Calvin Run is a long first-order tributary to Dunkard Creek in Greene County. This is the only stream of this name in the United States.
Calvin Run rises about 1.25 miles south of Kirby, Pennsylvania, and then flows southeasterly to join Dunkard Creek about 0.25 miles northeast of Mount Morris, Pennsylvania.
Calvin Run drains of area, receives about 43.1 in/year of precipitation, and is about 76.8% forested.