From what I've read die-hards sail all year round on the Columbia River. I sailed there around 14 years ago in their autumn. It was great! However even then the water was a bit cool so I would imagine a pretty good wetsuit and booties would be the go in spring. Coming from Queensland you may have to buy some cold weather gear.

sailed there over 20 years ago, in June but I recall it being all that cold. But it certainly wasnt 3 meter type stuff.
Great place to visit, trip around a bit while you are there, its a lovely area.
Cant say I get any enjoyment from anything stronger than about 30 knots though

Hi There
April is too early for the Gorge
The Gorge happens with thermals through the Gorge. You can rent gear there really easy about $60 US (Latest Boards & couple of sails) but you will need a car to get to the good spots. When it's happening it's awesome. You need a 3.7 for sure on the good days.
Here is a web site to start with
www.windance.com/
They sell gear but there are a bunch of links from their website & plus it will give you some idea's on pricing. The only catch with the Gorge is that you have to move around abit to catch the wind. It can be blowing 20 knots at the Event Site & then 35 at the Hatchery just across the river. Check out sites with the words "The Dalles" "Hood River" & this should help[ you out a bit
Cheers