The Westbourne or Kilburn, also known as the Ranelagh Sewer, is a culverted small River Thames tributary in London, rising in Hampstead and Brondesbury Park and which as a drain unites and flows southward through Kilburn and Bayswater to skirt underneath the east of Hyde Park's Serpentine lake then through central Chelsea under Sloane Square. It passes centrally under the south side of Royal Hospital Chelsea's Ranelagh Gardens before discharging into Inner London's old-fashioned, but grandiose combined sewer system, with exceptional discharges into the Inner London Tideway. Since the latter 19th century, the population of its catchment has risen further but to reduce the toll it places on the Beckton Sewage Treatment Works and related bills its narrow basin has been assisted by private soakaways, and public surface water drains. Its depression has been replaced with and adopted as a reliable route for a gravity combined sewer. The formation of the Serpentine relied on the water, a lake with a long, ornate footbridge and various activities associated, which today uses little-polluted water from a great depth.