Introducing Large Rivers. Avijit Gupta
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.
3.6 Conclusion
Existence of a large river requires a considerable amount of water and sediment. For this, its drainage basin needs to be big enough to collect a necessary volume of water, large enough to nourish the main stream. At least part of the basin should be in an area of high rainfall so that the main river can be sustained all along its course. The volume of discharge may be annually seasonal, and also may vary over a group of years as directed by climate drivers such as ENSO. The drainage basin may contain sources of high sediment production which commonly comes from a high relief and erosive fold mountain. Sediment grains in long rivers tend to demonstrate size-sorting and quartz enrichment in the downstream direction. Many large rivers transport and store sediment also in a lateral direction, producing dynamic growth and decay of floodplains. Very large volumes of water and sediment are generally needed to sustain large rivers but the patterns of their supply and distribution are different.
Questions
1 Explain the difference between discharge and runoff.
2 What are the sources of big discharges of large rivers? Give examples.
3 Explain the difference between barotropic and barclinic conditions. How do they relate to large rivers?
4 What is a climate driver? How does a short-term climate driver relate to floods and dry conditions?
5 Name five large rivers that flow through arid environments. How do they make it possible?
6 Explain the lateral transport and storage of sediment in large rivers. What effect does it have on floodplains?
7 Describe the changes in sediment