The source is the farthest point of the river stream from its estuary or its confluence with another river or stream. The farthest stream is called the headstream or headwater. They are usually cool waters because of shade and barely melted ice or rain. They may also be glacial headwaters. These are waters made by the melting of the ice of glaciers.
Where a river is fed by more than one source, it is usual to regard the highest as its source. The source is where a river begins, and the mouth is where it joins the sea.