At the estuary, it merges with the rivers Tavy, Plym and Lynher and the Tamar can't be considered in isolation.
Together, the Tamar, Tavy and Lynher are a designated Area of Outstanding Natural Beauty.
Flanked by ancient woodland along lengthy stretches, the rivers also provide rare habitat.
The intertidal systems are perfect for mudflats, saltmarshes and reedbeds - all home to birdlife, including the avocet.
The woodlands are also a haven for birds and butterflies as well as rare lichen and orchids.
The Tamar-Tavy Estuary and the Lynher Estuary are both protected Sites of Special Scientific Interest (SSSI) because of the habitat and wildlife.
There is important heathland up-river as well, where rare birds like the Dartford Warbler can be found.
The Tamar at Stonehouse, heading down towards the estuary
The location and climate have made the Tamar Valley an important area for market gardening.
At the height of the industry, the valley was covered with apple orchards, while cherries, strawberries and daffodils were also produced, not only for local consumption but for cities elsewhere in the country.
Market gardening still takes place in the valley, but on a much smaller scale.
The valley is historically important, with evidence of Stone and Bronze Age settlements - especially on the Cornish side of the river. Kit Hill is a good example.
All the way up the Tamar, there are magnificent medieval stone arch bridges - some of which are over 500-years-old.
Plymouth: The growth of a port city
At the mouth of the Tamar, on the Devon side, there is the port city of Plymouth, and Devonport Dockyard.
This is where there is a potential clash between industry and naval interests on the one hand, and the environment on the other - but the two have co-existed for centuries.
Devonport Dockyard's origins date back to 1691, when King William III (William of Orange) commissioned the building of a new dockyard to support the Royal Navy in the Western Approaches.
A World Heritage site
The River Tamar has always played an important role in industry and the region's economy.
Mineral extraction was the key industry in the Tamar Valley dating back many centuries, and in 2006, the Cornwall and West Devon's Mining Landscape won World Heritage Site status.
Tin, silver, lead, granite and copper were all mined in areas like Lopwell, Bere Alston and Morwellham.
In fact, Morwellham Quay on the Devon side of the river was a centre for shipping minerals for 1,000 years.
The minerals were transported down the river to the sea until the advent of the railways - and Brunel's amazing Royal Albert Bridge across the Tamar.
Brunel's bridge (right) and the road bridge alongside it
Isambard Kingdom Brunel's bridge was an extraordinary feat of Victorian engineering. It was completed in 1859 and it was built to bear the weight of three express trains. It's still the main rail link between Cornwall and the rest of the country.
The bridge opened Cornwall up for visitors - as did the Tamar road bridge, built more than a century later.
The Tamar Bridge was opened in 1961, and it was then the longest suspension bridge in the UK. The toll bridge was recently widened at a cost of £34 million.
Before such engineering feats were possible, the only way to cross the Tamar was via the little bridges further up the river - or by boat.
An Act of Parliament in 1791 granted the major landowners - like the Earls of Mount Edgcumbe and St Germans - permission to operate a ferry across the Tamar between Plymouth and Torpoint.
A succession of Torpoint ferries have operated during the past 200 years, and now there are three: the Plym, the Lynher and the Tamar.
Like the Tamar Bridge, the Torpoint Ferries are now run by a joint committee involving Plymouth City Council and Cornwall County Council.
A smaller ferry, the Cremyll Ferry, also runs from Admirals Hard, Plymouth to Mount Edgcumbe, Cornwall. The Cremyll Ferry was first documented way back in 1204 - so 2004 was its 800th anniversary.
These days, the Tamar is largely recreational - a place to visit, walk, enjoy a boat trip, and take in the scenery and wildlife. And, of course, it remains the most unique county boundary in England.
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