From Leigh, Greater Manchester to the River Mersey it forms the county boundary with Cheshire.

[1] It is 22 miles (35 km) long and its main tributaries are the Astley, Bedford, Hey, Pennington, Shaw and Westleigh Brooks.

[3] After picking up the waters of Bedford Brook, which runs southward from Leigh and the Black or Moss Brook coming west from Worsley via Chat Moss, the brook turns southward, ultimately draining into the River Mersey section of the Manchester Ship Canal near Cadishead.

The catchment drains the flat lowland around Leigh which reaches a maximum altitude of 158 mAOD.

The underlying geology is the sandstone and coal measures of the Lancashire Coalfield.