Why do rivers form deltas?

Rivers carry sand, silt and clay eroded from the landscape. When a river reaches the coast or a lake, its current slows dramatically and can no longer hold the sediment in suspension. The particles settle out, with heavier sand dropping first and finer clay carried further. Over time, sediment piles up and the river splits into multiple channels called distributaries, fanning out across the growing delta. The Nile Delta, which gave deltas their name because its shape reminded the ancient Greeks of the letter delta (Δ), covers about 24,000 square kilometres. Not all rivers form deltas; strong ocean currents or tides can sweep sediment away.