Why is "Missouri" pronounced with -z-

[Cohen, Gerald Leonard]

    I suppose this is very basic for the dialecticians in our group, but I've often wondered about it: Why is "Missouri" (with a double "s") pronounced with a
"z" (Mizzouree or Mizzourah)?  Similarly, "possess" is pronounced with a
"z" (after the "o").  But "Mississippi" is pronounced only with "s", as one would expect.

    I'd be very grateful for an insight into this.

Gerald Cohen