The Sydenham River is a river in Kent County, Lambton County and Middlesex County in southwestern Ontario, Canada flowing southwest from its source west of London, Ontario and empties into Lake Saint Clair. The length of the river is 100 kilometres and it drains a watershed of approximately 2,700 square kilometres. The river flows through the towns of Strathroy and Wallaceburg. Because this river flows through a large agricultural area, its waters collect silt and fertilizer runoff. In spite of this, the river provides habitat for 80 fish species and 34 species of freshwater mussels; these include:
- nine mussel species considered as "endangered" nationally
- Mudpuppy Mussel (Simpsonaias ambigua)
- Northern Riffleshell (Epioblasma torulosa rangiana)
- Rayed Bean (Villosa fabalis)
- Wavy-rayed Lampmussel (Lampsilis fasciola)
- Snuffbox (Epioblasma triquetra)
- Round Pigtoe (Pleurobema sintoxia)
- Kidneyshell (Ptychobranchus fasciolaris)
- Round Hickorynut (Obovaria subrotunda)
- Rainbow mussel (Villosa iris)
- two fish species and one turtle subspecies considered to be "threatened"
- Spotted Gar (Lepisosteus oculatus)
- Eastern Sand Darter (Ammocrypta pellucida)
- Eastern Spiny Softshell Turtle (Trionyx spiniferus spiniferus)
Unusual concretions, composed of calcite, can be found near this river. These are known as "kettles" because they resemble the bottom of a large kettle. There is also a second smaller Sydenham River in Grey County, Ontario which empties into Owen Sound harbour. Both rivers were named after Lord Sydenham, governor of Canada from 1839 to 1841.
