Vclaw
2
Is the river still mapped as an area in OSM? Can you give a link to it on the map (use the permalink option).
If its still mapped correctly in OSM, it suggests something has changed in the processing/generation of the Garmin map. You could contact ComputerTeddy and ask, or try using some other OSM Garmin maps.
If its no longer mapped as an area in OSM, you can check the history and find out who changed it. You can contact that user and ask why it was changed. It might just be an error that broke the rendering.