Boundary Bald Mountain is a remote peak in western Maine’s Somerset County, southeast of the Canada–U.S. border with Quebec, standing between Ironbound Mountain and Sandy Bay Mountain near the headwaters of major river systems. Its significance lies in its position on a drainage divide within Maine’s interior. Water from its north side flows through Alder Brook, Fish Brook, and Hale Brook into the South Branch of the Penobscot River, then to Penobscot Bay. Water from its south side runs through Churchill Stream, Upper Churchill Stream, and Heald Stream into the Moose and Kennebec rivers before reaching the Gulf of Maine.