The named river starts as compensation flow (that is, a guaranteed minimum discharge[3]) from Readycon Dean Reservoir in the moors above Denshaw.

The river flows generally south through Delph, Uppermill, Mossley, Stalybridge, Ashton-under-Lyne, Dukinfield, Haughton Green, Denton and Hyde.

The Division Bridge (which spans the river at Mossley), marks the meeting point of the traditional boundaries of Lancashire, Yorkshire and Cheshire.

The anti-pollution efforts of the last thirty years of the 20th century resulted in the positive fauna distributions listed below.

Portwood weir is 1¼ miles above the confluence with the Mersey and contains the great majority of the final flow (with the exception of waste water from a concrete facility).

[5] For its course after the Division Bridge in Mossley the river marks much of the historical boundary dividing Cheshire and Lancashire.

The lower reaches (near Reddish Vale Country Park) are home to coarse fish such as gudgeon (Gobio gobio), chub (Leuciscus cephalus), and roach (Rutilus rutilus); pike (Esox lucius) and perch (Perca fluviatilis) are also present.

The upper reaches (above Ashton) support brown trout (Salmo trutta) and smaller numbers of some coarse fish.