If you travel by boat up the Thames towards Richmond you will pass along Syon Reach, with Kew Gardens on your left hand and Syon House on your right. Then, just as Old Isleworth comes into view through the trees, you will enter the strangely named Sheen Gull.
Sheen Gull is the name given to the stretch of river that runs by and around Isleworth Ait, one of the largest islands on the Thames. Moses Glover’s map of 1635 shows the Ait as four separate islands but by 1740 it had shrunk to two islands. Surprisingly John Cary’s map of 1786 shows the Ait as three islands as does the Ordnance Survey map of 1865. By 1894 it was down to two islands and now it is just one, a single teardrop shaped, muddy, 3.5 hectares of densely packed trees.
The Ait was once used to grow osier, the heavily pollarded willow woven into baskets and crates. There was a regular demand for such containers from the many orchards and market gardens in the area. You can still see osier stands on the Middlesex bank of the island.
Now Isleworth Ait is a protected wildlife sanctuary and home for over 50 different species of birdlife, including the tree-creeper, kingfisher, spotted flycatcher, dunlin, swallow and swift. Two rare species of air-breathing land gastropods also live there; the Two-Lipped Door Snail which as its name suggests has a device to close its shell and the German Hairy Snail which captures its mate by first stabbing him/her with a long barbed dart. Apparently this helps reproduction. No wonder they’re rare! The London Wildlife Trust who manage the Ait discourage casual visitors but do arrange organised visits.
The Ordnance Survey Map of 1865 indicates a swimming pool on the island built by the Duke of Northumberland who owned the land for the use of local schoolchildren. In the 1930s the next Duke or two down the family line sold the Ait to the Metropolitan Water Board who clearly had plans. In 1936 the Board built a new sewage works at Mogden and installed outflow pipes beneath Isleworth and the Ait to discharge into the Thames. The pipes are still used today…
Rower notification from Thames Water: Mogden Sewage Treatment Works (STW) Timed: 14.06 13th October 2013
Within the next hour, Mogden Sewage Treatment Works will be discharging heavily diluted storm water into the River Thames.
Storm water is screened, settled in tanks and mixed with fully treated wastewater before it reaches the river.
Work has been carried out at Mogden to increase the treatment capacity during and after heavy rain in our catchment. Regrettably, there are still times that this capacity is exceeded, and with nowhere else for the excess storm sewage to go, these discharges to the river are legal and consented.
…If you walk along the Surrey towpath past Isleworth Ait you can sometimes see circular upwellings in the river. Although Thames Water, who now own and operate Mogden, reassure us that the discharging liquid has been cleaned at the sewage works local fishermen do see the occasional shoal of an unpleasant species they call “Mogden Trout.”
Once past Old Isleworth and its Ait you leave Sheen Gull and enter a stretch of river that has no distinctive name - and so it remains until the far side of Richmond where it becomes Horse Reach and later Cross Deep.
There are some who say that when Julius Caesar made his second expedition to Britain in 54BC he crossed the Thames not at Brentford as commonly believed but at Isleworth Ait where the river is shallower. I tried Googling “Caesar and the German Hairy Snail” and “Caesar and the Two-Lipped Door Snail” but had no results. It didn’t work with “Caesar and Mogden Trout” either.
 London Wildlife Trust:firstname.lastname@example.org