the borders of several of its reservoirs, which are used as the cheapest way to stop large plants growing and damaging the banks. Thames Tideway scheme Over centuries of London's growth from medieval times to the Victorian age, the natural tributary system of the Thames Tideway was converted first into public open sewers and then closed over into covered sewers which emptied directly into the River Thames. Joseph Bazalgette's remediation of the ensuing 1850s Great Stink renewed much of London's sewerage mains infrastructure during the period 1859 to 1865. However, the new design was not intended to cope with the doubling of London's population over the following 150 years. The concreting of huge amounts of London's green spaces causes substantial rainwater run-off into the drainage and sewerage systems which had been expected to soak into the ground. As a result, even small amounts of rainfall in certain circumstances can cause London's outdated Victorian sewerage system to fail over, and release untreated sewage mixed with rainwater directly into the Thames Tideway. Each year, on average, there are 50–60 such incidents and a total of , or 39 million tonnes, is released. In 2013–14, exceptional weather conditions and flooding caused a total release of , or 55 million tonnes. The released effluent follows the ebb and flow of the tidal Thames, and can take up to 3 days to exit the Tideway into the Estuary. For this reason, Thames Water advises against swimming in the Thames Tideway and, by extension, walking in the tidal strand area. Despite this pollution, large marine mammals are increasingly found in the Thames Tideway and Estuary, indicating some level of year-on-year improvement To mitigate and resolve the above problems, the Thames Tideway Scheme proposed a three-stage series of improvements. The first two stages of the improvements were upgrades to 5 sewage treatment works and construction of the Lee Tunnel, formally opened on 28 January 2016. Together, these are expected to result in an annual discharge reduction of 40%. This is equivalent to a reduction of or 16 million tonnes per year, down to about or 23 million tonnes of effluent per year. The third stage is the Thames Tideway Tunnel, which was proposed by the Thames Tideway Strategic Study, including Thames Water, as an effective solution to deal with most of the remaining problem. On 12 September 2014, planning consent was formally approved by the UK Government.