Who is this Mrs. Sippy Twain is talking about?

In the space of one hundred and seventy-six years the Mississippi has shortened itself two hundred and forty-two miles. Therefore ... in the Old Silurian Period the Mississippi River was upward of one million three hundred thousand miles long ... seven hundred and forty-two years from now the Mississippi will be only a mile and three-quarters long. ... There is something fascinating about science. One gets such wholesome returns of conjecture out of such a trifling investment of fact.
            — Mark Twain


("Mrs. Sippy" is a trite wordbird entitlement for any number of coffee dripperies and beangrinders in Middle Girth, as it turns out. Google on your own dime.)