I heard St Johns River in FLA is only river in NA that flows north? True or false? If true, why?

In addition to the St. John's River in FLA...the Monongahela in western PA (one of the three rivers that meet at Pittsburgh, and the busiest in the US) is another. Why? might have something to do with gravity...and flowing 'downhill'...and if downhill happens to be north..so be it...just following the law (of gravity) I noticed that POTUS also answers Yahoo questions, under the name of KNOWITALL....because he said the Nile River is another North American river that flows North...at least he didn't say "up"!!!