Occationally, late summer storms will swell front range creeks. More reliable late season water includes:
North Fork of South Platte: Baily and Foxton, augmented by Robert's Tunnel Diversions (there's a gage)
Waterton Canyon: at confluence of North fork and South Platte
Gore Canyon
Westwater (Utah border)
Lyons park and play (under specific irrigation conditions)
Bluegrass Creek in Wyoming
Royal Gorge of the Arkansas
Check out the Colorado Division of Water Resouces website for historical flow information and real time gage data:
Colorado Division of Water Resources