The Mole enters Surrey to the south of Horley, where it meets the Gatwick Stream, a tributary draining Worth Forest to the southeast of Crawley. The second-largest Sewage Treatment Works (STW) in the Mole catchment is located on the Gatwick Stream upstream of the confluence with the Mole: Crawley STW discharges of water per day, and in prolonged dry periods it accounts for up to 75% of the flow of the Mole downstream of the confluence. The mean flow measured at Horley gauging station ( above OD) is . The Mole passes Horley to the west, flowing north towards Sidlow and entering a largely rural area. south of Sidlow the mean flow is measured as at Kinnersley Manor gauging station ( above OD). The Earlswood Brook, a tributary draining the urban area of Reigate and Redhill, joins the Mole at Sidlow. The largest STW in the Mole catchment (Reigate STW) discharges up to per day into the Earlswood Brook.