Because of the shallow meandering nature of the river, and the variation in level of the water, there can be quite large chunks of driftwood perfectly posed on the sand.

Is it flotsam or jetsam? Well, the definitions seem to be a bit confusing. I am pretty sure it is not flotsam: "flotsam," as the first part of the word implies, is floating stuff. In a nautical court, "jetsam" is a ship's equipment or cargo that is purposely tossed overboard (as in "jettisoned") to lighten the load, but "jetsam" is also used to refer to debris that is washed up on the beach. So I'm going with "jetsam."