Yes, because river is part of the proper name.
All of them should be capitalized if they are at the beginning of a sentence. Drive, River, and Railroad would also be capitalized if they are part of proper nouns such as South Drive, Mississippi River, Edaville Railroad.
Missouri river
Yes it should be capitalized.
The Nile River is typically capitalized as it is a proper noun.
St. louise river
Missouri RiverMississippi River
Missouri RiverMissouri River; Illinois River
Missouri (and Columbia)
Yes, Mississippi River valley should be capitalized because it is a proper noun referring to a specific geographic region named after the Mississippi River.
NO. The Missouri river is the tributary
Missouri river
The Missouri River flows into the Mississippi River just north of Saint Louis, Missouri.