What Is a Triangle-Shaped Landform at a River Mouth Called?

It is called a **delta**.

Formation

As a **river enters a standing body of water (ocean, lake, reservoir)**, its **velocity drops sharply**. **Sediment (silt, sand, clay) carried in suspension** is **deposited**. Over time, **deposition builds a fan-shaped or triangular landform**—the **delta**—**prograding (building outward)** into the water body.

Why "Delta"?

**Greek letter Δ (delta)** — **Herodotus (5th c. BCE)** noted the **Nile's triangular shape** resembled the letter. **Name stuck.**

Key Components

• Apex: **River bifurcation point (top of triangle).**
• Distributaries: **Multiple branching channels** — **river splits to find paths** across its own deposits.
• Delta Plain: **Subaerial (above water) — levees, marshes, swamps, forests, farms, cities.**
• Delta Front / Prodelta: **Subaqueous (underwater) — steep slope, fine sediments, slump structures.**

Types (by dominant process)

• River-Dominated (Bird's Foot): **Mississippi** — **long distributaries, low wave/tide energy**.
• Wave-Dominated (Arcuate/Cuspate): **Nile, Rhône, São Francisco** — **waves smooth into arc**.
• Tide-Dominated (Funnel-shaped, linear sand ridges): **Ganges-Brahmaputra (Sundarbans), Fly, Ord** — **tidal currents shape channels**.
• Gilbert (Coarse, steep, lake): **Lake Bonneville deltas.**

Importance

• Fertile Soil: **Annual silt renewal → cradle of civilizations (Nile, Tigris-Euphrates, Indus, Yellow, Mekong).**
• Biodiversity: **Wetlands, nurseries, migratory birds.**
• Resources: **Oil/gas (Niger, Mississippi), sand/minerals.**
• Hazards:** Subsidence, flooding, saltwater intrusion, **sea-level rise (existential threat)**.
• Human Impact: **Dams trap sediment (Aswan, Three Gorges) → deltas STARVE & SINK.** **Levees prevent natural flooding → no new sediment.** **Subsidence + sea-level rise = drowning deltas.**