The River Thames, the longest river entirely in England, has more than 80 islands along its course. Starting in Gloucestershire and winding through cities like Oxford, Windsor, and in today's image, London, it flows for more than 340 kilometres before merging with the North Sea. With a range of freshwater and near seawater habitats, the river is a haven for an array of wildlife like mute swans and bottlenose dolphins. The lower stretches of the river, known as the Tideway, extend up to Teddington Lock, reflecting its extensive tidal reach. From Oxford to its estuary, the river descends 55 metres. Although the Thames flows through some of the drier regions of mainland Britain and is heavily tapped for drinking water, its discharge is relatively low compared to its size.