The Afon Clarach is a small river discharging into Cardigan Bay north of Aberystwyth. It flows through a largely agricultural and forested landscape except in its lowest reaches where its banks are crowded with holiday developments. In its headwaters it comprises a number of small tributaries including the Afon Stewy, and the Nant Seilo with the Clarach itself originating on high ground near Capel Dewi. Tha course of the river takes it through Garth Penrhyncoch and Penrhyncoch and then past Plas Gogerddan, the University of Aberystwyth experimental farm. Between Bow Street and Llangorwen, the river is joined by the Bow Street Brook.
In past centuries, the upper reaches of the river have been affected by lead and silver mining and the quality of the river has suffered as a result. There is some organic pollution from diffuse agricultural inputs and from the discharge of treated sewage from the Bow Street sewage treatment works
The river discharges to the sea over a beach into Clarach Bay. This limits the population of migratory fish able to access the river and as a result Salmon and sea-trout are uncommon. Brown Trout, brook lampreys, millers thumb and stone loach are however commonly found.