(5–10 m) buoyant plume which reaches more than 300 km offshore and about 1000 km to the NW entrained by the North Brazil Current (NBC). However, the plume is highly variable over time and space due to wind forcing and tidal variations in bottom drag and vertical mixing. There is also seasonality in discharge, with maximum discharge during May–June; peak sediment discharge precedes peak water discharge by about a month or more, so the volume‐specific rates of discharge vary considerably. Strong tidal currents and waves generated by the easterly trade winds dominate the Amazon shelf resulting in variable spatial and temporal distribution of sediments on the shelf. Intense reworking of sediment on the inner shelf allows only temporary deposition. Once resuspended, the NBC carries sediment far to the NW. Despite high alongshore current flux, erosion occurs along the shore although erosion–deposition episodes depend on the strength of the NBC. At least 50% of sediment accumulation takes place on the mid‐shelf (depth 30–50 m) seaward and NW of the mouth. Remaining sediment is probably stored in the tidal reaches of the lower river.
FIGURE 4.9 Major stages in sediment dispersal of river sediments in the coastal ocean.
Source: Wright and Nittrouer (1995), figure 3, p. 503. © Springer Nature Switzerland AG.
The Purari River system on New Guinea is different, being much smaller (Table 4.1) and having a mountainous watershed (Wright and Nittrouer 1995). The Purari delta is heavily vegetated by mangroves and crossed by an intricate network of interconnected channels which trap most of the river sediment load. Saltwater intrusion is prevented by large shallow and mobile sand banks within and outside the river mouth. Fine sediments are carried onto the inner Gulf of Papua shelf as muddy, low‐salinity plumes that are broken up by the coastal oceanographic regime which is dominated by onshore‐directed southeast trade winds for most of the year. Thus, much of the sediment remains trapped relatively close inshore as a turbid band and is advected alongshore. Plumes enter tidal channels on flood tides supporting the extensive mangroves within the delta. Some sediment is transported directly offshore especially during summer when the trade winds are weak, and rainfall is at its peak.
In the Ayeyarwady River system, little modern sediment accumulates on the shelf immediately off the delta. Instead, a major mud wedge with a distal depocentre of up to 60 m thickness has been deposited seaward of the adjacent Gulf of Martaban, extending to about 130 m into the Martaban Depression (Liu et al. 2020). However, no river‐derived sediment has been found in the adjacent deep Martaban Canyon. There is a mud drape wrapping around the narrow western Myanmar shelf in the eastern Bay of Bengal. Unlike other large river systems in Asia, such as the Yangtze and Mekong, there is a bidirectional transport and depositional pattern controlled by local currents and tides, and seasonally varying monsoonal winds and waves.
In the Ganges–Brahmaputra River system, approximately one‐third of total sediment discharge is sequestered within the flood plain and delta plain (Rahman et al. 2018). The remaining load appears to be apportioned between the accumulating subaqueous delta and the deep‐sea Bengal fan via a nearshore canyon. The roughly equal partitioning of sediment among the flood plain, shelf, and deep sea reflects the respective influence of an inland subsiding tectonic basin, a wide shelf, and a deeply incised canyon system (Rahman et al. 2018).
Plumes of other large tropical river systems may be dispersed laterally due to local coastal currents and hydrography. For instance, the typical seasonal orientation of the Zaire River plume is northward for most of the year, except during February–March when the plume has a large westward extension onto the narrow shelf (Denamiel et al. 2013). The northward extension of the plume is explained by a buoyancy‐driven upstream coastal flow and the combined influences of the ambient ocean currents and the wind. During February–March, the surface ocean circulation drives the westward expansion of the plume and the presence of the deep Congo canyon increases the intrusion of seawater into the river mouth.
Off the Mekong delta, a similar lateral plume occurs throughout most of the year, with a net deposition SW of the river mouth down the Ca Mau peninsula (Szczuciński et al. 2013). In summer, a large amount of fluvial sediment is deposited near the Mekong River mouth, but in the following winter, strong mixing and coastal currents lead to resuspension and south‐westward dispersal of previously deposited sediment. Strong wave mixing and downwelling‐favourable coastal current associated with the more energetic NE monsoon in the winter are the main factors controlling post‐depositional dispersal to the SW.
For tropical river plumes, coastal hydrography plays an important role in governing how the discharged sediment is dispersed onto the adjacent shelf (Wright and Nittrouer 1995; Hetland and Hsu 2014). Oceanic processes that resuspend and transport sediment act in concert with maximum plume outflow, causing sediments to be dispersed farther from the delta mouth. Sediments can be dispersed over relatively wide areas that extend considerable distances from the delta; some rivers do not have a subaerial delta or do not protrude much beyond the regional coastline. Alongshore dispersal of sediments takes place primarily on the inner shelf but near the delta mouth the depth is too shallow and energetic for fine‐grain sediments to be deposited, except temporarily (Hetland and Hsu 2014). Much of the material transported alongshore becomes sequestered within tidal currents, and off the Amazon, muds that are transported alongshore ultimately accumulate, forming expensive, accumulating mud banks hundreds of km to the NW of the mouth. Despite a high level of instability, these mud banks are colonised by mangroves. However, the Amazon is characterised by accumulations of sediments as subaqueous delta deposits at relatively large distances on the mid‐shelf. A large fraction of Amazon sediments reaches the mid‐shelf due to the energetic currents and waves that sustain sediments in suspension until they reach relatively deep water (Wright and Friedrichs 2006). Much of the observed diversity of sediment dispersal and accumulation is attributable to variations in coastal energy regimes and to the temporal sequencing of river discharge relative to oceanographic transport processes. Sediment transport in the southeast section of the Amazon coastal zone is greatly affected by tidal asymmetry, seasonal variation of the wind and wave regime, and river discharge (Gomes et al. 2020). Climate and geological configuration have resulted in numerous estuaries and large‐mangrove‐lined coastal plains that partially divided the estuarine basins, which are connected by tidal channels within the Amazon delta. Convergence of transported sediment occurs within a channel connecting the estuaries, resulting in mud retention and further delivery to the mangrove‐covered plains, with a net flux of suspended sediments between estuaries. The connectivity between estuaries via channels is a key process to redistribute muddy sediments along this coastal sector, which helps to explain the evolution and maintenance of the relatively homogenous and widespread progradation of mangroves along the coast.
Mechanisms that dominate the short‐term spreading and mixing of riverine sediment may differ from the mechanisms that determine the longer‐term dispersal of sediment. Sediment records from the South China Sea show that strong monsoons are associated with intensified reworking of pre‐existing floodplain sediment over millennial timescales (Clift 2020). Strong monsoons result in deposition of more altered material that is also delivered at higher rates than during drier periods. Millennial‐scale changes in monsoon strength result in changes in the weathering regime but not fast enough to account for the changes seen in the sediments preserved in Asian deltas; instead, monsoon‐modulated recycling dominates. Over longer time periods (>106 year) strengthening of the monsoon is linked to faster bedrock erosion and increased sediment flux to the ocean.
An additional complication is the fact that most tropical river systems are heavily affected by humans; few, if any, tropical rivers are pristine. Human disturbances such as the construction of dams and deforestation can greatly impact water and sediment discharge. Increased greenhouse gas emissions are projected to impact twenty‐first century precipitation distribution,
