 These discharge events kill fish, leave raw sewage on the riverbanks, and decrease the water quality of the river.To prevent the release of raw sewage and rainwater into the river, the Thames Tideway Scheme is currently under construction at a cost of £4.2 billion.This project will collect sewage before it overflows, before channeling it down a 25 km (15 mi) tunnel underneath the tidal Thames, so it can be treated at Beckton Sewage Treatment Works.