Big Moose Mountain stands within the watershed of the Kennebec River, which drains into the Gulf of Maine. The northeast side of Big Moose Mtn. drains into Moosehead Lake, the source of the Kennebec. The southeast end of Big Moose Mtn. drains into Squaw Brook, then into Moosehead Lake. The southwest side of Big Moose Mtn. drains into Indian Brook, then into Indian Pond and the Kennebec River.