River Thames
As a boundary

Until enough crossings were established, the river presented a formidable barrier, with Belgic tribes and Anglo-Saxon kingdoms being defined by which side of the river they were on. When English counties were established their boundaries were partly determined by the Thames. On the northern bank were the ancient counties of Gloucestershire, Oxfordshire, Buckinghamshire, Middlesex and Essex. On the southern bank were the counties of Wiltshire, Berkshire, Surrey and Kent.

Counting bridges to the far bank or to an island connected to such, the Thames has 223. From source to mouth a channel can be found with 138 bridges, plus the temporary footbridge often added during Reading Festival. The river is heavily splayed in Ashton Keynes and Oxford. Where the river is wide 17 tunnels that have been built, many of which for rail or notable electricity cables. The crossings have changed the dynamics and made cross-river development and shared responsibilities more practicable. In 1965, upon the creation of Greater London, the London Borough of Richmond upon Thames incorporated the former 'Middlesex and Surrey' banks, Spelthorne moved from Middlesex to Surrey; and further changes in 1974 moved some of the boundaries away from the river. For example, some areas were transferred from Berkshire to Oxfordshire, and from Buckinghamshire to Berkshire. In many river sports and traditions – for example in rowing – the banks are referred to by their traditional county names.
punctuation:no_comma, oulipo:avoid_letter[t]