Where do you live?
Predominantly for the south (well it appears that way to me!), there's boatertalk.com
For New England, npmb.com is a larger site.
As someone said for Colorado this is a great site!
Then there's club message boards. For instance, in Colorado Springs, Pikes Peak Whitewater Club has their yahoo messenging site. CWWA has a similar site and again back east (where I am initially from) has several whitewater club web pages where you can post.
Hope that helps,