There are two stories about the name. The respectable one, recorded in Ptolemy's Geography in the second century AD, holds that Tamar simply means "great water" — the same prehistoric root the Thames carries, possibly meaning "dark flowing." The Cornish one is better. A nymph named Tamara lived in the underworld and longed to walk in the upper world against her parents' wishes. She wandered Dartmoor, met two giants — Tavy and Tawradge — and led them a long flirtatious dance, darting away whenever they came close. Her father caught her near Morwenstow and, enraged, turned her into a spring. The spring became the river. Tavy, on waking, was likewise turned into a river and chased her down — finally finding Tamara and merging with her at the wide estuary above Plymouth, where the Tavy still meets the Tamar today. Tawradge ran the other way, became the Torridge and the Taw, and never found her at all. The Torridge does indeed rise barely five hundred metres from the Tamar before veering off in a vast arc north toward Bideford. The geography fits the story almost too well.

For most of its course the Tamar marks the official line between Devon and Cornwall. But the line is not as ancient or as fixed as it looks. In the eleventh century several parishes north of Launceston were transferred to Devon when the boundary briefly followed the River Ottery instead. The Counties (Detached Parts) Act of 1844 tidied up the messier exceptions. The full boundary was only restored to the Tamar in 1966, when North Petherwin and Werrington moved back into Cornwall. Three small exceptions to the river rule still exist, all in the upper course. Where the river is tidal, the border runs along the centre of the low-water channel — a definition so precise it can shift slightly with the seasons. Below the Tamar Bridge at Saltash, the counties officially extend only to their respective banks. The river itself, in those last few miles, belongs to no one and to both.

Twenty-two road crossings span the Tamar. The oldest still in use is the medieval Horsebridge of 1437 — slender stone arches, six and a half centuries old, still carrying cars across at a single-lane crawl. Greystone Bridge near Lawhitton followed in 1439. Gunnislake New Bridge, built in 1520 by Sir Piers Edgcumbe of Cotehele, was the lowest crossing of the Tamar for four hundred and forty-two years — the main route into south east Cornwall. Then in 1962 the Tamar Bridge opened beside Brunel's 1859 Royal Albert railway bridge at Saltash, and the centre of gravity shifted downstream. Saltash itself, sitting at the Cornish end of the bridge, calls itself the Gateway to Cornwall. It is hard to argue with.

The valley of the lower Tamar became, in the nineteenth century, one of the great mining landscapes of the world. Copper, tin, lead, silver and arsenic poured out of mines on both sides of the river. Morwellham Quay handled the export traffic — copper to South Wales for smelting, arsenic worldwide for cotton-field insecticide in the American South. The Tavistock Canal ran an inclined plane up from Morwellham, and barges of up to four hundred tons reached far inland. At the height of the boom, the Johnson Matthey smelting works at Weir Quay extracted silver and lead from ore brought in by sea from as far as Newfoundland. UNESCO recognised the Cornwall and West Devon Mining Landscape as a World Heritage Site in 2006, listing the Tamar Valley as one of its core areas. The mines are long closed. The engine houses still stand, hollow chimneys above the woods.

The Alliterative Morte Arthure, written somewhere around 1400, places the final battle of King Arthur and Mordred near the banks of the Tamar. The Cornish have always been keen to claim Arthur — Tintagel is just up the coast, and Slaughterbridge near Camelford is one of the candidates for the Battle of Camlann. Whether or not it was actually here, the Tamar valley has the right look for a final reckoning: steep, wooded slopes folding down to the slow tidal river, mist rising off the water at dawn. The valley was a working landscape of barges and miners through the early twentieth century, and the Royal Navy still keeps one of its three main bases at Devonport on the Hamoaze, where the Tamar finally widens and slips out to sea. The river has thus been continuously useful, from Bronze Age tin to nuclear submarines, for somewhere between three and four thousand years.