To this a (former) New England whitewater kayaker, it’s just the Dead River, but I concede that in a geographic/cartography context it’s the west branch of the Kennebec as the bit below the confuence at The Forks is the Kennebec. To apaddler,“West Branch” is taken to mean the west branch of the Penobscot. The only references to branches of the Kennebec paddlers use if the east outlet of the Kennebec (the section from Moosehead Lake to Indian Pond).