There are many great kayaking web sites that provide information about rivers and equipment. Many of them also have chat boards for discussion as well as a plethora of pictures.
The best kayaking sites in California are Boof and California Creekin'. The Boof site specializes in the rivers of the Sierra Nevada and has a well used chat board, tons of pictures, and links for gear. California Creekin' has mile by mile descriptions of over 150 different whitewater runs in the state. The sheer number and quality of pictures makes this an invaluable resource for both kayakers and rafters.
There are several smaller groups of kayakers that keep up to date web sites with information about their trips. Kev's Mom is a wacky site that has some good information about kayaking including pictures, stories, and a discussion board. The other classic site with a similar feel is CA-Chunk.
There are several great web sites with information about the sport of kayaking in general. Among the best is Wetdawg Kayaking which has a great chat board as well as many pictures and stories. The greatest part about Wetdawg is the up to date kayaking news and gear reviews. The other classic site is Boater Talk which has an amazing following of paddlers that discuss the sport of kayaking.
Both beginning and advanced kayakers that need kayak instruction should check out Current Adventures which offers beginning classes on the American River as well as advanced classes around the world.
In addition, many kayakers have posted great kayaking information on the web. Among the best of these pages are Craig's Wildwater Pages which includes a variety of information about rivers and geology.
Disclaimer: River descriptions and classifications may change due to natural events that may create new hazards or flows. C-W-R advises that any paddler that uses this site be additionally informed by seeking out local news and updates for changes on this river.