The starting point or origin of a river is known as the 'source' or sometimes the 'headwater'.
Source The source.
Source
tributary
River Indus.
Missippi River
That would be the Indus River.
spring
river
The source
The Illinois.
Feather River is a river in California. Flinders River is a river in Australia.
River Clyde in Scotland
it is called the mouth of the river
Xi River is the name of a river in China. Xingu River is the name of a river in Brazil.
There's the Nile River and the Niger River.