가람, garam, is what Korean called a river before 강 — borrowed from Chinese — took the job over.
It survives in place names and in old songs. You will find it in the name of a bridge, or a village, and no one living there will be able to tell you what it means.
Say the two words one after the other and they sit differently in the mouth. 강 is short, closed, the line on a map. Garam opens twice and keeps going, and sounds like the thing it is naming.
The river is the same river. It was only the word that went.