A delta grows where river sediment accumulates at the mouth faster than it is removed, while deposition keeps pace with available space.
A river entering a sea or lake spreads and loses some of its transporting power. Sediment is deposited near the mouth. A delta forms where enough river-borne material remains to build a partly landward and partly underwater accumulation. Strong waves, tides and currents can redistribute or remove that supply.
The delta plain is the low, relatively flat, mainly above-water part crossed by channels and wetlands. The delta front is the offshore sloping zone beyond it. The prodelta is the deeper, more distant underwater area where much of the finer sediment settles. These are parts of the deposit, not three separate rivers.
Mouth bars can divide flow into distributaries. Continued deposition may build the delta seawards, called progradation. Channels can switch to new routes, changing which areas receive fresh sediment. Coarser material tends to settle nearer the mouth, while finer particles can travel farther; salinity can promote the clumping and settling of fine clay.
River-dominated deltas show a strong river imprint. Waves can smooth and spread sediment along the shoreline. Strong tides can rework deposits into tidal channels and elongated bars. Most deltas reflect a mixture, so shape is evidence of the balance among processes rather than a fixed triangle template.
Sea-level rise and land sinking (subsidence) create accommodation space: extra room for sediment to accumulate. Deposition must keep pace with this increase to maintain the delta's height relative to the water. Uplift, basin geology, vegetation and changing discharge affect development. Dams and sand extraction can reduce sediment delivery, changing a delta that previously grew into one with retreating sections.
Step by step
Deliver sediment
The river brings load from its basin to the mouth.
Deposit and redistribute
Spreading flow deposits material; waves and tides reshape or remove it.
Assess the balance
Growth depends on retained sediment relative to erosion, subsidence and rising water level.
Worked example: A controlled delta comparison
In two illustrative mouths with equal river sediment supply, stronger coastal removal can prevent a lasting delta at one while the other accumulates sediment. If subsidence then accelerates, even the growing delta needs more deposition simply to maintain its elevation.
Watch out for this
A delta is always triangular and entirely above sea level.
Its shape varies with river, wave and tidal processes, and much of the deposit lies underwater in the delta front and prodelta.
Check your understanding
Which change is most likely to reduce delta growth, other things being equal?
- More sediment reaching the river mouth.
- A dam traps sediment while waves continue removing coastal deposits.
- A point bar appears far upstream without changing total sediment delivery.