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The River Thames rises at Thames Head in Gloucestershire ST9899 on the slopes of the Cotswolds and flows generally eastward to its mouth near Southend in Essex. At 215 miles long it is the second longest river in Britain after the Severn, and the longest entirely within England. Link

This route will connect Tower Bridge in London with Fakenham in Norwich. Passing through East London close to the Thames, Route 13 then heads north from Tilbury to Chelmsford in Essex. After a stretch on National Route 1 (also part of EuroVelo 2) National Route 13 resumes at Colchester and continues north to rejoin National Route 1 just south of Fakenham in Norfolk.
See Link and Link for more details.