Answer: tributary

AnswerCrossword Clue
TRIBUTARYFeeder stream
TRIBUTARYMany a headwater, relatively
TRIBUTARYRiver into another.
TRIBUTARYOhio to the Mississippi.
TRIBUTARYSubordinate
TRIBUTARYMissouri, to Mississippi
tributaryflowing into a larger body of water
tributarypaying tribute / a river that joins another river
tributarystream flowing into a larger body of water