River Indus.
Missippi River
One fabric beginning with P is poplin.
That would be the Indus River.
The name of a large flat fish beginning with "P" is a "Plaice."
spring
The name of the river of woe is Acheron :P
The source
The Illinois.
river
Feather River is a river in California. Flinders River is a river in Australia.
it is called the mouth of the river