Isleworth Ait is aLondon Wildlife Trust nature reserve, and is a teardrop-shapedisland in theRiver Thames in England, between 3.5-hectare (8.6-acre) and 9.370-acre (3.792 ha).^[1][2] The long ait is on theTideway facingOld Isleworth and the towpath alongside theRoyal Mid-Surrey Golf Club. These places are in the London Boroughs ofHounslow andRichmond upon Thames. The island faces Heron's Place and a number of commercial buildings. It is not open to the public, other than on organised event or volunteering days.

Isleworth Ait is covered by densely packed trees, and provides a sanctuary for a variety of wildlife. It floods regularly, but is home to more than 57 species of bird life, including the tree-creeper, kingfisher and heron. Two rare species of air-breathing landgastropods also live on the island, the two-lipped door snailBalea biplicata and the German hairy snailPseudotrichia rubiginosa, as well as several rare species of beetles. This variety of unusual wildlife makes the island one of theLondon Wildlife Trust's most remarkable reserves. It is alocal nature reserve^[3][4] and part ofThe River Thames and tidal tributariesSite of Metropolitan Importance for Nature Conservation.^[5][6]

Isleworth Ait was once a centre for the production ofosier - a willow which used to be harvested on the island to weave baskets to carry fruit and vegetables grown inMiddlesex to the markets in London. Much of the island has resulted from five pre-19th century neighbouring islands, overall covering a broader area and partially reduced by river erosion intensified by passing boat traffic.

TheRiver Crane flows into the Thames southwest of the island and itsdistributary to feed former mills and the lake inSyon Park, theDuke of Northumberland's River, to the northwest.

Thames Water owns the island, which houses the outfall from theMogden Sewage Treatment Works draining outer West London plusWraysbury andSpelthorne – effluent of more than 2,000,000 residents in all.^[7] Most of the outfall has always meant to have been treated. The local setup is the sewerage catchment mainly has separatesurface water drainage (drains for rainwater to flow into rivers and soakaways) which thus keeps untreated storm discharges from sewerage works low by not sending storm water to them. Such a system is more "desirable" and "preferable" though costlier to build.^[8] The older system alternative is that ofcombined sewers, having succeeded manyformer rivers in London. Excess discharges from all those older outfalls to the east are to be about 99% collected bya 2025 completion date tunnel. The Mogden works, which is a consolidation of many across its catchment, for a complete approach towards cleaning London's main watercourse, is being upgraded from the late 2010s to the early 2020s after billions of litres annually of untreated outfall from the island.^[7][9][10]

TheMetropolitan Water Board bought the ait from theDuke of Northumberland in the 1930s; visitor access is granted to local volunteers of theLondon Wildlife Trust.

• Islands in the River Thames

• Isleworth Ait volunteers

51°28′08″N0°19′15″W / 51.4690°N 0.3207°W /51.4690; -0.3207

• Geography of the London Borough of Hounslow
• Islands of the River Thames
• Local nature reserves in Greater London
• Nature reserves in the London Borough of Hounslow
• Islands of London
• London Wildlife Trust

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