Rapid Run is a tributary of Buffalo Creek in Centre County and Union County in Pennsylvania, in the United States. It is approximately long and flows through Miles Township in Centre County and Hartley Township, Lewis Township, West Buffalo Township, and Buffalo Township in Union County. The watershed of the stream has an area of . Some streams in the watershed are impacted by nutrients, sediment, E. coli, and thermal radiation and one unnamed tributary is designated as an impaired waterbody. Rapid Run is a freestone mountain stream in the ridge and valley physiographic province. It flows through a lake known as Halfway Lake and also passes through the Rapid Run Gap.
Rapid Run is one of the major tributaries of Buffalo Creek. Its watershed is mostly forested, but there is some agricultural land and impervious surfaces as well. Pennsylvania Route 192 runs alongside the stream for much of its length. A number of bridges have been constructed across the stream. The drainage basin is designated as a High-Quality Coldwater Fishery and a Migratory Fishery. Additionally, a reach of Rapid Run is Class A Wild Trout Waters. Various eels and freshwater sponges have also been observed in the stream. Rapid Run flows through Bald Eagle State Forest, R.B. Winter State Park, and Sand Bridge State Park.
Course
Rapid Run begins in a valley in Miles Township, Centre County. It flows in a generally east-northeasterly direction for a few miles and receives the tributary Yankee Run from the left before entering Hartley Township, Union County. In this township, the stream crosses Pennsylvania Route 192 and passes by a spring called The Little Bubbler. The stream then turns south-southeast, passing through Halfway Lake, where it receives the tributary Halfway Run from the left. From the southern end of the lake, it continues flowing south-southeast, crossing Pennsylvania Route 192 again and entering a considerably narrower valley that it shares with Route 192. It passes by Bake Oven Mountain and Seven Notch Mountain for several tenths of a mile before turning east-northeast to stay in the valley.
After more than a mile, Rapid Run enters Lewis Township, Union County. Here it continues flowing east in its valley, alongside Pennsylvania Route 192 and Jones Mountain. The stream crosses Pennsylvania Route 192 twice and spends several miles in Lewis Township before entering West Buffalo Township. In West Buffalo Township, it continues heading east for more than a mile before crossing Pennsylvania Route 192 one last time and turning east-southeast. The stream then leaves its narrow valley and flows alongside a much shallower, broader one that it shares with Stony Run. After a few miles, it enters Buffalo Township, where it continues flowing southeast. After several tenths of a mile (about 0.5 km), it reaches its confluence with Buffalo Creek.
Rapid Run joins Buffalo Creek upstream of its mouth.
Tributaries
Rapid Run has two named tributaries: Yankee Run and Halfway Run. Yankee Run joins Rapid Run upstream of its mouth and its watershed has an area of . Halfway Run joins Rapid Run upstream of its mouth and its watershed has an area of .
Hydrology
A total of of tributaries to Rapid Run are designated as impaired waterbodies. The cause of the impairment is nutrients and siltation. The source of the impairment is grazing-related agriculture. The total maximum daily load date is 2015. In addition to nutrients and sediment, the watershed is impacted by E. coli and thermal radiation. At the point where the stream drains , the average daily discharge is . Further downstream, where it drains , its average daily discharge is . At the point where it drains , the average daily discharge is .
In 2000, the sediment load in Rapid Run was , while in 2008, it was . This equates to a sediment load of slightly over per year. However, in the future, the load could be reduced by 54.86 percent to per year. In 2000, of sediment came from streambank erosion, came from row crops, came from low-density urban land, came from hay and pastures, and came from unpaved roads. Other sources contributed of sediment.
The nitrogen loads of Rapid Run in 2000 and 2008 were , respectively. This equates to less than per year. However, the annual load could potentially be reduced by 20.43 percent to . In 2000, of nitrogen came from groundwater/subsurface, came from row crops, came from hay and pastures, came from low-density urban land, and came from septic systems. came from unpaved roads, came from streambank erosion, and came from miscellaneous sources.