Generally, the source is melting snow and ice, rain can be a source too.
he Source of a river is the start of the river. It's often found in the mountains.
Mountains
In the upper course in the mountains and hills. the source of the river is rain.
the Russian river
Acarai Mountains
the Cambrian mountains
Kunlun Mountains
The source of the Yellow River is the Bayan Har Mountains in Qinghai Province in western China.
Usually a high source of water such as a lake or river high in the mountains that comes from snow melting.
The Arkansas River
A source in geographic terms means where the river begins in mountains or lakes.
the kunlun mountains