Lemnoideae means a green canal is a clean canal

On one of our regular canal patrols this morning our Wildlife Rangers (Shadwell Branch) met some very nice people who were taking more than a passing interest in the canal just by Shadwell Basin.

These gentlemen were commenting on the section of the canal underneath Garnett Street bridge which is, well, green.

Green but is it clean? Yes it is
Ducks enjoying the duck weed.

Our Wildlife Rangers explained their basic knowledge of the how the canal works and that just because a section is green does not mean it is stagnant or in any way nasty.

A few minutes later the Rangers bumped into local Wapping Hero Chris who has been looking after the canal for years and he explained properly.

Chris said that the ‘green stuff’ in the section of the canal under the bridge is duck weed (Lemnoideae) also known as water lens and that duck weed only thrives where the water is very clean.

And the water in Wapping canal is very clean indeed as it is pumped up 70m from an underground stream beneath Wapping Woods.

Chris said that if you dip your hand in duck weed you can see that it just floats on the surface.

Duck weed
Duck weed

And of course Chris was quite right. As you can see from the photo above it looks like cress without the stalks. Apparently it is a high-protein food source for waterfowl and some people eat it in Southeast Asia – it contains more protein than soybeans.

Duck weed can also be used for waste water treatment and controls the breeding of mosquitos.

Amazing what you can find out if you ask – and there we were thinking it was green slime.

Wapping canal schematic.
Wapping canal schematic.

But why does the duck weed only appear underneath the bridge?

Chris explained that this part of the canal is a self-contained system and the pump which keeps the water moving pumps from either end and the water ends up in the middle – nothing more complex than that.

Next time you walk past pause a moment, dip your hand in and see for yourself.

For more information

Duck weed (Wikipedia)

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