The Thames Conservancy, also known as the Conservators of the River Thames, was a body responsible for the maintenance and management of the River Thames as well as ensuring that it remained freely navigable. Prior to its formation the river between Staines and the Isle of Grain had been under the control of the Corporation of the City of London while the river above Staines was under the care of various commissions setup by Parliament. This situation continued until the 1840s when proposals for what would become the Victoria Embankment were being explored caused a dispute between the Crown and the City of London over who owned the bed and soil of the river. This was eventually settled when the City accepted that as a navigable river the bed of the Thames was Crown property.

In 1847 a bill was brought before Parliament proposing the creation of a body that would be responsible for ensuring the Thames was navigable, but this was delayed by the ongoing dispute between the City of London and the Crown so that it wasn’t until August 1857 that the Thames Conservancy Act received Royal Assent. This created the Thames Conservancy which not only controlled the river between Staines and the Isle of Grain, as the City had, but also received the Crown’s land rights, on the condition that one third of the revenue from rents and tolls went to the Crown whilst the rest was used to maintain the river. The new body soon introduced a number of new regulations to river traffic including a speed limit of 5mph, new tolls for steamships and a ban on netting for fishing between Richmond and Staines. At this time the river upstream of Staines was still the responsibility of the Thames Navigation Commission but this body was beginning to suffer financial difficulties as newly built railways were taking traffic away from the barges that it relied on for toll revenue. This resulted in the 1866 Thames Navigation Bill which transferred the right to control navigation, licence and approve the erection of any structure in or adjacent to the river, carry out maintenance and generally do any work to ensure the safety of the river and its users to the Conservancy but unlike the stretch taken over from the City of London this did not include ownership of the river bed which remained with the riparian owners, although they could not construct any structure in the river without consent. The Conservancy could acquire land via compulsory purchase if it was required for improvement work. Following the expansion of its power several improvement works were carried out to the locks and weirs that had been taken over which, along with the removal of sewage works, was paid for by a £1,000 per year charge on the five major water companies that used the river.

Despite these improvements there was still some concern as the Port of London was being hampered by the lack of a single body to coordinate the works being undertaken to improve the river and docks to allow for large ships to be accepted. In order to tackle this a Royal Commission was established to find a solution, and this recommend the creation of a new central authority, which would be established in 1909 as the Port of London Authority. The PLA would take over responsibility for the maintenance of the River Thames from the Conservancy below Teddington, with the Conservancy retaining the section from Teddington up to Cricklade.

Despite this reduction in its responsibility the Conservancy continued to improve many of the Thames’ locks, undertaking several such projects throughout the 1920s and 30s. Another major project undertaken at the time was the Desborough Cut which straightened the river between Weybridge and Walton, thereby reducing flooding and cutting the distance that needed to be travelled. The 1960s also say the introduction of hydraulically powered locks.

In 1973 the Water Act was passed which, as recommended by the Central Advisory Water Committee, abolished local authority control by removing river authorities, along with water and sewerage boards, and established ten Regional Water Authorities. As a result, the Thames Conservancy was merged into the Thames Water Authority as its Thames Conservancy Division. This continued until 1990 when the water authorities were privatised and management of the rivers in England and Wales passed to the National Rivers Authority and then, in 1996, to the Environment Agency. This did not include the Thames below Teddington which remained under the control of the Port of London Authority.