Did You Know? The Mississippi River forms most of Arkansas's eastern border, except in Clay and Greene counties where the St. Francis River forms the western boundary of the Missouri Bootheel, and in dozens of places where the current channel of the Mississippi has meandered from where it had last been legally specified.[5] Arkansas shares its southern border with Louisiana, its northern border with Missouri, its eastern border with Tennessee and Mississippi, and its western border with Texas and Oklahoma. Arkansas is a land of mountains and valleys, thick forests and fertile plains. The so-called Lowlands are better known by names of their two regions, the Delta and the Grand Prairie. The Arkansas Delta is a flat landscape of rich alluvial soils formed by repeated flooding of the adjacent Mississippi. Further away from the river, in the southeast portion of the state, the Grand Prairie consists of a more undulating landscape. Both are fertile agricultural areas.