noun, noun which may take the genitive case particle 'no'
fresh water
Refers to water that is not salty, such as river, lake, or rain water. Often contrasted with 海水 (seawater) or 鹹水 (salt water).
See also: 鹹水
この湖は淡水で、泳ぐのに適している。
This lake is fresh water and suitable for swimming.
淡
Freshwater fish are said to be harder to keep than saltwater fish.