The North Branch Raritan River rises in Morris County, in eastern Mendham Borough, and flows generally southward into Somerset County, around the southern end of the Watchung Mountains. At its end, it forms the border between Bridgewater and Branchburg Townships, and upon reaching the border of Hillsborough Township, joins the South Branch Raritan River to form the main Raritan River, which generally flows eastward from that point. This area where the branches converge was called "Tucca-Ramma-Hacking" by the Lenape, meaning the flowing together of water. It was called "Two Bridges" by the early European settlers, after a set of bridges built in 1733 that met at a small island (the island has washed away over time) on the North Branch. Today the area is generally referred to as "The Confluence". In the 1970s, the state discussed plans for a Raritan Confluence Reservoir, which have been shelved due to acquisition costs.
The river gives its name to the unincorporated village of North Branch, which lies on both sides of the river in Branchburg and Bridgewater Townships.