There isn't much running in March usually. Last year they were doing small releases on the Arkansas in March. It wasn't very much water, but it was boat-able and fun. Really about the only other things running in March are Shoshone on the Colorado and possibly Waterton on the South Platte.
My suggestion would be to wait a couple more months (the runoff typically begins in May in Colorado), but if you really want to come in March I would say come to Buena Vista. Fly in to DIA or Colorado Springs and drive the couple hours to the Ark Valley. The Arkansas is a class III-IV river (mostly class III at lower flows), and some of the runs on it include the Numbers, Fractions, Browns Canyon, Bighorn Sheep Canyon, and the Royal Gorge. Colorado Kayak Supply Kayaks, Kayak Accessories, Gear, Fishing Equipment, Kayaking Accessory
, is located in Buena Vista and can set you up with all the gear you need.
There is also great skiing nearby. 45-1 hr from Monarch, Copper and Cooper. 1.5 hrs from Vail, Beaver Creek, Keystone, Breckenridge, Arapahoe Basin and Loveland.