From the 《
礼记·
学记》 line
三王之祭川也，
皆先河而后海 ('the Three Kings, when sacrificing to rivers, sacrificed first to the rivers and only then to the sea'). The reasoning: rivers were thought to feed the sea, so they were honored first as the source. 'Opening the
先河' came to mean inaugurating something — establishing the headwaters from which others would follow.
