Big Spencer Mountain in Piscataquis County, Maine, stands beside Little Spencer Mountain within an unusually complex watershed. Different sides of the mountain send runoff into separate drainage systems. Some water reaches Lobster Lake and the West Branch of the Penobscot River by way of Kidney Brook, while other flow passes through Blood Pond and Maxfield Brook to the same lake. Part of the southeast side drains through Bear Brook, Ragged Lake, and a chain of lakes and streams to the West Branch Penobscot, but the southern part flows by Lazy Tom Stream to the Roach River and Moosehead Lake, source of the Kennebec River. The mountain thus forms part of the divide between the Penobscot and Kennebec watersheds.