The Fifteenmile Creek watershed in western Maryland and south-central Pennsylvania is approximately 64 square miles in area and is located in a rural part of the Ridge and Valley physiographic province. Fifteenmile Creek originates as a small stream on the flank of Ragged Mountain several miles north of the village of Artemas in Bedford County, Pennsylvania. The creek flows in a southerly direction, crossing into Maryland at the Mason-Dixon Line, and flowing under Scenic Route 144 and Interstate 68. Turning to the east and then to the southeast, the creek continues its 19.3 mile long journey through the Maryland countryside. Near the town of Little Orleans, Maryland—at which point it is a major 4th-order tributary—the creek flows under the Chesapeake and Ohio Canal aqueduct just before reaching its confluence with the Potomac River.
The Fifteenmile Creek watershed contains numerous major tributaries, including: Bear Camp Branch, Pine Lick, Little Pine Lick, Big Divide Run, Piclic Run, White Sulfur Run, Deep Run, Terrapin Run, Mud Lick, Spring Lick, and Flat Run. Fifteenmile Creek is one of the Potomac River’s most pristine tributaries, owing in part to the fact that 92% of its watershed is forested land, of which nearly two-thirds is protected state land: Buchanan State Forest in Pennsylvania, Green Ridge State Forest in Maryland, and Billmeyer Wildlife Management Area in Maryland.
The mainstem of Fifteenmile Creek is presently ungaged, but two major tributaries (Deep Run and Terrapin Run) have been gaged since 2005. Flow records reveal that streamflow in the basin is highly variable in nature. The watershed is capable of producing flash floods in response to intense rainfall and long periods of low (or zero) flow during persistent droughts. Prolonged periods of extremely low flow are relatively common in the hydrologic record, reflecting the combined influences of a rain shadow effect and the underlying bedrock lithology that is unable to sustain baseflow for long periods without rainfall. Under these conditions, much of the creek and its tributaries actually resemble dry cobblestone roads with the only surface water found in a few stagnant pools.