Lilley Run is a 3.66 mi (5.89 km) long 2nd order tributary to South Branch French Creek in Erie County, Pennsylvania.

[4] Lilley Run rises in Sparta Township, Crawford County, Pennsylvania, north of Crawford County and then flows north into Erie County through Concord Township where it meets South Branch French Creek.

Lilley Run drains 4.77 square miles (12.4 km2) of Erie Drift Plain (glacial geology).

The watershed receives an average of 46.8 in/year of precipitation and has a wetness index of 453.45.

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