Duckweed?

I think duckweed is smaller that this stuff. There's a whole lot of this at the reservoir.

[Mark Prince]

No, you are right. This is not duckweed, duckweed is less than half an inch across, but it does multiply extremely fast and can cover any size of pond very quickly. We call it Canadian Pondweed / Duckweed so I guess that's where ours came from.
This looks like Water Chestnut , an invasive species in the USA. It originates from parts of Africa and Asia.

[*lynn]

Interesting to read what Mark Prince said about this. I have pictures of frogs covered in duckweed and it is very small, but I had no idea what this is.

[Casablanca]

Could be Greater Duckweed, that can be half an inch across. Bigger sister to normal duckweed.