I am down at U of O right now (well in salem for the break) but we ride during the spring term quit a bit at Fern Ridge. Its not a very nice lake but its only 15 minutes away so it works great for us. There is also....
Foster 45 minutes
Green Peter 50 minutes
Cottege Grove 25-30 minutes
Dorena (sp?) 30 minutes
And there are a lot more around there. The water is going to be COLD until June, but there won't be anyone out until then. Will you be bringing a boat or just trying to find rides? Being that everyone is a college student there aren't a whole lot of boats that you will be able to find rides on. My friends and I usually take our parents boats down at keep them at Fern Ridge during the spring term cause its too cold still for our parents to get any use out of them. I am leaving (graduating) this summer so I don't know if I will have any time to board this spring
As others have mentioned, the "scene" in Portland is much more alive then down in Eugene, even Salem is a lot better then Eugene because it actually has wakeboat dealerships in town. The Willamette River in Salem (and more specifically) in Portland is where lots of wakeboarding happens. The river is a little on the dirty side, but almost always has glass to be found. Detroit Lake, Lake Billy Chinook, The Willamette River, Foster Lake, Prinevill Resivor, and Devils lake are some of the more popular riding spots in Oregon.
BTW- if you have any questions about the University or Eugene in general, feel free to ask.