Hi! It's a little difficult to see the rock you found in the images. I suspect that it might be a schist (metamorphic rock) because it seems to be shiny and made of thin plates of muscovite. It's an odd rock to find in a river because schist usually breaks easily. I suggest you take it to a local geologist for further identification. There are geologists in the earth science departments at your local universities and community colleges in California. It's a very shiny rock! Sharp eyes!