Hello and thank you for your report! I believe this is Water lettuce. It is listed as a prohibited invasive species in several states, but there is no status for it in Oregon. Still, given it is invasive in other areas, an eye should be kept on this. The usual reviewer for Lane County is on vacation right now but I will ask her to take a look at this when she is back. Thank you for taking the time to report this!
-Lindsey

Lindsey Wise
Sept. 24, 2020, 2:48 a.m.