noun
freshwater lake
A lake containing fresh water, as opposed to saltwater or brackish water.
琵琶湖は日本最大の淡水湖です。
Lake Biwa is the largest freshwater lake in Japan.
塩水湖 means a saltwater lake, the opposite of 淡水湖.
Compound of 淡水 (fresh water) and 湖 (lake).