climatic and geomorphic processes, all three working in an integrated fashion to determine the sediment of a drainage basin.

The data sets used for these conclusions may include measurements from hundreds of rivers but not exclusively from large rivers. We therefore may not only need to prioritise certain basin properties for all rivers but also determine the relative contribution of individual smaller rivers and sum them to construct the total discharge of a specific large river. High young fold mountains, such as the Himalaya or Andes, are directly associated with high sediment discharge because of tectonics. Older ranges such as the Rockies or the Urals produce less sediment because of lack of tectonics and hardness of the older rocks. The orographic effect on precipitation adds to enhanced discharge increasing both runoff and sediment discharge of such streams. Briefly, the sediment discharge of large rivers increases directly with relief, lithology, tectonics, precipitation, and basin area.

According to Milliman and Farnsworth (2011) most large rivers with high dissolved loads have large drainage basins, drain high mountains, and carry a high runoff. Their list of a dozen major rivers with the highest dissolved loads includes six Himalayan rivers: the Changjiang, Irrawaddy, Ganga, Mekong, Salween, and Brahmaputra. The Amazon, Mississippi, Danube, MacKenzie, Parana, and St. Lawrence complete the list. In contrast, a large river may carry very little dissolved load, given its basin geology and low precipitation. Certain major river basins are dominated by a single lithology. The Zhujiang drains about 80% carbonate rocks whereas more than 80% of the basin of the Yukon is on shale. More than half of the St Lawrence basin is on shield rocks (Amoitte-Suchet et al. 2003). In brief, sediment yield of large rivers depends on several environmental factors: basin elevation, tectonics, lithology, precipitation, and basin area. All these factors determine the nature of sediment load a large river would carry.