The River Wey is a main tributary of the River Thames in south east England. Its two branches, one of which rises near Alton in Hampshire and the other in West Sussex to the south of Haslemere, join at Tilford in Surrey. Once combined the flow is eastwards then northwards via Godalming and Guildford to meet the Thames at Weybridge. Downstream the river forms the backdrop to Newark Priory and Brooklands. The Wey and Godalming Navigations were built in the 17th and 18th centuries, to create a navigable route from Godalming to the Thames. The Wey drains much of south west Surrey (as well as parts of east Hampshire and the north of West Sussex) and has a total catchment area of . Although it is the longest tributary of the Thames (if the Medway is excluded), its total average discharge is lower than that of the Kennet and Cherwell. The river morphology and biodiversity of the Wey are well studied, with many places to take samples and record data. The main tributary is the Tillingbourne, which rises on the western slopes of Leith Hill and flows westwards to join the Wey to the south of Guildford, between Shalford and Peasmarsh. The name Wey is of unknown origin and meaning. Course Wey North The Wey north branch, sometimes referred to as the Alton Wey, has its official nomenclature source in Alton in Hampshire; however is exceeded by length and, in wet weather, in flow by the nearby Caker Stream rising in dendritic drainage spanning fields of Upper Farringdon and Hartley Mauditt, passing Chawton between these places. After the union in Alton the brook runs quite straight, east north-east through Upper Froyle and Bentley, turning southeast after Farnham's centre to Tilford. The steep-sided valley accentuates entering Surrey, between vast masses termed the Lower Greensand Group (south), then down the more easterly valley on both sides (east and west). Reflecting the crumbly nature of this material which has readily eroded, the valley falls from about entering Surrey west of Farnham to 60 feet lower at Tilford south-east of Farnham and changes from almost v-shaped to a more u-shaped alluvial plain. The upper parts of the branch were the start of the upper River Blackwater's catchment. The Wey captured this following cumulative flooding and deposition right up to around Aldershot. A vestige of this is that the upper Blackwater valley proper, north of