[1] It rises at Cwm y Saeson, and flows east past Cefn Llech to Sychnant, where it is joined by the Full Brook.

Here it turns north past the ford at Rhyd Myheryn at Glanyrafon, receiving the Rhydyclwydau Brook on the right bank.

Turning north once more, the river passes Upper Glandulas where it meets its last tributary the Afon Brochan, before joining the Severn to the south-west of Llanidloes at Felindre.

This long-term record shows that the catchment of 52.7 square kilometres (20.3 sq mi) to the gauging station yielded an average flow of 1.4 cubic metres per second (49 cu ft/s).

[4] The Dulas catchment upstream of the station has a substantial average annual rainfall of 1,269 millimetres (50.0 in) and a maximum altitude of 571 metres (1,873 ft) at Crugyn Llwyd hill on the eastern edge of the watershed, near to the Pegwn Mawr windfarm.