head of navigation

The head of navigation is the farthest point above the mouth of a river that can be navigated by

ships A ship is a large watercraft that travels the world's oceans and other sufficiently deep waterways, carrying cargo or passengers, or in support of specialized missions, such as defense, research, and fishing. Ships are generally distinguished ...

. Determining the head of navigation can be subjective on many streams, as the point may vary greatly with the size or the

draft Draft, The Draft, or Draught may refer to: Watercraft dimensions * Draft (hull), the distance from waterline to keel of a vessel * Draft (sail), degree of curvature in a sail * Air draft, distance from waterline to the highest point on a vesse ...

of the ship being contemplated for navigation and the seasonal water level. On others, it is quite objective, being caused by a

waterfall A waterfall is a point in a river or stream where water flows over a vertical drop or a series of steep drops. Waterfalls also occur where meltwater drops over the edge of a tabular iceberg or ice shelf. Waterfalls can be formed in several wa ...

, a low bridge that is not a

drawbridge A drawbridge or draw-bridge is a type of moveable bridge typically at the entrance to a castle or tower surrounded by a moat. In some forms of English, including American English, the word ''drawbridge'' commonly refers to all types of moveable ...

, or a

dam A dam is a barrier that stops or restricts the flow of surface water or underground streams. Reservoirs created by dams not only suppress floods but also provide water for activities such as irrigation, human consumption, industrial use ...

without navigation

locks Lock(s) may refer to: Common meanings *Lock and key, a mechanical device used to secure items of importance *Lock (water navigation), a device for boats to transit between different levels of water, as in a canal Arts and entertainment * ''Lock ...

. Several rivers in a region may have their heads of navigation along a line called the

fall line A fall line (or fall zone) is the area where an upland region and a coastal plain meet and is typically prominent where rivers cross it, with resulting rapids or waterfalls. The uplands are relatively hard crystalline basement rock, and the coa ...

. Longer rivers such as the

River Thames The River Thames ( ), known alternatively in parts as the The Isis, River Isis, is a river that flows through southern England including London. At , it is the longest river entirely in England and the Longest rivers of the United Kingdom, se ...

may have several heads of navigation depending on boat size. In the case of the Thames, that includes

London Bridge Several bridges named London Bridge have spanned the River Thames between the City of London and Southwark, in central London. The current crossing, which opened to traffic in 1973, is a box girder bridge built from concrete and steel. It r ...

, which historically served as the head of navigation for tall ships;

Osney Bridge Osney Bridge is a road bridge across the River Thames in Oxford, England, built in 1888 to replace a stone bridge which collapsed in 1885. It carries the Botley Road ( A420) from Botley into Oxford. The Thames Path crosses the river on this brid ...

Oxford Oxford () is a city in England. It is the county town and only city of Oxfordshire. In 2020, its population was estimated at 151,584. It is north-west of London, south-east of Birmingham and north-east of Bristol. The city is home to the ...

, which has the lowest headroom of any bridge on the Thames that generally restricts navigation to smaller vessels such as

narrowboat A narrowboat is a particular type of canal boat, built to fit the narrow locks of the United Kingdom. The UK's canal system provided a nationwide transport network during the Industrial Revolution, but with the advent of the railways, commerc ...

s and

cabin cruiser A cabin cruiser is a type of power boat that provides accommodation for its crew and passengers inside the structure of the craft. A cabin cruiser usually ranges in size from in length, with larger pleasure craft usually considered yachts. Man ...

s, and the long reach above St John's Lock, the first lock downstream of the river’s source, on the outskirts of

Lechlade Lechlade () is a town at the southern edge of the Cotswolds in Gloucestershire, England, south of Birmingham and west of London. It is the highest point at which the River Thames is navigable, although there is a right of navigation that contin ...

, where the river can become treacherously narrow and shallow for anything but small motorboats and human-powered vessels. {{Water-transport-stub Nautical terminology