This was posted on Boater Talk today, re-posting here:

"I just happened onto this by accident today . . .

If you pull up maps.google.com, and get "Directions" (from anywhere, to anywhere, like if you are going on a trip out-of-state) . . .
And then, if you put (into the google-search-bar) http://www.americanwhitewater.org/content/River/state-summary/state/xx/.kml but replace the xx with the two-character abbreviation for any state along your route, when you click the search-button (or press "Enter"), your travel route stays onscreen, AND the rivers (as listed on AW) will be indicated with colored dots, color-coded for runnable flows (where gauges exist, anyway). And, you can click on a dot, then click on a link to go to the river description.

Want to check out another state along your route? Change the two-character abbreviation to another state, click the button (or press "Enter"), and VOILA! you get the next state's rivers (and your route is still visible)! If you see a river some distance off your route, click+drag+drop on the route-line and Google will reroute to your desired detour (to the river). Doing this will cause the AW river-dots to disappear, but the url will still be in the Google-search bar, so you can click the search button again to show them."