|- location||East Knoyle, Wiltshire, England|
|Wardour, Wiltshire, England|
|98 m (322 ft)|
|Length||7.03 km (4.37 mi)|
|Basin size||25.6 km2 (9.9 sq mi)|
The River Sem is a natural waterway that flows through the ceremonial county of Wiltshire in England. It flows approximately 4 1⁄4 miles (7 km) from its source near East Knoyle to join the River Nadder near Wardour.
The source is Summerleaze Pond near East Knoyle, just east of the A350. The river flows southeast to Pondhead Copse, below which it flows through two large ponds: Brach Pond and Eeelstage Pond. It then turns east and flows under Savage Bridge and Share Lane before reaching the Nadder, northwest of Wardour.
The river is fed by several small waterways. On its south bank, a stream with a source near Whitebridge Farm southeast of Sedgehill, again close to the A350, flows east to join the Sem near Billhay Farm, above Pondhead Copse.
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.
- "Sem". Catchment Data Explorer. Environment Agency. Retrieved 25 October 2017.
- "Hamphsire Avon Demonstration Test Catchment Project". Retrieved 16 October 2017.