Yes, the song is partly based on the experiences of Bruce's older sister Virginia ("Ginny" is her nickname) and her husband. I'm not sure if Ginny really got pregnant before they were married, like the character "Mary" in the song, but I'm pretty sure her husband was a construction worker like the young man in the song. Evidently, Ginny and her husband -- just like the couple in the song -- had serious money problems after they got married, problems that dashed their hopes and dreams for the future.
My interpretation of the song is that the young couple later goes back to the river where they had once swam and made love together, in the days when they were still young, innocent, and full of hope. They know that the river is now dry, but they go there anyway, in a futile attempt to regain their lost innocence. The river really is "dry" -- the water is gone -- but I think the barren river also symbolizes the loss of their hopes and dreams.
It's a great song, but it includes one of Bruce's most enigmatic lines: "Is a dream a lie if it don't come true, or is it something worse?" I've never been able to figure that one out!
Bruce actually dedicated this song to his sister and her husband when he performed it at the 1979 "No Nukes" benefit concert in NYC. The performance was featured in the movie "No Nukes". It's also on Bruce's DVD video anthology, along with two more songs ("Thunder Road" and "Quarter to Three") from the No Nukes show.
