Nature · Earth Science
Where rivers surrender their load to the sea
At the very end of a river’s journey, where fresh water meets the sea, the current slows and drops the sediment it has carried for hundreds of kilometres. The result is a delta or an estuary — flat, fertile, water-laced landscapes that are among the richest ecosystems and most densely populated regions on Earth.
As a river enters still ocean or lake water, it loses the energy to carry sediment, which settles out and builds new land. Where deposition outpaces waves and tides, a delta grows seaward in branching distributaries; where tides are strong and the valley is drowned, a funnel-shaped estuary forms instead.
A river splits into distributaries and drops its load in layered beds, building new land out into the sea.
The classic fan-shaped delta
The textbook delta: a smooth, fanned arc of land where the river fans into many channels. The Nile Delta gave the landform its name, from the Greek letter Δ.
Long fingers of sediment
Where a sediment-heavy river overwhelms weak waves and tides, it builds long, narrow fingers of land stretching far out to sea — the unmistakable bird’s-foot of the Mississippi.
Drowned river mouths ruled by tides
Where strong tides prevent a delta from forming, the sea floods the river valley into a broad estuary. The daily mixing of fresh and salt water makes estuaries astonishingly productive nurseries.
Pointed and inland deltas
Where waves attack a delta evenly from both sides, they trim it into a pointed, tooth-like cuspate form. Rivers entering calm lakes build small, neat deltas of their own.