Downwindable.com website where paddlers can lookup/enter details about downwind runs anywhere in the world
Whatcom Paddlers Discussion Group group of avid racers and downwind paddlers based in Bellingham, WA. USA Apply for membership - getting and keeping access is usually straight forward as long as you are legitimately interested in paddling and not a spammer.
Rambo's Locker Ian Newland doesn't seem to be adding to this anymore but there are good past posts. (Is Rambo more transfixed with Ninja Warrior training now?)
Midwest Performance Paddling canoe and kayak racing (website can be a little hard to view - scroll down to get menus into view against background)