The term for a small penis/crotch. The term originate's from a male leaving the sea in his Y-front's and having a small crotch. The term can be used to describe someone whou you feel has a small penis but act's as if he has a large one. The "river" part of the word can be transferred to other word's for the same small effect.
Sunbather 1 - "Hey look at that guy, he's wearing tighty whitey's that are obviously a size too small."
Sunbather 2 - "I know, it would be more bearable but he as such a riverdick."
Person 1 - "Look at that guy acting like he's all that."
Person 2 - "I know, i bet he's got a right riverdick!"