Two Ways of Seeing a River
by Mark Twain
What the first responder gave you are known as similes which are basically the same as metaphors (in the way that they compare two things) except they use like or as.
Metaphors can be vague and open to interpretation.
The river itself is clearly a metaphor, as to what it is a metaphor for is unclear to me. I believe that each reader will choose as to what this metaphor means for themselves (and I think that will be based on experiences that they had in life).
To me the river is either life itself or perhaps more likely an experience in life (notice how he says it's unfamiliar from what he is used to at home). To me this speaks of a life experience perhaps a journey or maybe even a relationship.
One thing is clear and that is that what he is talking about he was excited to learn about at first, he was very focused and determined to learn more about it, this experience (whatever it may be) is something that was once of great interest to him. It intrigued him, astonished him and I might go as far to say he had some love/hope/belief for it at some point in time.
But as he "mastered" it or perhaps grew some knowledge about this river it started to loose it's magic. If the river is life I can see what he's saying, as I look at my niece she is filled with joy, she's untouched by the world, innocent, she's too young to understand a lot of things that happen. I noticed, myself, that as I grew older I began to question more, the more questions the more knowledge. I can't help but feel the more I know the more I loose of what my niece has (whatever this essence or river of hers is). It would seem, in my opinion, that as we grow older, life can loose the magic it once had.
Have you ever seen tourists in your area? they are fascinated by the attractions, they want to see them all and perhaps at one time locals did too, but as time goes by we hardly notice them or...