[3] It enters the English Channel at the western end of Lyme Bay, part of the Jurassic Coast, a UNESCO World Heritage Site.

The Permian and Triassic sandstone aquifer in the Otter Valley is one of Devon's largest groundwater sources, supplying drinking water to 200,000 people.

The river's source is north of Otterford, where a stream feeds the Otterhead Lakes: ST225152 and then through Churchstanton before entering Devon.

[2] The Otter Estuary Nature Reserve is a 57-acre (230,000 m2) Site of Special Scientific Interest (SSSI) consisting of tidal mudflats and saltmarsh.

The site was excavated by Channel 4's archaeological television programme Time Team, the episode "The Domesday Mill" being broadcast in 2007.

[citation needed] The Otter is the only river in England known to contain a wild breeding population of Eurasian beavers, a species that died out in Britain in around 1550.

A sport fishing industry lobbyist group, the Angling Trust, said "it would be irresponsible even to consider re-introducing this species into the wild without first restoring our rivers to good health.

"[8] This decision to immediately remove the beavers was protested by local residents and campaign groups, with environmental journalist George Monbiot describing the government and Angling Trust as "control freaks": "I'm an angler, and the Angling Trust does not represent me on this issue...most anglers, in my experience, have a powerful connection with nature.

"[9][10] The introduction of beavers to rivers has been encouraged by environmentalists, who have argued that beaver dams provide a habitat for birds and fish, reduce the strength of flooding by trapping water high up in a catchment area, away from homes further downstream, and could be a future tourist attraction.