Hi, looks like watercress to me, grows in streams of clean water, Derek.

I am not sure what it is but, if it grows 'quite quickly', then you have to make a choice between the plant and a nice clear stream!

It looks like watercress to me, too. If you trust the purity of the water, harvest it and eat it often, to keep it from smothering the stream, though a certain amount of it will help keep the water even purer. If the water isn't pure, thin it and bin it occasionally, as a sort of bio-remediation.

Does watercress smell citrusy? Never noticed that.

The alkalinity of this stream has increased for some reason. The presence and rapid growth of watercress is an indicator of that. The water that feeds this stream is now percolating through a chalk bed or limestone. Never noticed the smell of watercress but to me it has a mild radishy taste.

To me, the leaves of watercress smell lightly like oxalic acid when crushed, like french sorrel. That scent is easily mistaken for a lemon juice scent. The taste to me is like a mix of pepper, mustard, and a little lemon.

Good description Tug!