These high mountains are incised by steep river valleys and upland areas that border the central plain. A series of rivers, including the Nan, Ping, Wang, Yom, and Nan, flow southwards through mountain valleys and join to form the Chao Phraya in Nakhon Sawan Province in the central region. Sirikit Dam is on the Nan River in Uttaradit Province. The northeastern part is drained by rivers flowing into the Mekong basin, like the Kok and Ing.The four-region system includes the northern parts of the central plain as well as some mountainous areas bordering the western and the northeastern limits.