A river delta is a landform created at the mouth of a river where it flows into a relatively still body of water, such as an ocean, lake, or reservoir. When the river’s current slows, it loses the energy required to carry its suspended load of sediment, known as alluvium. This deposition of sand, silt, and clay builds up over time, gradually creating new land that often extends outward from the coast. Deltas are recognized for their high ecological productivity and are historically significant centers for agriculture and human population.
The Anatomy of a River Delta
A delta is composed of several interlocking geological features, most of which lie submerged beneath the water’s surface. The most visible portion is the delta plain, the broad, low-lying expanse of land above the water line. This plain is crisscrossed by a network of branching channels called distributaries, which divert water and sediment away from the main river stem. Distributaries flow out and spread the river’s load across the delta surface, unlike tributaries, which flow into a river.
At the end of each distributary, a mouth bar forms where the channel meets the open water, as sediment is dropped due to the rapid decrease in flow velocity. These accumulations can block the channel, forcing the river to find new paths and extending the delta’s reach. Seaward of the delta plain is the delta front, a submerged, steeply sloping area where the bulk of the coarser sediment settles. Finer particles, like clay, are carried further out and accumulate in a gently sloping zone known as the prodelta.
Common Delta Shapes and Their Appearance
The overall outline of a delta provides its primary classification, reflecting the dominant forces acting on the deposited sediment. The bird’s foot delta, exemplified by the Mississippi River Delta, is characterized by long, thin, projecting lobes that resemble the outstretched toes of a bird. This shape develops when the river’s current is stronger than the coastal forces, allowing distributaries to build straight outward into the sea with minimal lateral reworking of the sediment.
The arcuate delta presents as a curved or bowed shape, convex toward the sea, similar to a fan. The Nile River Delta is the classic example, where numerous distributaries spread the sediment widely. Moderate wave action smooths the coastline into a continuous, arching margin. This shape is more rounded and less dissected by channels than the bird’s foot type.
The cuspate delta appears pointed or tooth-shaped and forms a symmetrical bulge along a straight coastline. This form develops when the river has a single or few main channels. Strong wave energy pushes the deposited sediment back toward the shore and distributes it evenly on both sides of the river mouth, creating a distinctive arrowhead appearance that juts out into the water.
External Forces That Mold Delta Appearance
The look of a delta is determined by the balance between three geological forces: the river, waves, and tides. River-dominated deltas occur when the momentum of the river’s flow and its sediment supply overwhelm marine forces. This scenario encourages the formation of the bird’s foot shape because the sediment is deposited straight into the basin before waves or tides can redistribute it.
Wave-dominated deltas form where coastal waves are the strongest influence, constantly reworking and smoothing the shoreline. The energy from these waves disperses the river’s sediment along the coast, which smooths the delta front and results in arcuate or cuspate shapes. The waves prevent the creation of long, projecting river lobes.
Tide-dominated deltas are shaped by the regular ebb and flow of tidal currents, as seen in the Ganges-Brahmaputra system. These currents erode and reorient the sediment, creating funnel-shaped channels perpendicular to the coast and filled with linear tidal sand ridges. This movement of water creates a highly dissected, dendritic pattern of channels and tidal flats.