The country's mountainous interior is surrounded by narrow coastal plains to the west, south, and east and by upland desert to the north along the border with Saudi Arabia. The Tihamah is a nearly long, semidesert coastal plain that runs along the Red Sea and is part of the Arabian Peninsula coastal fog desert ecoregion. The highland regions are interspersed with wadis, or river valleys, that are dry in the winter months (Yemen has no permanent rivers.) Most notable is the Wadi Hadhramaut in eastern Yemen, the upper portions of which contain alluvial soil and floodwaters and the lower portion of which is barren and largely uninhabited. Both the eastern plateau region and the desert in the north are hot and dry with little vegetation.