The name of the River Lea was first recorded in the 9th century, although is believed to be much older. Spellings from the Anglo-Saxon period include Lig(e)an in 880 and Lygan in 895, and in the early medieval period it is usually Luye or Leye. It seems to be derived from a Celtic (brythonic) root lug-meaning 'bright or light' which is also the derivation of a name for a deity, so the meaning may be 'bright river' or 'river dedicated to the god Lugus'. A simpler derivation may well be the Brythonic word cognate with the modern Welsh "Li" pronounced "Lea" which means a flow or a current.
The spelling Lea predominates west (upstream) of Hertford, but both spellings (Lea and Lee) are used from Hertford to the River Thames. The Lee Navigation was established by Acts of Parliament and only that spelling is used in this context. The Lee Valley Regional Park Authority also uses this spelling for leisure facilities. However, the spelling Lea is used for road names, locations and other infrastructure in the capital, such as Leamouth, Lea Bridge, the Lea Valley Walk and the Lea Valley Railway Lines. This spelling is also used in geology, archaeology, etc. to refer to the Lea Valley. The divergent spellings of the river are also reflected in place-names, including Leagrave, the suburb of Luton where the source of the river is located, and of Luton and Leyton: both mean "farmstead on the River Lea".
The source is usually said to be at Well Head inside Waulud's Bank at Leagrave, but there the River Lea is also fed by Houghton Brook, a stream that starts 2 miles (3.2 km) further west in Houghton Regis. The river flows through (or by) Luton, Harpenden, Welwyn Garden City, to Hertford where it changes from a small shallow river to a deep canal at Hertford Castle Weir, which then flows on to Ware, Stanstead Abbotts, Hoddesdon, Broxbourne, Cheshunt, Waltham Abbey, Enfield Lock, Ponders End, Edmonton, Chingford, Tottenham, Walthamstow, Upper Clapton, Leyton, Hackney Wick, Stratford, Bromley-by-Bow (past Fish Island), Poplar, Canning Town and finally Leamouth where it meets the River Thames (as Bow Creek). It forms the traditional boundary between the counties of Middlesex and Essex, and was used for part of the Danelaw boundary. It also forms part of the boundary between Essex and Hertfordshire.
For much of its distance the river runs within or as a boundary to the Lee Valley Park. Between Tottenham and Hackney the Lea feeds Tottenham Marshes, Walthamstow Marshes and Hackney Marshes (the latter now drained). In their early days, Tottenham Hotspur and Leyton Orient played their matches as football amateurs on the Marshes. South of Hackney Wick the river's course is split, running almost completely in man made channels (originally created to power water mills, the Bow Back Rivers) flowing through an area that was once a thriving industrial zone.
Inside Greater London below Enfield Lock the river forms the boundary with the former Royal Small Arms Factory, now known as Enfield Island Village, a housing development. Just downstream the river is joined by the River Lee Flood Relief Channel. The man-made, concrete-banked watercourse is known as the River Lee Diversion at this point as it passes to the east of a pair of reservoirs: the King George V Reservoir at Ponders End/Chingford and William Girling Reservoir at Edmonton known collectively as the Chingford Reservoirs; and to the west of the Banbury Reservoir at Walthamstow. At Tottenham Hale there is a connected set of reservoirs; Lockwood Reservoir, High Maynard Reservoir, Low Maynard Reservoir, Walthamstow Reservoirs, East Warwick Reservoir and West Warwick Reservoir. It also passes the Three Mills, a restored tidal mill near Bow.