Fall Brook is a tributary of the Lackawanna River in Susquehanna County and Lackawanna County, Pennsylvania, in the United States. It is approximately long and flows through Clifford Township in Susquehanna County and Fell Township, Carbondale Township, and Carbondale in Lackawanna County. The watershed of the stream has an area of , making it one of the largest tributaries of the Lackwanna River. It is not designated as impaired, but does experience flow loss. The stream begins on the Allegheny Plateau and passes through the Fall Brook Gap. It also flows over the Fall Brook Falls, which are high. Fall Brook is situated within the Coal Region.
The upper reaches of the watershed of Fall Brook are mainly forested. However, some disturbed land is in the watershed's lower reaches. Lakes in the watershed include Fall Brook Lake. A number of sawmills were built along the stream in the 19th century and a number of bridges were constructed across it in the 20th century. Projects such as channelization and riprapping have been done on reaches of the stream in the second half of the 20th century. Fall Brook is a Coldwater Fishery and a Migratory Fishery. Wild trout naturally reproduce within it.
Course
Fall Brook begins in a wetland near Birchtown in Clifford Township, Susquehanna County. It flows south-southwest for a few tenths of a mile before crossing Pennsylvania Route 247 and turning south. The stream then turns south-southwest again for several tenths of a mile, receiving an unnamed tributary from the right and exiting Clifford Township and Susquehanna County.
Upon exiting Susquehanna County, Fall Brook enters Fell Township, Lackawanna County. It continues south-southwest for more than a mile, passing through another wetland and receiving several more unnamed tributaries: three from the left and one from the right. The stream then turns south-southeast for several hundred feet before turning south-southwest again. After more than a mile, it turns southeast for a short distance and receives another unnamed tributary from the right before turning east-northeast. For the next several tenths of a mile, the stream flows east-northeast alongside Pennsylvania Route 106. The stream then turns east-southeast, entering a water gap and passing through Fall Brook Lake. After several tenths of a mile, it begins meandering south through the water gap for more than a mile, crossing Pennsylvania Route 106 several times. At the southern end of the water gap, it turns south and very briefly passes through Carbondale Township before entering Carbondale. A few tenths of a mile further downstream, the stream turns south-southeast. After several tenths of a mile, it turns south-southwest and receives an unnamed tributary from the right. The stream then turns south and reaches its confluence with the Lackawanna River.
Fall Brook joins the Lackawanna River upriver of its mouth.
Tributaries
Fall Brook has no officially named tributaries. However, it has a number of unofficially named tributaries. These include "Mountain Mud Pond Run", "Sandy Banks Run", "Unnamed trib 1", "Unnamed trib 2", "Finch Hill Run", "Crystal Lake Creek", and "Unnamed trib 3".
Hydrology
Fall Brook is not designated as an impaired stream. However, the stream experiences flow loss to underground mine pools in Carbondale. The flow loss is caused by past deep mining and surface mining. In the summertime, reaches of the stream are completely dry in low flow conditions. Additionally, the lower reaches were affected by acid mine drainage as of the early 1990s. Around this time, the pH was found to be 6.9.
At its mouth, the peak annual discharge of Fall Brook has a 10 percent chance of reaching per second. It has a 2 percent chance of reaching per second and a 1 percent chance of reaching per second. The peak annual discharge has a 0.2 percent chance of reaching per second.
In the early 1900s, waste water from Murrins Colliery was discharged into Fall Brook. However, most of the stream's length was fairly clear at that time, despite having one culm deposit in its vicinity. The city of Carbondale has had an NPDES permit to discharge stormwater into Fall Brook.
Geography and geology
The elevation near the mouth of Fall Brook is above sea level. The elevation of the stream's source is between above sea level.