A river’s source is a single spring vein. As the hike down the mountain grows on its way down towards the sea. If the water remained at spring vein… it would just be a smelly puddle.
Ryu-ha (流派) means a “branch of the current.” Anyone who founded a Bugei Ryu-ha (classical martial arts tradition; 武芸流派) made it originally for his own survival’s sake. Later, students, family members and close relatives who developed this ryu-ha’s ability to survive after life-changing experience, and it grew into a large flow.
The first, who founded the tradition, came of their descendants be called Ryuso (parent of the current; 流祖). Those who inherited his insights about the tradition are called Soke (family head; 宗家).