There are options for whitewater rafting near Mammoth Lakes just over 2 hours away, one of the closest being the East Fork Carson to the north, close to South Lake Tahoe. However this is not a dam controlled river and therefore has a variable season dependant on the snow pack. The Truckee River in North Lake Tahoe and the American River are dam controlled and available all season long!
Not offered by Tributary, but other Rafting options close to Mammoth: The Merced (spring only), and Tuolumne (challenging class 4 rapids) are both within 2.5 hrs. but they are on the opposite side of Yosemite National Park and accessible via Tioga Rd which is subject to closure in the spring due to snow. Visit the Tioga Road page on the Yosemite NP site for latest conditions. If this road is closed, travel time would be more than 3.5 hrs.
The Mammoth Lakes area offers outdoor adventurists many options. From Hiking, Mountain Biking, Fishing, and of course white water rafting you are sure to find something for everyone! Incorporate sightseeing as well with your adventure with a visit to some amazing local wonders: