Deltas are low flat deposits of alluvium at the mouths of streams and rivers that form broad triangular- or irregular-shaped areas that extend into bays, oceans, or lakes. They are typically crossed by many distributaries from the main river and may extend for a considerable distance underwater. When a stream enters the relatively still water of a lake or the ocean, its velocity and its capacity to hold sediment drop suddenly. Thus the stream dumps its sediment load there, and the resulting deposit is known as a delta. The term delta was first used for these deposits by Herodotus in the fifth century b.c.e., for the triangular-shaped alluvial deposits at the mouth of the Nile River. The stream first drops the coarsest material, then progressively finer material further out, forming a distinctive sedimentary deposit. In a study of several small deltas in ancient Lake Bonneville in utah, Nevada, and idaho, Grover Karl Gilbert in 1890 recognized that the deposition of finer-grained material farther away from the shoreline also created a distinctive vertical sequence in delta deposits. The resulting foreset layer is thus graded from coarse nearshore to fine offshore. The bottomset layer consists of the finest material, deposited far out. As this material continues to build outward, the stream must extend its length and forms new deposits, known as topset layers, on top of all this. Topset beds may include a variety of subenvironments, both subaqueous and subaerial, formed as the delta progrades seaward.
Most of the world's large rivers such as the Mississippi, the Nile, and the Ganges, have built enormous deltas at their mouths, yet all of these are different in detail. Deltas may have various shapes and sizes or may even be completely removed, depending on the relative amounts of sediment deposited by the stream, the erosive power of waves and tides, the climate, and the tectonic stability of the coastal region. The distributaries and main channel of the rivers forming deltas typically move to find the shortest route to the sea, and this causes the shifting of the active locus of deposition on deltas. Inactive areas, which may form lobes or just parts of the delta, typically subside and are reworked by tidal currents and waves. High-constructive deltas form where the fluvial transport dominates the energy balance on the delta. These deltas are typically elongate, such as the modern delta at the mouth of the Mississippi, shaped like a bird's foot, or they may be lobate, such as the older Holocene lobes of the Mississippi that have now largely subsided below sea level.
High-destructive deltas form where the tidal and wave energy is high and much of the fluvial sediment gets reworked before it is finally deposited. In wave-dominated high-destructive deltas sediment typically accumulates as arcuate barriers near the mouth of the river. Examples of wave-dominated deltas include the Nile and the Rhône deltas. in tide-dominated high-destructive deltas, tides rework the sediment into linear bars that radiate from the mouth of the river, with sands on the outer part of the delta sheltering a lower-energy area of mud and silt deposition inland from the segmented bars. Examples of tide-dominated deltas include the Ganges and the Kikari and Fly River deltas in the Gulf of Papua, New Guinea. other rivers drain into the sea in places where the tidal and wave current is so strong that these systems completely overwhelm the fluvial deposition, removing most of the delta. The orinoco River in south America has had its sediment deposits transported southward along the south American coast, with no real delta formed at the mouth of the river.
Where a coarse sediment load of an alluvial fan dumps its load in a delta, the deposit is known as a fan-delta. Braid-deltas are formed when braided streams meet local base level and deposit their coarsegrained load.
Deltas create unique, diverse environments where fresh and saltwater ecosystems meet, and swamps, beaches, and shallow marine settings are highly varied. Deltas also form some of the world's great est hydrocarbon fields, as the muds and carbonates make good source rocks and the sands make excellent trap rocks.
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