The pain of a lost kingdom - just like a river of spring water flowing east, endless.
Tips
history
Closing line of 虞美人 by 李煜 (last ruler of Southern Tang). Full couplet: 问君能有几多愁?恰似一江春水向东流。Written as a captive of the Song - reportedly the line that got him poisoned by Song Taizong.
culture
Covered in 邓丽君 Teresa Teng's song 几多愁 (1983), which brought the whole 词 into modern pop consciousness.