Because if you leave the bag it will be down to the water. Fish under water may accidentally eat it and if unfortunately you have lots and lots of people may cause spoilage and river again.
Because if you leave this piece. It is down to the water. Fish under water might accidentally eaten it. And if, unfortunately, a lot of people do, you may make the river is dirty and spoiled it.
If you dropped a bag of this piece. It into the water. The fish in the water may have eaten it into them. And if, unfortunately, someone like you a lot may be dirty river and rotten.