As we are talking, Kathy Hornig, an animal welfare officer, is on a secret mission further downstream at a place called Black Bridge.Odd.
Here, another immigrant camp has been built amid high bushes near the river, and the occupants have been seen repeatedly taking swans from the water.
With the camp residents scattering as she approaches with other animal welfare officers, her mission is a success: she discovers the carcass of a mature, adult swan inside a plastic bag hanging from a tree branch by the camp.
Acutely conscious of the potential for tensions between locals and immigrants over the slaughter of British wildlife, the investigators removed the carcass of the dead swan for further tests to determine how it died.
'We've been aware of this [killing swans] going on for some time,' Ms Hornig said. 'The people trying to catch swans are causing them extreme distress. But I cannot say with 100 per cent proof that the swan was killed for food, nor will I speculate about why it was here. I do not want to make any further comment.'
The authorities aren't usually so reticent when they suspect people of heinous animal-related crime.