The Lehigh Valley, which includes all of Lehigh and Northampton counties, is bounded on the north by Blue Mountain, a ridge of the Appalachian mountain range with an altitude of 1,300 to 1,604 feet (396 to 489 m), and on the south by South Mountain, a ridge of 700 to 1,100 feet (210 to 340 m) that cuts through the southern portions of the two counties.
The highest point in Lehigh County is Bake Oven Knob, a mass of Tuscarora conglomeratic rocks that rise about 100 feet (30 m) above the main ridge of the Blue Mountain in northwestern Heidelberg Township.

Lehigh County is in the Delaware River watershed.
 While most of the county is drained by the Lehigh River and its tributaries, the Schuylkill River also drains regions in the south of the county via the Perkiomen Creek and the northwest via the Maiden Creek.
The Lehigh Valley, which includes all of Lehigh and Northampton counties, is bounded on the north by Blue Mountain, a ridge of the Appalachian mountain range with an altitude of 1,300 to 1,604 feet (396 to 489 m), and on the south by South Mountain, a ridge of 700 to 1,100 feet (210 to 340 m) that cuts through the southern portions of the two counties. The highest point in Lehigh County is Bake Oven Knob, a mass of Tuscarora conglomeratic rocks that rise about 100 feet (30 m) above the main ridge of the Blue Mountain in northwestern Heidelberg Township.
Lehigh County is in the Delaware River watershed.  While most of the county is drained by the Lehigh River and its tributaries, the Schuylkill River also drains regions in the south of the county via the Perkiomen Creek and the northwest via the Maiden Creek.
Lehigh County, Pennsylvania