This is probably a hypomelanistic specimen of Northern Water Snake(Nerodia sipedon). Hypomelanism is a genetic mutation, which is actually rather common in many Water Snake species, that reduces the amount of black pigment in the skin. One form of hypomelanism is similar to the same gene that is responsible for chocolate Labrador Retrievers and "red-nose" American Pit Bull Terriers. The snake in my avatar is my hypomelanistic Florida Banded Water Snake(Nerodia fasciata pictiventris). In addition to the Northern/Midland Water Snake, Missouri also has Broad-Banded Water Snakes(N. Fasciata confluens), which can also be subject to the hypomelanism gene. Here's a pic of my own Northern/Midland Water Snake, a hypomelanistic female, so you can compare her to the snake you saw: img.photobucket.com
img.photobucket.com
Here is a pic of my hypomelanistic female Banded Water Snake, which is closely related to the Broad-Bandeds in MO, so I guess that species would look similar when influenced by this gene:
img.photobucket.com
img.photobucket.com