| environ. |
delta A delta is a vast, fan-shaped creation of land, or low-lying plain, formed from successive layers of sediment washed from uplands to the mouth of some rivers, such as the Nile, the Mississippi and the Ganges. The nutrient-rich sediment is deposited by rivers at the point where, or before which, the river flows into the sea. Deltas are formed when rivers supply and deposit sediments more quickly that they can be removed by waves of ocean currents. The importance of deltas was first discovered by prehistoric man, who was attracted to them because of their abundant animal and plant life. Connecting waterways through the deltas later provided natural routes for navigation and trade, and opened up access to the interior. Deltas are highly fertile and often highly populated areas. They would be under serious threat of flooding from any sea-level rise (三角洲是广大、扇形的新生土地或低地平原，是从高地冲刷到一些河口的沉积物一层层堆积形成，如尼罗河、密西西比河和恒河。 营养丰富的沉积物被河流堆积在它的入海口。 三角洲的形成是因河流提供的沉积物超过洋流波浪所带走的。 三角洲的重要性最早被史前人类发现，他们被此处丰富的动植物生命所吸引。 后来透过三角洲所连接的水路更提供了航海和贸易的天然路线，并开辟了进入内地的捷径。 三角洲土地肥沃，也常是人口密集地区；尽管它们处于海平面可能上升而被淹没的严重威胁之下。) |
