Head of navigation
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 . 
<triplet> London Bridge <subj> River Thames <obj> crosses <triplet> Osney Bridge <subj> River Thames <obj> crosses <triplet> Oxford <subj> River Thames <obj> located in or next to body of water <triplet> Lechlade <subj> River Thames <obj> located in or next to body of water