water level gauge; watermark
Refers to a physical marker or gauge that shows water level, such as on a riverbank, reservoir, or in a boiler. The English 'watermark' here means a mark indicating water level, not a digital watermark.
Looking at the water level gauge on the riverbank, the water level was higher than yesterday.
The water level gauge at the dam is installed to monitor the water storage.
Compound of 水位 (water level) and 標 (mark, sign). The exact historical derivation is uncertain, but the word is a straightforward technical term.