The Tonge is formed at the Meeting of the Waters, where Astley Brook, flowing from the Smithills area in the west, meets the southerly-flowing Eagley Brook. From there, the Tonge flows southwards, to the east of Bolton, past Springfield, where it is joined by Bradshaw Brook, at the end of its route from the Jumbles and Wayoh reservoirs, close to Tonge Fold. The Tonge joins the River Croal at Darcy Lever, shortly before the Croal's confluence with the River Irwell.
Tonge Bridge section is a part of the river north of Tonge Bridge designated as a site of special scientific interest (SSSI). The site is 0.7 hectares (1.8 acres) and comprises the steep west bank of the river. It was designated an SSSI 1987 for its geological interest, principally the fluvial sandstone which has yielded pteridosperm seeds, known as Trigonocarpus. The well preserved nature of the seeds make it of considerable sedimentological and palaeogeographic interest. It is one of only seven geological SSSIs in Greater Manchester.