low water mark

例句：

1. 1. The low water mark is just above the surface of the lake.
   低水位线就在湖面以下。
2. 2.The boat was close to the low water mark when it hit a rock.
   船在撞上岩石时离低水位线很近。
3. 3. The fishermen were careful not to disturb the low water mark on the riverbed.
   渔民们小心地不碰触河床上的低水位线。