Delta

A delta is the landform that results where a subaerial channelized flow (e.g., a river) enters a basin of standing water (e.g., the sea, a lake). In deltaic areas, flow velocities drop and sediment transport stalls, which results in bifurcation of rivers, a downstream widening of floodplains, and the buildup of deltaic plains that make the coastline protrude. Many deltas have become densely populated today. Industrial and urban uses of deltaic areas must deal with issues of flood hazard mitigation.

The word delta resembles the character “Δ” in the Greek alphabet. Its shape mimics that of the largest delta known in ancient Greece (that of the Nile River in Egypt as viewed from the north). Deltas come in many forms, and the term is used in various contexts. Fluvial landforms such as river channels, levees, and floodplains; lagoonal landforms, including swamps, wetlands, and lakes; and coastal features such as beach ridges, spits, salt marshes, mangrove, tidal creeks, and tidal rivers (estuary channels) can all be part of a larger area typified as a “delta,” which can further contain isolated “islands” of inherited, partly eroded, and/or buried older landforms.

Deltas grade upstream into feeder valleys, with no strict definition of their inland limits. At their downstream end, deltas terminate in open water and grade laterally into nondeltaic segments of coast—again without strictly defined limits. The word is applied to describe features at a range of spatial scales, from the small size of a flume experiment or a mouth bar at the end of a natural channel (typical in engineering textbooks) to the much larger area occupied by a full network of divergent river channels between the most-up-stream bifurcation (“delta apex”) and the most-downstream subaqueous deposition as can be traced offshore (typical in geomorphological textbooks) to even larger areas based on the presence of deltaic deposits that a river has delivered over geological time over thousands (Holocene) if not hundreds of thousands (Quaternary) or millions of years (Cenozoic) (typical in sedimentary geology textbooks).

For example, the Netherlands as a whole is geologically considered a Quaternary delta of the [Page 703]Rhine and Meuse within the shallow southern North Sea basin in northwest Europe. But the central part of the Netherlands is also geomorphologically considered the Holocene Rhine-Meuse Delta. Furthermore, areas where the deltaic Rhine branches terminate in the Netherlands’ central lagoon host small deltas, which are considered elements of the larger Rhine-Meuse delta. The urbanized westernmost part of the delta, Randstad Holland, a conurbation along the mouth of the rivers Rhine and Meuse (Rotterdam), the coastal barrier (The Hague) and the central lagoon (Amsterdam), is commonly referred to as a “delta metropole.” The Randstad conurbation covers the downstream part of the Rhine delta and adjacent coastal and lagoonal landforms. In the Rhine delta, the raising of dikes has restricted deltaic sedimentation in the past 800 years to narrow embanked flood-plains. Consequently, the modern floodplain has a width that is only a fraction of the preembanked situation—with impacts on style of sedimentation (allowing the flood-plains to be mined extensively for bricks) and river management. In contrast, the dike-protected flood basins were starved from sediment and became subject to land subsidence owing to artificial water table lowering—a common phenomenon and management problem in urbanized deltas.

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