It flows approximately 4+1⁄4 miles (7 km) from its source near East Knoyle to join the River Nadder near Wardour.

It then turns east and flows under Savage Bridge and Share Lane before reaching the Nadder, northwest of Wardour.

Also on the south bank, it is fed by a stream that has its source in Bennet's Copse near Hart Hill Stud Farm; this stream flows northeast through Sem Hill and Billhay Pond before joining the Sem at Pondhead Copse.

Faults between the two layers give rise to the springs that provide the source of the water for the river catchment.

There are several components that are used to determine this, including biological status, which looks at the quantity and varieties of invertebrates, angiosperms and fish.