Just about every fishable river in California is a navigable waterway. If it can be boated with a kayak (even if only during certain times of the year) it is navigable. If only sections of a river can be kayaked it is a navigable waterway. This has been tested and proven time and again.
For a good quick primer in navigable waterway law go to
http://www...r/nors/us-law-public.htm
If you think your favorite creek is too crazy or thin to be a navigable waterway check out:
http://youtube.com/watch?v=4QMGozRCIoQ
(I can't figure out how to post the video directly into this page. Maybe someone else can post it and let me know how it's done)