The river begins in the northeastern part of Burlington in northcentral Middlesex County Massachusetts and heading generally southeasterly, it passes through the towns of Wilmington (where once the important Middlesex Canal passed over it on an aqueduct), Reading, thence to North Reading, where it is joined by the left bank tributary Skug River — which originates in North Andover and Andover southwest of Boston Hill in a large Beaver Pond and marshland, situated north of Gray Road-jct.-Gray Street (very near the townlines of the two towns) where it crosses via culvert south of Rt. MA 125 and west of Rt. MA 114 (historically, and by street name, the Salem Turnpike), and thence, about a mile south, gains volume draining all of Harold Parker State Forest in the tri-town corner of Andover, North Andover, and Middleton — excepting the few acres draining to Ipswich tributary, Boston Brook along the other side of MA 114.