Let's Go Punting In Mirkwood?

[Kalimac]

You can probably guess what this is about - it's one of those minor points which are fascinating (well, to me anyway) since they're not really explained. In "The Hobbit" when Bilbo and the Dwarves come across the dark stream, they very fortuitously happen to find a little boat at the other side of the stream, which they haul in with a rope. It wasn't abandoned, since it was still tied up with a painter, but although Tolkien tells us that Thorin "kept his bow ready" in case the owner of the boat appeared, they never found out whose it was.

So where *did* that boat come from? As far as I can tell they were still a fair distance from where the Elvenking lived, and Mirkwood doesn't exactly seem to have had a large population of anything that would be interested in or capable of using a boat (and it must have been a fairly substantial little thing, too, if it could hold the weight of three or four Dwarves at a time). So was it kept by Elves, just in case, or abandoned, or just a (forgive me) Eru Ex Machina? Just curious, since it never does get explained otherwise.