From The Pueblo Chieftain (Ron Van Valkenburg):
The waters offer year-round fishing opportunities and are especially important to fly-fishers who pursue trout throughout the winter. Unlike the waters above the reservoir, called freestone rivers, which get jammed with ice in the winter, tailwaters remain fishable for several miles below the dam and reservoir, which provides a constant water source. The quality of the tailwater depends on the depth of the reservoir, water temperatures, the gradients of the stream channel, and the amount of water released from the reservoir.
The greatest advantage to a tailwater fishery is this controlled source of water. Regardless of drought, there is a constant flow with ideal water temperatures, a weed-rich stream bottom with high levels of nutrients, and diverse aquatic insect populations — all key ingredients for growing large, healthy fish.
More Colorado Water 2012 coverage here.