I believe the answer has to do with currents perpendicular to the main flow of the river. In temperate climates, lakes freeze over winter; in the spring, the lake "turns" because the melting ice water is colder than the insulated under layers. In a river, you might not get such a dramatic movement all at once, but you might still get some of the same effect.
In the winter, the water is low and only the central channel is covered by water. It may be cold, but this land is warmer than the low banks that are frozen solid. When the snow melt comes, it fills the river so that the lower banks now form the edges of the riverbed. Since the central channel is warmer than the edges, the water naturally wells up from the center and sinks by the edges. This current would be enough to drive a floating object to the edges.
In the summer, the opposite occurs. The shallow portions of the river on the edges are warmer than the deep central channel. The perpendicular currents reverse and the log moves to the middle.
I've never studied this, but it sounds pretty darned reasonable to me. :)