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The River Lea (or Lee) originates at Marsh Farm Leagrave, Luton in the Chiltern Hills and flows generally southeast, east, and then south to London where it meets the River Thames. The river is 42 miles long.
Wikipedia: Link

The River Thames rises at Thames Head in Gloucestershire on the slopes of the Cotswolds and flows generally eastward to its mouth near Southend in Essex. At 215 miles long it is one of the longest rivers in Britain, and the longest entirely within England. It is one of the most important rivers in Britain. Link