Regardless of size, a great variety of physical processes, ultimately driven by climate, make these tropical coastal margins unique compared to coastal settings of higher latitude. The climate of the equatorial region is characterised by high rates of rainfall, solar insolation, and temperature. By virtue of these characteristics and global position, Coriolis forces are small, and winds are dominated by easterly trade winds (Chapter 2). These physical forces, coupled with the enormous loads of freshwater and sediment draining from the land, produce extensive buoyant plumes, in some instances, extending beyond the shelf edge. Rapid rates of sediment accumulation and high rates of nutrient flux and primary productivity are but a few of the unique characteristics of these river‐dominated coastal systems.
Tropical rivers worldwide drain a variety of geologic/geomorphologic settings: (i) orogenic mountain belts, (ii) sedimentary and basaltic plateau/platforms, (iii) cratonic areas, (iv) lowland plains in sedimentary basins, and (v) mixed terrains (Latrubesse et al. 2005). These types of rivers show high but variable peak discharges during the rainy season and a period of low flow during the dry season; some rivers show two flood peaks during the year, a main one and a secondary flood peak.
Tropical rivers exhibit a variety of channel forms and, consequently, a variety of different delta and mouth morphologies (Latrubesse et al. 2005). In most cases, rivers morph from one form to another over time so they are difficult to classify. Two main settings are rivers that discharge onto a tectonically active margin and those that discharge onto a passive margin (Leithold et al. 2016). Active margins are narrow and passive margins are wide.
Exactly how and where the discharged sediment deposits onto the adjacent shelf are not well understood, although it appears that sediments being discharged onto a wide, passive margin deposit in shallow waters often proximal to the river mouths, while a significant, if highly variable, proportion of sediments discharging onto narrow, active margins are transported to the adjacent shelf slope and deep sea (Wright and Nittrouer 1995; Leithold et al. 2016). The fate of sediment seaward of river mouths involves five stages: (i) supply via plumes, (ii) initial deposition, (iii) resuspension and transport by marine hydrographic processes, (iv) sediment that comes back on shore from far away and/or via tidal pumping, and (v) long‐term net accumulation (Figure 4.9). These processes vary greatly with river regime and coastal physics. The Amazon plume extends along the NW portion of the coast and far seaward. And although tide range is large and mixing processes are relatively intense, the enormous volume of outflow results in the effluent filling the entire water column beyond the mouth, before
