The area is the site of a major archaeological site of the University of Illinois along the nearby Sangamon River of the homestead of Ira Sarlls, an early pioneer farmer to the area from New York[3] According to the 2010 census, the township has a total area of 67.39 square miles (174.5 km2), of which 67.33 square miles (174.4 km2) (or 99.91%) is land and 0.06 square miles (0.16 km2) (or 0.09%) is water.

Both streams are tributaries of the Sangamon River which runs from northeast to southwest through the township.

Willow Branch is part of Townships 17 and 18 North, Range 4 and 5 East of the Third Principal Meridian.

Robert Allerton Park, owned by the University of Illinois, is located in parts of Sections 20, 21, 22, 27, 28, 29, 30; T.18

The township contains eight cemeteries: Ater, Cronninger, Pioneer, Stringtown, Van Meter, Warfield Family, West France and Willow Branch.