water moccasin or cottonmouth, highly venomous snake of the swamps and bayous of the S United States.
A good climber, the water moccasin often relaxes on branches overhanging the water. If startled it erects its head and shows the white interior of its mouth?hence the name cottonmouth. It eats both warm-blooded and cold-blooded animals. It is aggressive in the wild state but may become quite tame in captivity.
This is the very same type of snake that fell off of a branch of a tree and landed coiled up right in front of my feet. 
And this was beside my husband's uncles house down in the country. Down behind his house though was a swampy type pond. It's odd he wasn't relaxing up in a tree above that pond instead of by that house.
I did a huge jump backward as did my mother-in-law and then I went inside and got my husband's uncle and he came out with his shotgun and shot it twice. The poison smell was so strong it was unbelievable. My husband could not believe that it fell from any tree.