Deltas
Deltas are landform comprised of sediments (sand, clay, silt, gravel) found at the mouth of the river, where the river enters a standing body of water (lake, pond, reservoir, or ocean). Herodotus, a Greek Historian, considered by many as the “Father of History”, studied this great geologic feature. He is credited with coining the term “delta” for this is type of landform because of its triangular shape reminded him of the Greek letter Δ (delta).
Delta Formation
Delta forms when river carrying sediment reaches a body of standing water. When the flow enters the standing water, it expands in width. This flow expansion results in a decrease in the flow of velocity, which diminishes the ability of the flow to transport sediments. As a result, sediment drops out of the flow and deposits. The sediment deposited by this running water is called alluvium. These sediments pile up into several layers called beds. The process in which there is a progressive deposition of sediments by rivers in a delta is called progradation.
Figure 1 Figure 2
As additional sediment is transported to the river mouth, the delta is extended farther out into the water body. Force to travel through its own deposits, the river lengthens and often divides into a series of smaller channels known as distributaries or distributary channels (refer Figure 1), so called because they distribute the river’s flow.
On the Mississippi Delta such channels known as passes (refer Figure 3). The larger passes of delta are dredged to maintain deep channels for ship and barge traffic.
Distributaries can be numerous and ephemeral because river will seek the most and least resistant path to the ocean, the ultimate base level. Whenever the channels receive an excess of water, they flood, breaching the natural levees (channel boundary) and depository sandy, fan-shaped splays of sediments more or less perpendicular to the channel.
Delta Formation
Delta forms when river carrying sediment reaches a body of standing water. When the flow enters the standing water, it expands in width. This flow expansion results in a decrease in the flow of velocity, which diminishes the ability of the flow to transport sediments. As a result, sediment drops out of the flow and deposits. The sediment deposited by this running water is called alluvium. These sediments pile up into several layers called beds. The process in which there is a progressive deposition of sediments by rivers in a delta is called progradation.
Figure 1 Figure 2
As additional sediment is transported to the river mouth, the delta is extended farther out into the water body. Force to travel through its own deposits, the river lengthens and often divides into a series of smaller channels known as distributaries or distributary channels (refer Figure 1), so called because they distribute the river’s flow.
On the Mississippi Delta such channels known as passes (refer Figure 3). The larger passes of delta are dredged to maintain deep channels for ship and barge traffic.
Distributaries can be numerous and ephemeral because river will seek the most and least resistant path to the ocean, the ultimate base level. Whenever the channels receive an excess of water, they flood, breaching the natural levees (channel boundary) and depository sandy, fan-shaped splays of sediments more or less perpendicular to the channel.