I'm going to end an almost dead animals suffering
How do you plan on ending the animal's suffering?
I will shoot it in the head
If you are witnessing the dying animal suffer then shooting the animal may be a good option. An animal will continue to suffer until it dies if the injury is bad enough. Shooting the animal would quickly allow it to die without having to writhe in pain any longer.
I'm at a river and I'm putting it in the water
It is ok to want to end a dying animals suffering. It is not right to put it back in the water source. You could contaminate the water with bacteria from decomposition.