Don't know if I am answering your question correctly. But to my understanding we call them "sub-rivers" or "tributaries" which will merge into the main rivers.
A rivulette :)
Tributarys.
Small creeks, streams and rivers that flow into larger rivers are called tributaries
tributaries
Tributaries are the smaller rivers and where they join is called a confluence
Tributaries
a lake ? It is a tributary
name four rivers flow into the Hudson bay
name four rivers flow into the Hudson bay
rivers
The River Seine is the main one. the one used most in crosswords is the River Somme
Fernand Rivers's birth name is Fernand Large.