Private property. No public access without permission. Friend or foe? Has been in our creek-line for many years. Doesn't seems to spread. Grows submerged in rock pools. Light green, soft foliage.
Thanks Michael for the suggestion. I'll pull up a sample to get a closer photo of the leaves and roots etc. I've checked all the aquatic weed lists and can't find anything similar. We've not seen it elsewhere upstream or downstream. Bit of a mystery.
If you are going to pull up a sample, it would be a good idea to keep chscking until you see fruiting bodies (0.5-2 mm wide) at the bases of the leaves, and then pressing the specimen, so that a positive ID can be made by the Herbarium. https://vicflora.rbg.vic.gov.au/flora/taxon/6249454c-1e7c-4331-84de-ac59be7956e1#&gid=1&pid=1