Introducing Large Rivers. Avijit Gupta
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4.4 and 4.5). Plate tectonics control the location and lithologic and topographic frameworks of the Amazon (Potter 1978). The basin was delineated following the Miocene uplift of the tectonically and volcanically active Andes due to the subduction of the Nazca Plate below the South American Plate. Exploration for petroleum has revealed the underlying structure along the channel and floodplain of the Andes (Mertes and Dunne 2007). Evidence from deep cores indicates an east–west crustal sag underneath the basin axis at a depth of 6000 m that links with a graben, the Marajó Rift, roughly located underneath the mouth of the river. It is a very large river with low gradient and limited power (Baker and Costa 1987), even at the average peak discharge (12 W m−2), which generally carries a huge amount of sediment, but finer than about 0.5 mm.
4.3.2 Hydrology
The discharge of the Amazon comes essentially from precipitation although some snowmelt is derived from the Andes. The precipitation pattern of the region is controlled by the annual shifting of the Intertropical Convergence Zone (ITCZ) and the South Atlantic Convergence Zone (SACZ). Average annual basin precipitation is about 2000–2500 mm, which is near-uniformly distributed over most of the basin. The maximum precipitation of 7000–8000 mm falls on the lower eastern slopes of the Peruvian Andes and, in contrast, the extreme northern and southern parts of the basin are relatively dry. The rain arrives first over the southern basin in November to December, and then moves north. The annual hydrograph of the river (Figure 3.2) is unimodal and damped. The river floods regularly (Chapter 5), but the rising of the river stage is slowed by the sheer size of the basin, length of the drainage network, and storage of water in the enormous floodplains which has a cumulative size approaching 100 000 km2. The precipitation is affected by El Niño Southern Oscillation (ENSO) and so is the discharge. Low flows of the Amazon occur in the El Niño years (Mertes and Dunne 2007). Such climatic fluctuations affect flooding and sedimentation (Aalto et al. 2003) but their effect on morphology and behaviour of the regional smaller rivers is yet to be understood.
4.3.3 Sediment Load
Only 800 000 km2 of the Andes and Sub-Andes falls within the 7 million km2 Amazon Basin but this area contributes almost the entire sediment load of the river. Throughout the Late Cenozoic, 500–600 million tonnes of sediment arrived annually from this source, although the foreland basins trapped approximately half of the amount to build fans (Mertes and Dunne 2007, referring to Guyot 1993). The foreland basins sag in response to the rise of the Andes, and its erosion, reducing the river gradients to very low figures. Only fine sediment (<0.5 mm) reaches the lowland Amazon flowing within Brazil. No significant sediment is supplied from the old rocks of the Brazil and Guyana shields, but from there a huge volume of water reaches the river. In brief, the main stem discharge of the Amazon is augmented cumulatively by a number of tributaries draining different parts of the basin, but the sediment supply comes almost exclusively from mountains of high relief at the head of the basin with tectonically fractured rocks and oversteepened slopes as described by Milliman and Syvitski (1992). Almost the entire sediment arrives either from the Peruvian Andes along the Amazon main stem or from the Bolivian Andes via the Madeira (Meade 2007). A number of large tributaries have been described as clearwater streams, rising below the Andes they bring little sediment to the Amazon.
Figure 4.4 The Amazon: Generalised geology and course.
Source: Dunne et al. 1998.
Figure 4.5 The Amazon from satellite imagery.
Source: NASA Worldview application (https://worldview.earthdata.nasa.gov), part of the NASA Earth Observing System Data and Information System (EOSDIS).
The average annual sediment load of the Amazon measured at Óbidos, approximately at the tidal limit, is about 1200 million tonnes. A bigger load is carried only by the combined flow of the Ganga and Brahmaputra. However, as calculated by Dunne et al. (1998), who studied sediment transport though the 2010 km of the Amazon in lowland Brazil, a higher amount of sediment is transferred laterally between the channel and the floodplain, and then passed downstream (Figure 3.3). The lateral exchange of sediment involves bank erosion, bar deposition in the main channel, settling from overland flow on the floodplain, and sedimentation in channels within the floodplain. Much of the sediment that leaves the channel in suspension during floods and enters the floodplain is deposited there before clearer water returns to the Amazon during the falling stage of the annual hydrograph. Mertes et al. (1996) estimated that in the reach between the confluences of the Jutai and Madeira with the Amazon, the mean recycling time is between 1000 years and 2000 years. This allows progressive enrichment of quartz grains downstream.
4.3.4 Morphology
The axial trough of the central Amazon Basin exhibits a remarkable suite of fluvial landforms. The lowland Amazon has a straight and anastomosing channel within its floodplain. The main channel of the Amazon in Brazil has a sinuosity of 1.0–1.2 for most of the course. Over a measured length of 2000 km, the low water width of the river increases from 2 to 4 km and the corresponding depth from 10 to 20 m (Mertes and Dunne 2007). The width of the flooded Amazon is much bigger and the morphology and ecology of the river in flood is discussed in Chapter 5.
The regional pattern of the channel of the Amazon depends on its changing discharge, sediment, and slope. Steep channels in bedrock and gravel characterise the valleys in the mountains. In the downwarped foreland zone with high sedimentation, rivers begin to build bars and shift smaller channels. Low-gradient meandering channels in fine material characterise the central trough of the Amazon. The channel and floodplain of the Amazon are incised into the central low trough of the basin displaying a complex pattern of channels of various size, scroll bars and levees, and lakes. Towards the east, tributaries are dammed by alluvium of the trunk river, forming characteristic river-mouth lakes. The gradient of the lower river being very gentle, tidal effects extend about 1000 km up the Amazon to Óbidos.
Beyond the Andes foreland, the floodplain of the Holocene Amazon lies below a landscape of low hills under thick forest cover. The forest is interspersed with savanna, and recent deforestation is visible towards the northern and eastern margin of the basin. The channel and the floodplain continue between discontinuous terraces which are about 5–15 m above the general flood surface. The Holocene sediment in the channel and the floodplain consists of medium sand and finer sediment, weathered to clay minerals and enriched quartz grains (Johnsson and Meade 1990).
The river tends to undercut the cohesive terrace above the floodplain at channel bends. Avulsions related to flow switching are common. A dense network of channels cut across the floodplain. Some of these channels are as big as the large tributaries, and floodplain channels about 100 m wide are seen everywhere. The Amazon, its anabranches, and floodplain channels, all shift. As a result, numerous scroll bars and depressions mark the surface of the floodplain. Mertes and Dunne (2007) summarised the Amazon as an entrenched river that is confined to its valley, remains straight, and is relatively immobile over hundreds of kilometres. Large-scale channel migrations or avulsions are common. This huge low-gradient river flows through a 40–50 km wide floodplain decorated with considerable complexity of anabranches, levees, and scroll