The Black River begins on a hill in Upper Saucon Township, Lehigh County. It flows north-northeast for a few tenths of a mile, then continues north-northeast for several tenths of a mile as the border between Salisbury Township and Lower Saucon Township. It passes through an unnamed pond and then turns east for several tenths of a mile, entering Lower Saucon Township, Northampton County. It then heads in a southeasterly direction for over a mile, crossing Interstate 78. Eventually, it gradually turns northeast, passing through another pond before heading in an easterly direction and entering the city of Bethlehem. Some distance later, the river turns east-northeast for a few tenths of a mile before turning north for a few tenths of a mile and reaching its confluence with Saucon Creek. Black River joins Saucon Creek upstream of its mouth.
Hydrology
Black River is not designated as an impaired waterbody. At its mouth, the peak annual discharge has a 10 percent chance of reaching. It has a 2 percent chance of reaching and a 1 percent chance of reaching. The peak annual discharge has a 0.2 percent chance of reaching. In April 1959, measurements of Black River near a private road from Wydnor found the creek's discharge to be.
Geography and geology
The elevation near the mouth of the Black River is above sea level. The elevation near the river's source is above sea level. The confluence of the Black River with Saucon Creek is approximately north of the confluence of Silver Creek with Saucon Creek. The Black River is described as a "miserable ditch" in Trout Unlimited's Guide to Pennsylvania Limestone Streams. It is a very small stream with only a trickle of stormwater runoff flowing through it. However, it is a limestone stream.
Watershed
The watershed of Black River has an area of. The mouth of the river is in the United States Geological Survey quadrangle of Hellertown. However, its source is in the quadrangle of Allentown East. The mouth of the river is at Hellertown. Most of the watershed of Black River is in Lower Saucon Township, Northampton County. However, smaller areas occupy parts of Upper Saucon Township, Lehigh County; Salisbury Township, Lehigh County; and Bethlehem, Northampton County. The northernmost corner of the river's watershed borders the southernmost part of Fountain Hill. Black River flows in the vicinity of the Saucon Fields tract of Lehigh University's campus in some reaches.
The drainage basin of the Black River is designated as a Coldwater Fishery and a Migratory Fishery. Wild trout naturally reproduce in the river from its headwaters downstream to its mouth. However, Trout Unlimited's Guide to Pennsylvania Limestone Streams said in 2000 that it was "not trout water". In 2006, the Black River was identified as one of the places in the Saucon Creek watershed most in need of habitat restoration. The river has been affected by Lehigh University.