SOLVED. Guy enters Grand Central station and suddenly he is in the woods and sees a dragon [s]

[Ray]

The dragon chooses him to be his Rider.
The world is populated by Elves.
He is a Human an oddity in this world,rare but not unheard of.
"They plug up the Gugglerun to flood out the Castle."

One of three maybe four books.

[Aerulan]

Long shot, but what you describe rang faint (really faint) bells for Dragon Companion by Don Callander. The damming the Gugglerun to flood the Castle bit it what is ringing the bells.
But I read it probably 15+ years ago, so my recollection is really hazy.

Edited: I went and got my copy -hooray for well organized bookshelves- and they damn the Gugglerun around page 42.

[Ray]

That's it! Thank you.

[Aerulan]

You're welcome!