What is a River Delta, and What Types Exist?
Deltas are the result of interacting fluvial (river) and, usually, marine systems. However, they can form anywhere a stream flows into shallower open water.
from Wright, 1978
Delta was coined by Herodotus (the father of history, 484-425 BC) after the Greek letter delta because of the deltoid-shape at the mouth of the Nile.
Delta definition: Coastal accumulations, both subaqueous and subaerial, of river-derived sediments adjacent to, or in close proximity to, the source stream, including the deposits that have been secondarily molded by various marine agents, such as waves, currents, or tides. [L. D. Wright, 1978]
Deltas occur throughout the world, except at the poles (see slides). They all have three characteristics in common:
1. The presence of a large catchment, or drainage, basin (the area where all run-off water drains to the river). The top 30 river deltas all have catchment basins in excess of 1,000,000 sq km.
2. They all are at the mouth of large river systems that carry large quantities of clastic sediments (soils or portions of rocks that have been moved by water from where they formed).
3. They are not near geologically active coastlines. In order to have a large catchment basin, a very complex tributary system is necessary. These long, complex systems take a long time to develop, so they are very rarely situated on tectonically active coasts.
ANATOMY OF A DELTA:
- Subaqueous (=under the water) delta - That part of the delta that is below the low-tide mark. Seaward, finer and finer soil particles are found. The seaward-most portion has the finest soils (clays) and is called the prodelta.
II. Subaerial (=under the air, or, above water) delta - That part above the low-tide mark.
A. Lower delta plain - That part governed by riverine/marine interaction. Extends landward to the limit of tidal influence. All salt and brackish flora and fauna are within this area.
B. Upper delta plain - That part governed by riverine depositional processes.
On the same figure, one can see the following two additional categories:
1. Active delta - That part that is still active and has water channeling through it carrying sediment.
2. Abandoned (or inactive) delta - That part that was once active, but now has reduced or no fluvial activity.
TYPES OF DELTAS:
1. Arcuate (fan-shaped) delta - e.g., Nile River. Has many active, short distributaries taking sediment to their mouths. The receiving (ambient) waters are rather shallow and have relatively even wave action arriving perpendicular to the shore with minimal longshore current. As the sediment exits the many distributary mouths, the waves push it back, so the coastline is rather smooth.
This and the following delta figures are from Strahler.
- Bird-foot (shaped like a bird foot) delta - e.g., Mississippi River. Tend to have one or a very few major distributaries near their mouths. The receiving basin has currents that carry the sediment away as it exits the distributary mouth. There is a broad, shallow shelf that deepens abruptly, so the trend is to grow long and thin like a bird's toe.
3. Cuspate (tooth-shaped) delta - e.g., Tiber River of Italy. Usually has one distributary emptying into a flat coastline with wave action hitting it head-on. This tends to push the sediment back on both sides of the mouth, with a "tooth" growing out onto the shelf.
4. Estuarine delta - e.g., Seine River of France. This type of delta has a river that empties into a long, narrow estuary that eventually becomes filled with sediment (inside the coastline).
Progradation - The river deposits sediments faster than the sea is able to remove them, so the delta grows outward into the sea (=progrades).
Aggradation - The river deposits build up (elevation) via overbank flooding, accumulation of biotic remains, etc., so the delta grows upward.
Transgression - The retreat of the delta, usually by the loss of sediment caused by continued wave attack or the reduction of nourishment soils.
It follows that deltas have four options:
1. They stagnate, i.e., remain in equilibrium.
2. They prograde.
3. They aggrade.
4. They transgress.