The four largest tributaries of the First Fork are: Tarkiln Run with a watershed of 2 sqmi or 15.4% of the First Fork watershed; Mud Run with 2.22 square miles (5.75 km 2 or 12.6%); Dog Run with 1.82 square miles (4.71 km 2 or 10.3%); and Marsh Run with 0.96 square miles (2.49 km 2 or 5.5%).