Envelope can be used for Protect. Protect is a different concept from Save. Save is related to Conservation. 
Dam is a type of Structure. you are likely to find Water in/at/on Dam. Water is related to Island. 
Envelope --UsedFor--> Protect <--DistinctFrom--> Save <--RelatedTo--> Conservation 
Structure <--IsA-- Dam <--AtLocation-- Water <--RelatedTo--> Island 