The song was inspired by what happened to the songwriter. He woke up from a dream but he does not remember what he dreamt about.
Just like in life, we experience losing something that we knew we had but cannot remember what. The river represents the lost of knowledge. We must face the river to regain what we lost. In life, in order to find what we lost, we must first know what it is we lost so that we can plan on how to retrieve it.