Chattanooga is the fourth-largest city in Tennessee, and the seat of Hamilton County. Located in southeastern Tennessee on Chickamauga Lake and Nickajack Lake, which are both part of the Tennessee River, Chattanooga lies approximately 104 miles (167 km) to the north-northwest of Atlanta, Georgia, 120 miles (190 km) to the southwest of Knoxville, about 135 miles (217 km) to the southeast of Nashville, and about 148 miles (238 km) to the northeast of Birmingham, Alabama. Chattanooga abuts the Georgia border, and the region is where three major interstate highways, I-24, I-75, and I-59, meet. The city, which has a downtown elevation of approximately 680 feet (210 m), lies at the transition between the ridge-and-valley portion of the Appalachian Mountains and the Cumberland Plateau. The city is therefore surrounded by various mountains and ridges. The official nickname for Chattanooga is the Scenic City.