The source is the farthest point of the river stream from its estuary or its confluence with another river or stream. Rivers are usually fed by many tributaries. The farthest stream is called the headstream or headwater. There is sometimes disagreement on which source is the headwater, hence on which is the true source. Headwaters are usually in mountains. Glacial headwaters are made by melting glaciers.
The source is where a river begins, and the mouth is where it joins the sea. The source of a river generally discharges water with less force leading to the formation of Interlocking spurs.