I think your use of graphics and music really succeeded in capturing the atmosphere of rafting down a woodland river nicely. I was especially impressed by the amount of graphical variety and detail. The innovative control scheme makes steering the raft and navigating the river a proper challenge, though I wish the movement of the raft would be a bit more responsive. I often found myself in a situation in which a sharp bend came up so suddenly that it was too late for me to react to.
Alternatively it may have been better to give the player a larger view of what's coming up ahead, so that they have the opportunity to preplan their actions. As such, it felt as if one has to memorise the river through trial and error in order to progress, but I think there could be potential in further emphasising the strategic aspects in navigation that your game already has!