Loyalsock Creek is a 64-mile-long (103 km)[1] tributary of the West Branch Susquehanna River located chiefly in Sullivan and Lycoming counties in Pennsylvania in the United States.

The name is a corruption of a word in the language of the local indigenous peoples meaning "middle creek" (the original was something like Lawi-sahquick).

The area surrounding the confluence with the Susquehanna River has been flooded numerous times over the past decade, devastating many local homes and businesses.

Because Loyalsock Creek is in a sandstone, shale, conglomerate, and coal mountain region, it has a relatively low capacity to neutralize added acid.

This makes it especially vulnerable to increased acidification from acid rain, which poses a long-term threat to the health of the plants and animals in the creek.

The record flood of September 8, 2011 was due to a convergence of rare events, which included moisture from three tropical cyclones: The Loyalsock valley is deep and narrow, and the 435 sq.

Her village at the mouth of Loyalsock Creek on the West Branch Susquehanna River, now part of Montoursville, was an important stopping point for the Moravian missionaries who were spreading the gospel throughout the wilderness of Pennsylvania during the 1740s.

Jennings Brothers Lumber Co. built a narrow-gauge logging line in the area in 1890, and another in 1891 up the creek into Wyoming County, to supply their sawmill at Lopez.

In 1892, the Loyalsock Railroad crossed the creek here, building south to bring coal from the Bernice area southward to Harvey's Lake.

Traffic gradually dwindled on the Lehigh Valley; their line south of Lopez was abandoned in 1939, and it was cut back to Bernice in 1944.

Abandonment of the line to Lopez marked the end of rail traffic on the Loyalsock, except for the Reading Railroad crossing near its mouth at Montoursville.