I have just returned from a pleasant half hour wading around the dam and pond getting rid of Cumbungi. Cumbungi is a water weed that fills the runoff dams in Manangatang. When we had active dams here there were a few infestations too. It grows all around in the shallower water and makes it very difficult to get to the edge - also speeds water use and harbours mosquitoes etc.
I spotted the first plants in the dam a few weeks ago (they stood out as the only green things in it) and they have been spreading amazingly quickly. Every time I went down to the dam there seemed to be twice the number as a few days before. Then I checked the pond and found a similar infestation there. I think the birds must have brought in seeds from the Manang dams. The seeds are fluffy and would travel with birds quite easily. That would also explain both the dam and the pond having a similar scale of infestation.
I had to do something today before it became too late (and before it rained and the water in the dam was too deep to do anything)! I pulled out the weeds and as much of their root systems as I could. The root systems are quite impressive:
and there was a fair bit of material (the pile on the left is from the dam and on the right from the pond. The dam plants are probably greener because of the better nutrient supply):