The Port of London is the United Kingdom's second largest port by tonnage. Today, little commercial traffic passes above the Thames Barrier. Central London is visited occasionally by cruise ships or warships, which moor alongside HMS Belfast, and a few smaller aggregate or refuse vessels, operating from wharves in the west of London. Most trade is handled by the Port of Tilbury, roll-on/roll-off ferry terminals at Dagenham and Dartford, and petroleum products handling facilities at Purfleet, Coryton and Canvey Island. There is a speed limit of west of Wandsworth Bridge and in tributary creeks, and except for authorised vehicles, between Wandsworth Bridge and Margaretness. The tidal river is used for leisure navigation. In London sections there are many sightseeing tours in tourist boats past riverside attractions such as the Houses of Parliament and the Tower of London, as well as regular riverboat services provided by London River Services. This section is not suitable for sporting activity because of the strong stream through the bridges. Rowing has a significant presence upstream of Putney Bridge, while sailing takes place in the same area and also along the coasts of the Estuary. The annual Great River Race for traditional rowed craft takes place over the stretch from Greenwich to Ham. Thames meander challenges along the length of the Thames from Lechlade often pass through the London sections and finish well downstream, for example at Gravesend Pier. The Grand Union Canal joins the river at Brentford, with a branch – the Regent's Canal – joining at Limehouse Basin. The other part of the canal network still connecting on the Tideway is the River Lea Navigation via Bow Lock. Environment Narrow low-lying belts beside the tidal section of the Thames regularly flood at spring tides, supporting brackish plants. One such example is at Chiswick Lane South, where the river, as pictured, overflows this road a few times per year. (Picture taken in 2006). Although water quality has improved over the last 40 years and efforts to clean up the Tideway have led to the reintroduction of marine life and birds, the environment of the Tideway is still poor. Heavier rainfall in London causes overflows from pipes on the river banks from the standard type of sewer in the capital, the combined sewer. Around or 39 million tonnes of untreated sewage mixed with rainwater are released into the Tideway each year from sewage treatment works and