Question: Feildes Weir is a weir on what river that is one of the largest rivers in London and the easternmost major tributary of the Thames?

Context paragraphs:
Yuan River: The Yuan or Yüan River (Chinese:  ,  "Yuán Jiāng" or , "Yuán Shuǐ ") is one of the four largest rivers in Hunan province in southeast-central China.  It is a tributary of Yangtze River.  It is 864 km long and rises in Guizhou province in the Miao Mountains near Duyun and is navigable.  The upper stream is called the Longtou River, and downstream it is called the Qingshui ("Ch'ing-shui") River.  It becomes the Yuan River after its confluence with its northern tributary, the Wu River.

River Cherwell: The River Cherwell ( , particularly near Oxford, or , particularly in north Oxfordshire) is a major tributary of the River Thames in central England.  It rises near Hellidon in Northamptonshire and flows south through Oxfordshire for 40 mi to meet the Thames at Oxford.  It adds a significant discharge to the Thames—when entering Oxford, the Thames's discharge is 17.6 m³/s (622 cu ft/s), but after leaving and consuming the Cherwell it has increased to 24.8 m³/s (876 cu ft/s).  The river gives its name to the Cherwell local government district and "Cherwell", an Oxford student newspaper.

Feildes Weir: Feildes Weir is a weir on the River Lea located near Hoddesdon, Hertfordshire at the confluence of the River Lea and River Stort.  The weir marks the start of the Lower Lee.  A channel of the man-made River Lee Flood Relief Channel is incorporated into the weir.

Longest rivers of the United Kingdom: There seems to be little consensus as to the lengths of rivers in published sources, nor much agreement as to what constitutes a river.  Thus the River Ure / River Ouse can be counted as one river system or two rivers.  If it is counted as one, the River Aire / Yorkshire Ouse / Humber system would come fourth in the list, with a combined length of 161 mi and indeed, the River Trent / Humber system would top the list with their combined length of 222 mi (357 km).  Also, the Thames tributary, the River Churn, sourced at "Seven Springs" adds 14 miles to the length of the Thames (from its traditional source at Thames head).  The Churn/Thames' length at 229 mi (369 km) is therefore greater than the Severn’s length - 220 mi (354 km).  Thus, the combined "Churn/Thames" river would top the list.  Sue Owen et al. in their book on Rivers generally restrict the length to the parts that bear the correct name.  Thus the River Nene is quoted at 100 mi , but would be around 5 mi more if the variously named sources were included.  Many of the above lengths are considerably different from Sue Owen's list, some longer and some shorter.

Rickmansworth: Rickmansworth is a small town in south-west Hertfordshire, England, situated approximately 20 mi northwest of central London and inside the perimeter of the M25 motorway.  The town is mainly to the north of the Grand Union Canal (formerly the Grand Junction Canal) and the River Colne.  The nearest large town is Watford, approximately 5 mi to the east.  Rickmansworth is the administrative seat of the Three Rivers District Council; the local authority is named from the confluence of three rivers within Rickmansworth's borders; the River Gade and the Grand Union Canal join the upper River Colne near Rickmansworth's eastern boundary and are joined by the River Chess near the town centre from where the enlarged Colne flows south to form a major tributary of the River Thames.  The town is served by the Metropolitan line of the London Underground and Chiltern Railways from London Marylebone to Aylesbury.

Huai River: The Huai River, formerly romanized as the Hwai, is a major river in China.  It is located about midway between the Yellow River and Yangtze, the two largest rivers in China, and like them runs from west to east.  Historically draining directly into the Yellow Sea, floods have changed the course of the river such that it is now a major tributary of the Yangtze.  The Huai is notoriously vulnerable to flooding.

Feildes Weir Lock: Feildes Weir Lock (No5) is a lock on the River Lee Navigation located close to Hoddesdon.

Stort Navigation: The Stort Navigation is the canalised section of the River Stort running 22 km from the town of Bishop's Stortford, Hertfordshire, downstream to its confluence with the River Lee Navigation at Feildes Weir near Rye House, Hoddesdon, Hertfordshire.

Waihopai River (Marlborough): The Waihopai River—the more northerly of two New Zealand rivers of that name—runs through the South Island's Marlborough Region and is a major tributary of the Wairau River.  The river has its source in the Raglan Range of mountains, some 70 km northwest of Kaikoura.  It runs in a generally northeastward direction for all of its length, the first section of which is through narrow alpine valleys.  After joining its major tributary, the Spray River, the valley begins to widen, becoming a broad valley by the time the second main tributary, the Avon River is reached.  For the last 15 km of its length the Waihopai turns northward, reaching the Wairau 7 km west of Renwick.

River Lea: The River Lea in England originates in Marsh Farm, Leagrave, Luton in the Chiltern Hills and flows generally southeast, east, and then south through east London where it meets the River Thames, the last section being known as Bow Creek.  It is one of the largest rivers in London and the easternmost major tributary of the Thames.  Its valley creates a long chain of marshy ground along its lower length, much of which has been used for gravel and mineral extraction, reservoirs and industry.  Much of the river has been canalised to provide a navigable route for boats into eastern Hertfordshire, known as the Lee Navigation.  While the lower Lea remains somewhat polluted, its upper stretch and tributaries, classified as chalk streams, are a major source of drinking water for London.  A diversion known as the New River, opened in 1613, abstracts clean water away from the lower stretch of the river for drinking.  Its origins in the Chilterns contribute to the extreme hardness (high mineral content) of London tap water.

Supporting paragraphs: Feildes Weir River Lea
Answer: River Lea