A river source is the place from which a river begins to flow. Types of sources include lakes, bogs, springs, streams, collections of snow or rainwater, glaciers, swamps and other rivers.
Sources for rivers are typically found at higher elevations than downstream portions of the river. A river or stream that flows into a larger river may also be called a tributary. The main portion of the river often travels through a floodplain, leading out to the river's mouth, where the river deposits its water into a larger body, such as an ocean or a lake. The source of the river is the furthest point from the mouth of the river, also known as the estuary.