River Mole (Sussex)

River Mole (Sussex)
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Name	River Mole (Sussex)
Other name	Mole
Country	England
Region	Sussex
Length km	25
Source	Near Gatwick
Mouth	River Arun
Basin size km2	120

River Mole (Sussex) is a chalk-stream tributary of the River Arun in the county of West Sussex, England. Flowing roughly northwest to southeast from its headwaters near Gatwick Airport to its confluence at Billingshurst and Pulborough environs, the Mole traverses a landscape shaped by South Downs geology, historic settlements and 19th-century infrastructure. The river has been central to local industry, recreation and conservation efforts involving national bodies and local trusts.

Course and geography

The Mole rises in the uplands close to Gatwick Airport and the parish of Charlwood before passing through or near settlements such as Horley, Betchworth, Dorking, Box Hill, Fetcham and Capel en route toward the River Arun floodplain near Pulborough and Billingshurst. It crosses administrative boundaries between Surrey and West Sussex and skirts the eastern flanks of the North Downs and the western margins of the Weald. Major transport corridors including the A24 road, the A264 road and the London–Brighton railway intersect its valley, while historic routes such as the Pilgrims' Way and the South Downs Way run nearby high ground. The corridor includes floodplain meadows, riparian woodland and fragments of traditional watermeadow, all within reach of conservation sites such as South Downs National Park buffer areas and local Sites of Special Scientific Interest like Wotton Common.

Hydrology and tributaries

The Mole is a perennial chalk stream with baseflow predominantly sustained by groundwater from the Upper Greensand and Chalk Group. Its regime is influenced by seasonal aquifer recharge, storm runoff from urban catchments including Gatwick Airport environs, and abstractions for public water supply managed by companies such as South East Water and regulatory oversight by the Environment Agency. Principal named tributaries and influent streams include the River Grom, the Pipp Brook, the River Arun feeders in the lower reaches, and smaller brooks draining from commons like Reigate Heath and Leith Hill. Historic gauging sites and modern hydrometric monitoring have recorded variability in discharge linked to droughts affecting the Thames Basin and intense precipitation events that impacted surrounding basins such as the Rivers Medway and Wey.

Geology and catchment

The Mole’s catchment lies across lithologies including chalk, greensand, and Wealden clays, reflecting the complex stratigraphy described in surveys by the British Geological Survey. Springs rising from chalk outcrops feed the upper reaches, producing characteristic clear, alkaline waters rich in calcium carbonate similar to other southern chalk streams like the River Itchen and River Test. The valley morphology shows incision into the Weald anticline with terraces and fluvial deposits mapped alongside Quaternary alluvium studied by regional geomorphologists associated with institutions such as the University of Sussex and University of Brighton. Soil associations including Rendzinas and Gleysols influence land use patterns dominated by mixed arable and pastoral holdings, historic hop gardens around Billingshurst and remnant heathland.

History and human use

Human interaction with the Mole dates back to prehistoric and Roman-era occupation in the valley evident at sites studied by archaeologists from the Museum of London Archaeology and county records held by West Sussex County Council and Surrey County Council. During the medieval period mills and watermeadows were developed under manorial systems tied to estates like Nonsuch Palace-era holdings and later estate owners such as the families associated with Box Hill and Leith Hill House. The Industrial Revolution and Victorian transport expansion brought railways constructed by companies such as the London, Brighton and South Coast Railway and roads that altered floodplain connectivity. 20th-century urbanisation, notably around Gatwick Airport and commuter towns like Horley and Dorking, increased demand for water abstraction and prompted regulatory responses from bodies including the Water Research Centre (UK) and post-privatisation water companies.

Ecology and wildlife

As a classic southern chalk stream, the Mole supports assemblages of aquatic plants such as water crowfoot (documented in surveys coordinated by the Centre for Ecology & Hydrology) and invertebrates including mayfly and caddisfly species listed in regional biodiversity action plans by organisations like the Sussex Wildlife Trust and Surrey Wildlife Trust. Fish communities historically included brown trout and European eel, with migratory patterns influenced by barriers and connectivity to the English Channel via the River Arun. Riparian habitats host birds recorded by the Royal Society for the Protection of Birds such as kingfisher and grey heron, while adjacent grasslands and wetlands comprise habitats for amphibians monitored by the Amphibian and Reptile Conservation Trust.

Conservation and management

Conservation and management involve multi-stakeholder collaboration between statutory agencies like the Environment Agency, county councils, non-governmental organisations including the Sussex Wildlife Trust, community groups and private landowners. Key measures have included bank restoration, removal or modification of weirs championed by river trusts, regulation of abstractions under the Water Resources Act 1991 framework, and catchment-sensitive farming initiatives promoted by the National Farmers' Union. Monitoring programmes by academic partners such as King's College London and applied conservation projects funded through schemes involving the Heritage Lottery Fund address issues of invasive species, water quality improvements, and habitat connectivity to support species migrations to links with larger systems like the River Arun and coastal habitats managed by agencies including Natural England.

Category:Rivers of West Sussex