The Ongi River (Mongolian: Онги гол) flows through the endorheic Ongi River Basin in Mongolia and past Arvaikheer. In some particularly wet years, it used to empty into Ulaan Lake, in most years it dries up earlier. In recent years it has been additionally threatened by 37 mining operations within the basin, but successful pressure by Tsetsgeegiin Mönkhbayar and the Ongi River Basin Movement helped convince 35 of the operations to cease explorations and harmful activities in the region.