[5] 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.

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.

[6] Beneath the catchment are chalk, Bagshot Formation sand and greensand aquifers, covered by varying depths of London clay-based soil, mostly very weakly permeable.

[7] The Blackwater Valley SSSI covers 86.55 acres (35.03 ha), comprising unimproved alluvial meadows, swamp, and wet woodland.

[10] The river is crossed by a small part of the Basingstoke Canal SSSI, which supports the widest variety of aquatic species in England.

[16] From its source up until the three-county tripoint east of Blackwater station, the river almost exactly marks the longstanding boundary between the counties of Surrey and Hampshire.

[17] On the left bank is Aldershot sewage treatment works that in dry weather supplies more than half of the local flow[18] opposite is Willow Park Fishery.

The river passes under an aqueduct carrying the Basingstoke Canal, and is then bordered by Lakeside Nature Reserve to the east and Gold Valley Lakes fishery to the west.

After the M3 junction, the river is joined by Cove Brook, which starts as a series of streams near Farnborough Airport and flows northwards.

For much of its route it is bordered by housing on both sides, but then passes beneath the North Downs railway line to reach the junction.

This was the cause of a serious pollution incident in 2012, when a malfunction resulted in partially treated effluent released, contaminating the river for 1 mile (1.6 km) downstream.

It is part of a Site of Special Scientific Interest (SSSI) because of its populations of Dartford warbler, nightjar and woodlark, and is a nationally important habitat for dragonfly and damselfly.

[23] The stream then flows through the grounds of the Royal Military Academy Sandhurst,[24] entering Bathing Pool, and then passing through a series of engineered ponds and weirs, which create a significant obstacle to the movement of fish, to reach the Lower Lake.

[27] The river then flows through Shepherd Meadows, an SSSI noted for the Black Poplar tree, and a variety of wild flowers, including sneezewort, knapweed, betony, meadowsweet and great burnet.

[27] As the river curves to the west, it passes under the North Downs Line railway bridge, flows past Sandhurst sewage treatment works on the right bank and enters Trilakes Country Park.

Former gravel pits flank the river on both sides, and as it passes the village of Finchampstead, the channel is briefly called Long Water.

They were subsequently used as a restaurant, with the restored mill wheel, gears and machinery visible from the seating area, encased behind glass screens.

It is a Special Protection Area in view of the number of bird species to be found there, and also has large populations of damselflies, dragonflies and silver studded blue butterflies.

The park was enclosed by King Edward III in 1354, and remained the property of the monarch until the late 16th century, after which it was privately owned until 1965.

[38] Thk Blackwater used to flow through a brick culvert under the embankment,[39] but major reconstruction took place over the winter of 1994-95 as part of a project to route the A331 road along the river valley.

Reasons for the quality being less than good include discharge from sewage treatment works; drainage from roads and transport infrastructure; contaminated land adjacent to the river; physical barriers such as weirs and impounding of the water which prevent the free migration of fish and other species; and the presence of the North American signal crayfish, an invasive species.