[2] The North Branch Raritan River rises in Morris County, in eastern Mendham Borough rising out of Raritan Pond, and flows generally southward into Somerset County, around the southern end of the Watchung Mountains.

[2] 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.

In the 1970s, the state discussed plans for a Raritan Confluence Reservoir, which have been shelved due to acquisition costs.

[citation needed] Both North and South branches of the Raritan run nearly parallel southwards, east and west of one another, but receive their names from the direction each one flows from at their confluence.