Looks poisonous to me, but hard to tell from photo. A fat triangle for a head means bad news. My gut reaction was Cotton Mouth. But the more I look at it, the more unsure I get...
Any other help?
I am from Kansas. It is not poisonous - look at the round pupil. All native snakes to this area have an eleptical or cat-like pupil. I am assuming that you saw in near water. It is one of the common species of water snake - genus Nerodia.
They have been known to act mean and often mistaken for the cottonmouth or water moccasin.
Do you have any better shots of the head?
These are easy to confuse, and it's really easy to jump to poisonous!
I'm thinking cotton mouth or Water snake of some kind.
Check you memory against these photos: http://mdc.mo.gov/nathis/herpetol/snake/snake1.htm
Funny isn't it Rod that had you NOT had your camera you would have steered completely clear of this guy/gal! We are so brave/foolish when we have our cameras with us Nice shot, hopefully with a long lens
Nice shot, hopefully with a long lens 