As the flood dissipates downstream particles will fall out of suspension and/or wash ashore. They're all the same shape and density so the conditions in which

As the flood dissipates downstream particles will fall out of suspension and/or wash ashore. They're all the same shape and density so the conditions in which they drop out is going to be about equal. If they were all swept up from the same starting point they could very well all end up washing up in the same place. It's made more likely if the river has something like a natural lake for the flood to dissipate into. The dead stuff is free to float about and get naturally sorted by the wind and mild currents.