The official website of the City of London300 Dufferin Avenue519-661-CITY (2489)3-1-1 (within London)

Discolouration in Thames River identified as common water dye

The City of London received a report this morning of a green discolouration in the water along the banks of the Thames River. City staff investigated the area along the river’s south branch near the Meadowlilly Bridge, and can confirm that the green discolouration reported was a common water tracing dye used to track water flows.

A contractor working for the City of London released a volume of water containing the dye to trace water flows on February 12, 2020. This was noticed as a florescent green colour along the bank of the river.

This dye is designed to be safe for the environment. The Ministry of the Environment, Conservation and Parks has been notified of the report.

This green dye may be observed in the Thames River again as water tracing is ongoing.

Residents can continue to contact the City of London about issues through Service London online at or by emailing