1. A short river, a mere 346 kilometers (215 statute miles), but exceedingly famous by virtue of the fact that it flows through London, England and connects that city to the sea. The source of the river is in the Cotswolds, flows through Oxford (where it is called the Isis), Maidenhead, Windsor, London proper, Greenwich, then Dartford before entering the sea.
About 90 kilometers from the sea, further inland from London, the river begins to exhibit signs of tidal activity as the nearby sea begins to affect it. London was reputedly made capital of Roman Britain at the spot where the tides reached in 43 AD, but a variety of factors has pushed this spot up river in the 2000 years since then. The Romans called the river Thamesis. At London, the water is slightly brackish with sea salt.
By the 18th century, the Thames was one of the world's busiest waterways, as London became the center of the vast, mercantile British Empire. The coming of rail and road transportation, and the decline of the Empire in the years following 1914 have reduced the prominence of the river. London itself is no longer a port of any note, and the Port of London has moved downstream to Tilbury. In return, the Thames has undergone a massive clean-up from the filthy days of the late 19th and early- to mid-20th centuries, and life has returned to formerly dead waters.
See also: Thames crossings
3. A river in Connecticut.