My understanding is that Hydrilla does not exist as of yet in Oregon. You probably have Elodea.
When I first found Elodea in my pond I thought it was Hydrilla but later determined it was Elodea.
We used a combination of Reward and Cutine plus on the Elodea and it worked wonders. I don't know if these herbicides are allowed in Oregon though.
Break off a strand and take a close up photo of the leaves. Then an identification can be made.
Here is a two page adobe file that will help you identify
Hydrilla versus Elodea. 
