Читать книгу Introducing Large Rivers - Avijit Gupta - Страница 35

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.