The South Branch Machias River is a 13.6-mile-long (21.9 km) river in Aroostook County, Maine. From the outflow of Center Pond (46°31′25″N68°44′49″W) in Maine Township 10, Range 8, WELS, the river runs east to its confluence with the Machias River in T.10 R.7 WELS. Via the Machias and Aroostook rivers, it is part of the St.