the surname "Pohl" does not mean "by the river or
by the lake". Pohl is a German surname. I found out two
different explanation for its origin. One says "someone who
trades with Poles" the other derives the name from
the word "Pfuhl" which is an obsolete word for "swamp".
Pohl is in Germany quite a common name, but it originally came
from the former Eastern German provinces.
There is no connection to the name "Hitler" which is an Austrian
name that was probably derived from "Hiedler".
If you want to know more about your ancestry
you should find out from which German town your
family once came. With this information you can contact
the Mormones who have thousands of German "churchbooks"
(I don't know the correct English expression, I mean
the book, where the local priest wrote down births, marriages etc.).