Ask nicely from other sites like yours. 'Resource' or 'Links' pages are your friend.
Ask your friends to link from their blogs. If your friends are tech folks, they probably have more than one blog or site they can add your site too.
If you post your site I'll give you some specific strategies and examples.
As far as link opportunities, this seems to me like something that could go in: - lists of google maps mashups - lists of cartographic resources - lists of outdoors resources - On the resources page of every rafting club and outdoors club in the country - especially clubs at colleges.
Is the whitewater data you're laying over the Google Map yours, or does it come from somewhere else?
See http://www.seomoz.org/blog/how-i-would-do-seo-for-hipmunkcom for Tom Critchlow's take on it. (Did this post inspire your question? Seems coincidental.)