Larry, is it tidal where you are discussing?
My experience is stuff sloshes back and forth as the high tides dislodge it from its first wash up and the clocking winds blow it back into the current.
Usually on most smaller rivers and coastal creeks, after a dozen or so cycles with some strong clocking winds or one particular high tide and the larger debris drop off dramaticalky.
But some long rivers always take a bit longer...depending on how quickly they drop in volume.
Yes, it's tidal but it's also slow moving. The St John's River has a lot of smaller rivers and large drainage areas feeding it. The total fall from Stanford to it's mouth, about 200 miles, is ~30'. No wonder they call it the lazy river.