Alder Brook is a long tributary to West Branch French Creek classified as a 1st order stream by the EPA Waters Geoviewer.
Alder Brook rises in Venango Township in eastern Erie County, Pennsylvania, and flows east to meet West Branch French Creek north of Lowville.
The Alder Brook watershed is part of the Erie Drift Plain (glacial geology) and covers an area of . The watershed receives an average of 46.2 inches per year of precipitation and has a wetness index of 473.57. About 41% of the watershed is forested.