In most years, the Sierra snowpack meltoff gives us a few months of great river rafting, with a few rivers continuing to run into the late summer. The number of rivers lets one choose a wide range of difficulty ranging from the beer float on the lower American River to cascades where a bad line will flip the boat over.
Required people - 1 . The outfitter will fill up the boats. Obviously, it's more fun with a group of friends, both in the drive out and on the river. Most boats take 5-7 customers, so a group of that size is particular good. On the easier rivers, anyone can do it. On the harder ones, participants should be able to swim well and have decent conditioning.
Required gear - generally none. The outfitter will give you the paddle, PFD, and if necessary, a helmet. Wetsuits are either provided or rented where needed, but if you have your own, you should bring it. Those rental suits get a lot of abuse, and often have 'colorful' designs to boot. If you have a better PFD or helmet, you might want to bring those as well. I usually wear tight swim trunks and a thermal shirt under my suit, along with a worn pair of neoprene booties. Some people wear shorts over their wetsuit for better friction on the seat. Hats seem too difficult to work with the helmet. Sunglasses can be a similar problem at the ears. Definitely bring the sunscreen.
Required time commitment - full day with a long drive. Since you'll be tired at the end of the day, it may make more sense to stay overnight in the area before or after. We've done hiking, wine tasting, and cavern exploration. There is the option of multiday river trips, with overnight camping. However, many take a more leisurely tour down the same stretch of river, so I'd rather do two separate day runs.
Costs - prices range considerably based on the length of the trip and the exclusivity of the river. Figure 80-just under 200 for a single day trek, another 100 or so for a 2 day. Weekday prices may be significantly lower for obvious reasons, and websites may have specials on short notice availability.
Rivers are rated on a 6 pt system. Class I is essentially calm, II has some minor rapids. These are the pleasant rivers for Fourth of July party junkets, with many rivaling boats full of water guns and coolers. Class VI is unrunnable. In between, you have the sport of river rafting.
The South fork of the American river is the one most people have experienced. It is a Class III river, which had rapids, rocks, descents. The guide needs to steer the course, but the penalty if the rowers make mistakes is pretty low. There is a III+ element called Troublemaker (or widowmaker) that can turn over a portion of boats, but generally you only get in water voluntarily. Anyone can do this river, and this degree of difficulty. It's a good river for large group (company) outings as the calm sections between the rapids allow for raft to raft water wars. On the downside, it's tame enough that I never got terribly excited.
The Merced River is the easiest of the Class IVs, and a suitable entry for any adventurous types. It's a relatively wide, fast moving river and even has a few spots where you can jump off and run the rapids by yourself. Not too rough - the only time we manage to lose people was when we tried a tricky stunt - didn't quite pull it off.
The year before last we did the Stanislaus River. Some of us in the lead boat got in the drink 4 times, including a spectacular boat flip towards the end. (The second boat of our people barely got wet at all - once I think. But we had more fun) Unlike the Merced, the Stanislaus is much narrower, with many rocks, requiring more nimble manuervering. Lots of one side reverse paddling followed immediately by reversing, and then back again.
This year we will finally make it onto the Tuolumnee River after failing to secure a reservation the past 2 years. Some of these rivers have but a 2 month season in May and June, and on weekends might only be able to have a half dozen boats permitted to enter. You need to be planning these trips in February/March, or be very flexible about the date. The Tuolumnee is one of the harder IV+ rivers, with one V element. We hope to have a blast.
Tuolumnee proved to be big water, but actually easier than the Stan. The outfitter switches to a combo paddle/oar boat, with the guide have two rowboat like oars that allow him to steer and reverse the boat. We hardly ever needed to steer left or right, just straight through the big holes in the rapids. As a result, no one went overboard, and with very few close calls. It's a long 18 mile river, with pretty constant pattern of rapids and calm. But for our money, I think we'd prefer to go back to the paddle only boats.
There is only one Class V rated river - Cherry Creek. It's a pretty expensive trip and has numerous V rated elements. Our group might try it in another year. Apparently the outfitter does some stress testing the night before to make sure all participants are ready for it. This is not a river where a raft can have someone onboard not pulling their weight.
links: These two are among the better of the outfitter guides for the more technical rivers. For the American, I'd suggest soliciting recommendations from friends. I did it many years ago and did notice that some outfits looked much more confident than others. You could still go with these 2 companies, but you might find a good deal elsewhere.
AO Rafting - our preferred
Z Rafting - try them if AO can't deliver the date you want.