Head of navigation is the farthest point above the mouth of a river that can be navigated by ships. Determining the head of navigation can be subjective on many streams, as this point may vary greatly with the size of the ship being contemplated for navigation and the seasonal water level. On others, it is quite objective, being caused by a waterfall, a low bridge which is not a drawbridge, or a dam without navigation locks. Several rivers in a region may have their heads of navigation along a line called the fall line.
On longer rivers such as the River Thames, there may be several heads of navigation depending on boat size, in the case of the Thames, this includes London Bridge, which historically served as the head of navigation for tall ships, Osney Bridge in Oxford, which has the lowest headroom of any bridge on the Thames, generally restricting navigation to smaller vessels such as narrowboats and cabin cruisers, and the long reach above St John's Lock - the first lock downstream of the river’s source - on the outskirts of Lechlade, where the river can become treacherously narrow and shallow for anything but small motorboats and human-powered vessels.