Is this a saying in Norwegian?

Foss remains in some northern English dialects and as a place name - High Force, Falling Foss, Thomason Foss.

Like, she runs at the mouth?

We have the same saying in German. ☺

Love foreign idioms.

In certain parts of America we would say "she is a motor mouth"

In English there is something similar: she's like a babbling brook.

like this http://www.europeanwaterfalls.com/wp-content/uploads/Kjosfossen-7-G.jpg like my mom

Ironically, I learned about norsklærer Karense from the comments on these lessons and I first learned this idiom from her. I love the idiom, and I used to have a best friend named Sam Foss.

Does this mean she talked a lot, or quickly?

This little puddle of consciousness, treading water...

Verbal diarrhoea !