>it will be more water on a hillside than many of those hillsides can bear
A good example was the Oso mudslide. Nearby Darrington gets 80 inches a year. (Twice as much as Seattle!) March 2014, a storm drops 3 to 5 inches on soil saturated by six months of rain, and a hill lets go. Killed 43 people: https://en.wikipedia.org/wiki/2014_Oso_mudslide