How do deltas form as river mouths slow down?

• A. Deltas form by tectonic uplift alone
• B. Rivers deposit sediment in suspension and as bedload; velocity decreases at mouths causing deposition and delta formation with distributaries
• C. Deltas form primarily through glacial carving
• D. Deltas form only at high-lall sea levels

Answer: (B)

When a river enters a still body of water, its flow slows dramatically, reducing its ability to carry sediment. That drop in velocity causes the river to deposit the material it’s transporting, both the fine stuff in suspension and the coarser bedload. The coarser sediments settle out first near the main channel, while the finer sediments settle farther out, building up a broad, low-lying area that fans out into multiple channels. Over time these deposits create distributary networks that split the flow and deliver sediment across the delta, allowing it to prograde into the sea or lake. Other processes like tectonic uplift, glacial carving, or the notion that a delta only forms at high sea level don’t drive this landform; the essential mechanism is sediment deposition as the river slows at the mouth, forming a delta with distributaries.