of this is the Gulf Coastal Plain, a broad feature that begins at the Gulf of Mexico and extends northward into southern Illinois. The plain begins in the east with low rolling hills and wide stream valleys, known as the West Tennessee Highlands, and gradually levels out to the west. It ends at steep loess bluffs overlooking the Mississippi embayment, the westernmost physiographic division of Tennessee, which is part of the larger Mississippi Alluvial Plain. This flat wide strip is commonly known as the Mississippi Bottoms, and contains lowlands, floodplains, and swamps. Hydrology Tennessee is drained by three major rivers, the Tennessee, Cumberland, and Mississippi. The Tennessee River begins at the juncture of the Holston and French Broad rivers in Knoxville, flows southwest to Chattanooga, and exits into Alabama before reemerging in the western part of the state and flowing north into Kentucky. Its major tributaries include the Clinch, Little Tennessee, Hiwassee, Sequatchie, Elk, Beech, Buffalo, Duck, and Big Sandy rivers. The Cumberland River flows through the north-central part of the state, emerging in the northeastern Highland Rim, passing through Nashville, turning northwest to Clarksville, and entering Kentucky east of the Tennessee River. Its principal branches in Tennessee are the Obey, Caney Fork, Stones, Harpeth, and Red rivers. The Mississippi River drains nearly all of West Tennessee. Its tributaries are the Obion, Forked Deer, Hatchie, Loosahatchie, and Wolf rivers. The Tennessee Valley Authority (TVA) and the U.S. Army Corps of Engineers operate many hydroelectric dams on the Tennessee and Cumberland rivers and their tributaries, which form large reservoirs throughout the state. About half the state's land area is in the Tennessee Valley drainage basin of the Tennessee River. The Cumberland River basin covers the northern half of Middle Tennessee and a small portion of East Tennessee. A small part of north-central Tennessee is in the Green River watershed. All three of these basins are tributaries of the Ohio River watershed. Most of West Tennessee is in the Lower Mississippi River watershed. The entirety of the state is in the Mississippi River watershed, except for a small sliver near the southeastern corner traversed by the Conasauga River, which is part of the Mobile Bay watershed. Ecology Tennessee is within a temperate deciduous forest biome commonly known as the Eastern Deciduous Forest. It has eight ecoregions: the Blue Ridge, Ridge and Valley, Central Appalachian, Southwestern Appalachian, Interior Low Plateaus, Southeastern Plains, Mississippi