The area around the mouth of the Sagami-gawa River is also called Banyu-gawa River (write:馬入川, kanji meaning 'horse,' 'enter,' 'river') and it is said that the name originated from Yoritomo's horse suddenly acted up and entering the river, and Yoritomo fell from the horse.
また相模川河口付近は馬入川とも呼ばれており、頼朝の跨った馬が突然暴れて川に入り、落馬に至った事に由来するとも伝わる。