Khar Us Nuur (15,800 sq km, average depth 4m) is a vast marshy delta and Mongolia's third biggest lake by area, home to wild ducks, geese, wood grouse, partridges and rare sea birds. Twitchers, however, may be disappointed: the lake is difficult to reach because of the marshes, and locals know little about the birdlife. Best to go with a national park worker in May or late August and head for the delta where the Khovd Gol enters the lake.
The outflow from Khar Us Nuur goes into a short river called Chono Khairkhan, which flows into another freshwater lake, Khar Nuur (Black Lake), home to migratory pelicans. The southern end of Khar Nuur flows into Dörgön Nuur, a large alkaline lake that's good for swimming; its eastern side is an area of bone-dry desert and extensive sand dunes.
For day trippers (or if you're just passing through and fancy a peek), a convenient place to see the Khar Us Nuur is off the paved Khovd–Altai Rd near the southwest corner of the shore, where a metal watchtower has been set up to view the nearby reed islands. Better still, travel around the southern tip, where the going is less marshy and the road (a sandy track, What3words: enthralling.punks.stem) closes to within 200m of the shore for about 15km northwards towards the end of the Jargalant Khairkhan Uul massif.
The lake is plagued by mosquitoes in summer.