The River Blackwater is a tributary of the Loddon in England and sub-tributary of the Thames. It rises at two springs in Rowhill Nature Reserve between Aldershot, Hampshire and Farnham, Surrey. It curves a course north then west to join the Loddon in Swallowfield civil parish, central Berkshire. Part of the river splits Hampshire from Surrey; a smaller part does so as to Hampshire and Berkshire. The source is locally rare heath within the Thames Basin Heaths Special Protection Area, due to the Farnborough/Aldershot Built-up Area. After the Blackwater is joined by the Whitewater near Eversley. The river gives its name to the town of Blackwater, extending back from the bank facing Camberley, and the wider urban area including Aldershot, Farnborough, and Camberley is sometimes collectively referred to as the Blackwater Valley. Naming This article reverses the term found by Ordnance Survey mapmakers, old and continued there, Blackwater River. A stretch west of Finchampstead is called Long Water in 1897 and on recent maps. The adjacent final section from the Whitewater to the Loddon is known by alternative names. In Ordnance Survey maps: the 1:50,000 calls it as in general; the 1:25,000 calls it (the) Broadwater; the 1:2500 calls it as in general at each end and Broadwater in the middle, reflecting local broadening. This last option was the convention in the 1899 (forerunner) map at that scale. Blackwater Valley The river runs along the centre of the Blackwater Valley, which is maintained as a largely tree-planted open space, with some bog, marsh and water-meadows. It runs for approximately from the source at Rowhill Nature Reserve near Aldershot in the south, northwards to Swallowfield where the river is joined by the "Whitewater" and then joins the River Loddon. The Loddon flows into the Thames on the southern borders of Wargrave. For many miles banks of the lower half are semi-rural or rural. The upper half of the river for more than a mile each side is highly urbanised – the river passes Aldershot, Ash, Ash Vale, Frimley, Farnborough, Camberley, Blackwater, Sandhurst and Yateley; the continuous urbanisation extends to Fleet in the west and to Farnham in the south, with a combined population of over 300,000 people at the 2011 Census. Beneath the catchment are chalk, Bagshot Formation sand and greensand aquifers, covered by varying depths of London clay-based soil, mostly very weakly permeable. Some of the water flows from springs