If you make periodic snapshots of a volume, the snapshots are incremental so that only the blocks on the device that have changed since your last snapshot are incrementally saved in the new snapshot. Even though snapshots are saved incrementally, the snapshot deletion process is designed so that you need to retain only the most recent snapshot in order to restore the volume.
http://docs.amazonwebservices.com/AWSEC2/latest/APIReference/ApiReference-query-DeleteSnapshot.html
How to understand, which of snapshots I can remove? Can I remove any? Can I remove first?