Typically, rivers have their beginnings in glaciers, springs, underground geysers, etc. Usually, they end up discharging into lakes, seas, oceans, or other rivers. They provide the bulk of the useable water for people and wildlife, making them the most significant aspect of the water cycle. The river basin is the collective term for the surrounding regions through which a river flows.

Although the Amazon river system is the second biggest in the world after the Nile, it has the greatest river basin due of its numerous tributaries and distributaries. Based on the distance travelled from a river's source to its mouth, its length is determined.

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