Khar-Us, black water, is a freshwater lake located south of the city of Khovd, at an altitude of 1,157 meters. It stretches 72 km long and 37 km wide. It is remarkably shallow: between 2 and 4 meters deep on average. It contains 3,432 km³ of water. It is the upper lake in an interconnected lake system, known as the Great Lakes Depression.

The Khovd, Buyant and Tsenkher rivers flow into the lake, while the Chono Kharaikh river originates there.

Lake Khar-Us has distinctive geological features, with its marshy shores overlooked by peaks covered in perpetual snow. In the middle of the lake lies the island Ak-Bashi, meaning "white head" in the Kazakh language, which divides the lake into two sections: the South section and the North section. This island is located 272 meters from the lake's edge.