Flowing south through Cambodia's eastern regions is the Mekong River. East of the Mekong the transitional plains gradually merge with the eastern highlands, a region of forested mountains and high plateaus that extend into Laos and Vietnam. In southwestern Cambodia two distinct upland blocks, the Krâvanh Mountains and the Dâmrei Mountains, form another highland region that covers much of the land area between the Tonle Sap and the Gulf of Thailand.