low-water mark
/ˈləʊˈwɔːtə(r)/noun
1- the level reached by the sea at low tide, or by a lake or river during a drought or dry season低潮线; 低水位线。
1.1
- a minimum recorded level or value最低点, 最低程度：
the market was approaching its low-water mark.
行情已经接近最低点。