Question: The Herford East branch line is part of a collection of London consumer routes called the Lea Valley lines that run along which river?

Context paragraphs:
Brimsdown Industrial Estate: Brimsdown Industrial Estate is located to the east of the residential part of Brimsdown in the London Borough of Enfield.  The estate, which lies in the Lea Valley, is bordered to the west by the West Anglia Main Line portion of the Lea Valley Lines and to the east by the River Lea and King George V Reservoir.  A number of businesses are located here including Warburtons Enfield bakery, warehousing and retail, and heavier industry such as Johnson Matthey, UOP and the Enfield Power Station.

Chingford branch line: The Chingford branch line is a railway line between Clapton Junction (just west of Clapton station) and Chingford station.  Services currently operate between Liverpool Street station and Chingford.  The branch is currently part of the Lea Valley Lines network.

Southbury Loop: The Southbury Loop (formerly known as the Churchbury loop) is a line linking Edmonton Green, in north-east London, to Cheshunt.  It was opened by the Great Eastern Railway in 1891 although initially it was not very successful and was closed to passenger traffic in 1910.  Goods trains continued to use the line although in World War I passenger services were reinstated for munitions workers.  Once the war finished the line returned to its goods-only role although was occasionally used for diversionary purposes when the West Anglia Main Line was closed south of Cheshunt.  Electrification of the line and the reintroduction of passenger services in 1960 saw the line become busy with regular suburban services as part of the Lea Valley Lines.  The line became part of London Overground in May 2015.

Lea Valley lines: The Lea Valley lines are two commuter lines and two branches in North East London, so named because they run along the Lower Lea Valley of the River Lea.  They were part of the Great Eastern Railway, now part of the "Anglia Route" of Network Rail.

Hertford East branch line: The Hertford East branch line is a railway line in Hertfordshire in the United Kingdom, running between Hertford and Broxbourne .  The line follows the route of the Lea Valley, serving intermediate towns and villages.  It branches off the West Anglia Main Line north of Broxbourne , and could be seen as part of the Lea Valley lines—a collection of commuter routes into London.

Wood Street railway station: Wood Street railway station in Upper Walthamstow in the easterly part of Walthamstow, north east London.  It is in Travelcard Zone 4, and the station and all trains are operated by London Overground.  It is also occasionally known as Walthamstow Wood Street.  The station is located on the Chingford Branch Line and is part of the Lea Valley Lines network.

Edmonton Green railway station: Edmonton Green is a railway station on the Lea Valley Lines which form part of the West Anglia Main Line, located in Edmonton in the London Borough of Enfield, north London.  It is 8 mi down-line from London Liverpool Street and is situated between to the south and (on the branch) and (on the branch) to the north.

Spital Brook: Spital Brook is a minor tributary of the River Lea which rises in Hoddesdonpark Wood in the county of Hertfordshire, England.  Spital Brook flows eastwards from Hoddesdonpark Wood, passing through Barclay Park, the former Hoddesdon Common, on its journey to the Lea, while an unnamed parallel stream to the south has been diverted to form ornamental waterbodies located within the former parkland of Broxbournebury.  The stream crosses the Roman road, Ermine Street, before going under the A10 and Cock Lane.  The original crossing place or ford of Spital Brook in Cock Lane remains and the original route it took ran parallel to the brook in a north-south direction.  The brook passes through its namesake area, Spital Brook and marks the traditional boundary between the two parishes of Hoddesdon and Broxbourne.  The stream runs parallel to the Lea Valley Lines by Broxbourne Station until it meets the River Lea close to Broxbourne Riverside, the former Broxbourne Lido.

Highams Park railway station: Highams Park railway station is in the district of Highams Park, north east London.  It is in Travelcard Zone 4, and the station and all trains are operated by London Overground.  The station is located on the Chingford Branch Line and is part of the Lea Valley Lines network

St. James Street railway station: St. James Street railway station is in Walthamstow in the London Borough of Waltham Forest, east London, and is in Travelcard Zone 3, on the Chingford Branch Line of the Lea Valley Lines.  The station, and all trains serving it, are operated by London Overground.

Supporting paragraphs: Hertford East branch line Lea Valley lines
Answer: River Lea