ABSTRACT
Coastal deltas are accumulations of allochthonous (terrigenous) sediment deposited where rivers enter into the sea. River sediments may also accumulate at the head of coastal embayments if the coastline is drowned (bayhead delta), but these deposits are generally controlled by estuarine processes and will not be discussed here (see Section 7.2.1). This chapter focuses on situations where the delta is sufficiently large to cause the adjacent coastline to prograde. This implies that sediments must be delivered by the river faster than they are dispersed by waves, tides and ocean currents. For a river to deliver sufficient sediment to cause coastal progradation, the drainage basin usually needs to be large, although high rates of precipitation and catchment denudation are also sufficient conditions. Large drainage basins are mostly restricted to the tectonically passive trailingedge of continents where the drainage divide is well inland from the coast. High rates of precipitation and catchment denudation mostly occur in mid to low latitudes. Inman and Nordstrom’s (1971) census of major deltas placed 57% along trailing-edge coastlines (e.g., the Amazon), and 34.5% along coasts fronting marginal seas that are often protected from ocean waves by island arcs (e.g., the Klang). Many in this latter group are located in the mid to low latitudes (e.g., the Fly).
