The River Otter rises in the Blackdown Hills just inside the county of Somerset, near Otterford, then flows south for some 32 km through East Devon to the English Channel at the western end of Lyme Bay. 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:
The river flows through a predominantly rural area, with small cattle, sheep and dairy farms. The largest town in the Otter Valley is Honiton. Tourism and leisure play important roles in the economy; hundreds of private cottages and farms offer B&B and rented holiday accommodation.
The river passes through or by Upottery, Rawridge, Monkton, Honiton (then below the A30 trunk road), Alfington, Ottery St Mary, Tipton St John, Newton Poppleford, Otterton and reaches the coast to the east of Budleigh Salterton, after flowing through the 57-acre Otter Estuary Nature Reserve - a Site of Special Scientific Interest (SSSI) - consisting of tidal mudflats and saltmarsh. There is no public access to the estuary itself but footpaths lead alongside with two viewing platforms on the west and a hide on the east. The wintering population of wildfowl and waders includes redshank, greenshank, dunlin, common sandpiper, ringed plover, grey plover, curlew, snipe, water rail, wigeon, teal, shelduck, brent goose, red-breasted merganser and little grebe. Reed warbler, reed bunting and sedge warbler breed on the reserve.
A small tributary is the River Tale, with the confluence NW of Ottery St Mary. This small town (associated with Coleridge, Sir Walter Raleigh, and an annual tar barrel rolling event) is the site of an unusual circular weir, known as the Tumbling Weir.
At one time there were as many as fifty watermill powered by the River Otter. One remaining working mill, thought to date from the 17th century, is Tracey Mill near Honiton. In the 1970s, fish ponds were dug around the mill, fed by the leat; over a million gallons of fresh water flow though these ponds every day, helping the commercial production of trout 'without the need for antibiotics or added oxygen'.
One mill, at Dotton, is known to have operated from around 1100 to 1960, after which the building was demolished. The site was excavated by Channel 4's Time Team, the programme The Domesday Mill being broadcast in 2007. This mill is mentioned in the Domesday Book. A weir diverted water to the mill's leat, supplying the vertical breast-shot wheel. Dotton (now liitle more than one farm) is 4.5 miles from the mouth of the Otter, and about one mile north of the village of Colaton Raleigh.
At the picturesque village of Otterton (once a seaport on a larger Otter Estuary) there is a working watermill in this case over 1,000 years old; it was one of the three largest mills in Devon as recorded in the Domesday Book of 1086. It is powered by water diverted through a leat. Immediately north of the leat, a fish pass (aka ladder/staircase) has been constructed beside a river-wide weir, restoring migratory fish runs to the river after a break of over 100 years.