Hammersley Fork begins in central Eulalia Township, Potter County, near the Sinnemahoning Creek watershed.

It flows southeast to Elk Lick Knob, where it receives the tributary Black Mark Hollow.

Immediately after crossing Pennsylvania Route 144, the stream reaches its confluence with Kettle Creek.

[5] Hammersley Fork joins Kettle Creek 19.90 miles (32.03 km) upstream of its mouth.

[6] The concentration of alkalinity in Hammersley Fork ranges from 0.011 to 0.015 ounces per cubic foot (11 to 15 mg/L).

[7] A spot on Hammersley Fork near its mouth is the only location in the Kettle Creek watershed that has been channelized.

[14] Hammersley Fork flows over bedrock or large cobbles for much of the stretch between Dry Hollow and Nelson Branch.

From the mouth of Nelson Branch to 2 miles (3.2 km) downstream, the stream mostly flows in the central part of its valley.

[13] The mouth of the stream is in the United States Geological Survey quadrangle of Tamarack.

[11] A number of camps and residences are on the left bank of Hammersley Fork 0.2 miles (0.32 km) upstream of the mouth.

[15] Hammersley Fork was entered into the Geographic Names Information System on August 2, 1979 and its identifier is 1199877.

[14] A two-span steel stringer/multi-beam or girder bridge carrying T545 over Hammersley Fork was built in 1933 and repaired in 2010.

[18] The Cherry Springs Civilian Conservation Corps camp was built near the headwaters of Hammersley Fork in 1933.

[19] Wild trout naturally reproduce in Hammersley Fork from its headwaters downstream to its mouth.

[20] Great blue herons have been observed at the headwaters of Hammersley Fork, despite the fact that they are more commonly found in large river valleys.

The Pennsylvania Department of Environmental Protection also classifies the watershed as an exceptional-value area.

[8] The canopy coverage of the section of Hammersley Fork between Dry Hollow and Bunnell Run is 62 percent on average.

The upper part of the stretch of the stream between Nelson Branch and Dry Hollow has a canopy coverage of 93 percent.

The tree coverage at the confluence of Nelson Branch with Hammersley Fork is 70 percent.

Tree species in this stretch include hemlock, red maple, river birch, sycamore, and willow.