Most of the Arkansas river including the Pueblo tailwater - is chocolate milk, running above 2,000 cfs, and is an all around bundle of joy for rafters.
The South Platte - is running between 70-100 cfs depending on which stretch you're fishing, is low and clear, and full of beat-up highly pressured fish. Flies of choice for anyone nymphing are: Barr's Emergers bwo 22, JuJu Baetis and midges - the smaller the better, Caddis Candy size 18, Black Beauties size 22-24, Top Secret Midges size 22-24.
Anyone throwing dry flies should carry Barr's Visaduns bwo 20-24, Parachute Adams 20-26, tan Elk Hair Caddis 16-18, Matt's midge 22-24, Griffiths Gnat 22-26.
Start out nymphing in the morning. After a good lunch be prepared to cover a lot of water throwing dry flies.