The Khorat region is a saucer-shaped low platform of approximately 60,000 square miles drained to the southeast, with intermittent lakes in the north and flooded land during the rains in the south. It is bordered on the north and east by the Maekhong River and the mountains of Laos, on the west by the Phetchabun Mountains and Dong Phrayayen, and on the south by the Sankamphaeng and Dong Rek scarps. The interior of the region is undulating and dotted here and there by low hills and small shallow lakes. Large areas are flooded during the wet season, but the region suffers in the dry season for lack of water. The soils for the most part are thin and poor.
The altitude of the northern and western borders of the plain is from 400 feet to 700 feet, while Ubon near the southeast corner is hardly over 200 feet. The main portion of Khorat stands between 300 and 650 feet above sea level. A group of hills rises several hundred feet above the plains along the north and east, and a few flat-topped mountains near the Maekhong exceed 1,600 feet.
Almost the entire region is drained by a single river system, the Mun or La Moon, which is a tributary of the Maekhong. Only on the northern and eastern edges are there a few small tributaries which drain directly into the Maekhong. Khorat rivers generally flow through wide, shallow valleys but their beds cut sharply into the floodplains. In the rainy season the rivers quickly fill their channels and overflow the wider valleys. Because of the low relief, the high water flows off very slowly and therefore the lower portions of the lowlands cannot be used for rice agriculture. There is annual flooding along the lower course of the rivers which flow into the Maekhong. The Maekhong itself floods the lowlands along the lower courses of the tributary streams and, particularly in the northeastern part of Khorat, the high waters form a series of lakes which are dammed by quantities of silt left when the current is slowed. Each year these lakes shrink considerably during the dry season and expand during the summer high-water season of the Maekhong.
The surface of Khorat is characterized by numerous shallow ponds, which appear to be related to the solution of the salt lenses that are believed to occur in the red sandstone formation. The high salt content of the soils of Khorat, particularly around the margins of these ponds, can hardly be explained otherwise. Rising ground water in the summer rainy season dissolves salt and brings it up into the superficial layers. In the dry season an accumulation of salt develops at the surface of the soil through evaporation and by capillary rise of moisture from the deeper layers. Then, crusts of salt develop on the surface in many localities.
The marginal areas of Khorat are of special morphological interest. In the west and south the mountain zone is 20 to 25 miles wide and very considerable altitudes are attained. Along the western rim, Khao Phanghoei, 15° 38′ north latitude, is about 3,000 feet, while the Pasak River Valley floor at the west foot of that peak is only about 300 feet above sea level. At the southwestern corner of Khorat, where the western and southern rims meet, Khao Laem rises to 4,300 feet and Khao Kamphaeng to more than 3,300 feet. Farther east, along the escarpment dividing Khorat from the plains of Cambodia, an altitude of 2,300 feet is attained, while along the Maekhong, below the mouth of the Mun River on the Indo-Chinese side, stands a group of mountains, the Phu Pasak, which rises to an altitude of 4,600 feet.