The Thornapple rises in northeast Sawyer County in the Chequamegon National Forest at 45°56′21″N 90°45′29″W / 45.93917°N 90.75806°W / 45.93917; -90.75806 (Thornapple River (source)).
One is a right-side tributary entirely within Sawyer County near the boundary with Rusk County, flowing from 45°40′17″N 90°52′31″W / 45.67139°N 90.87528°W / 45.67139; -90.87528 (Little Thornapple River (source)) primarily to the west and slightly south into the Thornapple River at 45°39′09″N 90°58′52″W / 45.65250°N 90.98111°W / 45.65250; -90.98111 (Little Thornapple River (mouth)).
[2] The other Little Thornapple River is a left-side tributary that rises in southern Sawyer County at 45°38′30″N 91°01′39″W / 45.64167°N 91.02750°W / 45.64167; -91.02750 (Little Thornapple River (source)), less than 2 miles (3.2 km) west-southwest of the mouth of the other Little Thornapple River.
[3] Largely undeveloped, the river is a popular destination for canoeing and kayaking.
[4] Small rapids and good fishing conditions are attractions.