RIO MAINSTREAM TROUT SERIES FLY LINE INTERMEDIATERIO Mainstream Intermediate Fly Line WF3I to WF8I The RIO Mainstream Intermediate series is crafted for fly anglers seeking a slow sinking line that offers better control in deeper water or when wind and surface chop make floating lines less effective. Designed with a moderate sink rate of 1. 52 inches per second, its ideal for fishing just below the surface, where many fish feed. Featuring a slightly heavier, easy loading head and a short front