As we were walking along we came across a river we couldn't pass, so we turned back.

turned 是动词 turn 的过去式，turn 的基本意思是"转动、转向"。 back 在这里是副词，表示"向后、往回"的方向。 turned back 这个短语表示"转身往回走"，即"返回"的意思。