Several people have written about rafting the waters of Merced River in Yosemite Valley. Here is a very good description:
Here is a short but nice video showing the peaceful water and Yosemite Falls (I believe) in the background.
The flow of the river is too slow to be exciting for adults, but it can be a really great paddle to do with young kids. A good water fight will make it fun for everyone. Make sure to bring water cannons.
Kayaking the Merced River in Yosemite Valley has to be one of the tops in terms of beauty in surroundings. To say Yosemite is beautiful is probably an understatement. Out-of-this-worldly Valley of the Gods is more like it. Add to it kayaking down the beautiful river running through the middle. Words probably can't describe it.
Rafting season starts in June, which also opens the river for kayaking (if you bring your own kayak). The exact date varies by the combination of water temperature, air temperature, and height of the water. The rafting page on Yosemite's web site has more information and current status:
You can call the Curry Village Recreation Center to find out details, and a live person will actually answer:
Here you have it, all the information you need to kayak the Meced River in Yosemite Valley. If you go, let me know how it is because one of these days, I will make it out there, too.